Chapter 15 Classification of Matter 1. Composition of Matter
a. Pure Substances—materials are either pure substances or mixtures. A pure substance has fixed composition. Pure substances can be elements or compounds. Examples: helium, aluminum, water, and salt.
i. Elements—all substances are built of atoms. If all of the atoms that make up a substance are the same, we call this an element. Example: The graphite in a pencil is all carbon. Carbon is an element. Copper is an element. Mercury is an element. Oxygen is an element. Nitrogen is an element. They can be found on the periodic table. P. 518-519
ii. Compounds—Two or more elements can combine in fixed proportions to form a compound. Examples: water is a compound of hydrogen and oxygen in a fix 2:1 proportion…2 hydrogen to 1 oxygen (H2O1 or just H2O); Carbon dioxide is a combination of one carbon and two oxygen (C1O2 or just CO2). Chalk is Calcium carbonate. One calcium joins with one carbon and three oxygen to make this compound (Ca1C1O3 or just CaCO3). Salt is the combination of sodium and chlorine. Sodium is normally an explosive metal and chlorine is a deadly poisonous gas.
b. Mixtures—soda and pizza are mixtures. Soda is homogenous and pizza is
heterogeneous. These are mixtures because they can be separated by physical mean. No chemical reactions need to take place.
i. Heterogeneous mixtures –compounds are combinations of elements that have a fixed proportion (H2O1). A compound will look the same throughout. Mixtures are not compounds because they do not have a fixed proportion. Some mixtures like granite, concrete, pizza are not the same throughout. You can see the mixture of different materials.
ii. Clothing labels show you that clothing is a heterogeneous mixture. You might not be able to see the mixture with your own eyes, but each individual fiber is made up of a combination of fibers. Ex. 90% cotton 10% polyester. Picture on page. 453
some are transparent (can see through easily), some are translucent (can see through with difficulty).
iv. Solutions--If a homogenous mixture is easy to see through like white distilled vinegar or air, then we call it a solution. A solution is easy to see through because the particles that are mixed in a very, very small. They are essentially invisible.
v. Colloids—milk is a colloid. The particles in a colloid are small, but not as small as a solution. They can be see, but they are too small to settle out. A colloid is a type of mixture with particles that are large enough to detect with our eyes, but not heavy enough to settle out. Examples: milk, paint, fog, smoke, mayonaisse vi. Detecting Colloids—Some are easy to see and are opaque (like milk or glue or
mayonnaise). Some are translucent. They may appear almost clear, but light will be scattered. The Tyndall effect is the scattering of light by the suspended particles in a colloid. Ex. Sun shining through the clouds makes sun rays; headlight on cars will appear blurry in the fog, a laser will make a beam when it shines through a colloid.
vii. Suspensions—some mixtures where the particles are large and do eventually settle down to the bottom are called suspensions. Suspensions can look like colloids if they have just been shaken about. Examples include a muddy stream. If you collect the water in a cup, the sand, mud and silt will settle to the bottom leaving clear water above. Orange juice is another example.
Table 1 on page 456 summarizes solution, colloid, and suspension. Coffee: colloid or suspension?
Self Check p. 456 Answers #1. Compounds and homogenous mixtures are both made of more than one
element. They are also homogeneous. They are different because compound must be made of elements in fixed composition and have chemical bonds between them. Homogenous mixtures can be in different, non-fixed compositions. #2. A substance must be either an element or a compound. Example:
Aluminum, water, salt. A mixture can be made of combinations of compounds or elements. Example: bread, pizza, orange juice, milk. #3. Colloids have small particles that do not settle out. Suspensions have larger particles, they can settle out. #4. You have to shake orange juice because it is a suspension. It doesn’t taste good if all of the suspended particles are down at the bottom of the jug. This works for Tabasco, Tepatio, and salsa. #5. Homogenized milk is a colloid because the particles of fat and protein do not settle out of the mixture. OJ is a suspension because the pulp particles are large enough to settle out. You will want to shake the OJ to keep from getting a mouthful of pulp at the bottom of the jug.
a. Physical Properties—any characteristic of a material that you can observe without changing the identity of the material. Ex. Stretchiness of a rubberband, wetness of water, hardness of a rock, color of gold
i. Appearance—the way things look. Pencil lead is dark. Coffee is brown. Milk is white. Gold can be yellow. How much?
ii. Behavior—how something acts can help us identify it. Iron is magnetic, aluminum is not magnetic. Ductile—the ability to be shaped into a wire
(copper). Malleable—the ability to be hammered into a shape (gold). Viscosity— the ability to flow (syrup vs. water).
iii. Using Physical properties to Separate—We can exploit difference in physical properties to separate mixtures. Size can be used to separate a mixture of different kinds of seeds (Figure 12 p. 459). Iron filings can be separated from a mixture of sand or water by using a magnet.
b. Physical Change—you can change a physical identify without changing a substance— melting glass, chewing gum.
i. The Identity Remains the Same—freezing, boiling, evaporating, condensing are all changes, and the underlying chemical has not changed. This is called a physical change. Physical changes involve changes in energy, but not the kind of substance. Certain physical properties cannot be changed and they help identify the substance. Examples: density, specific heat, boiling point, melting point. (Intensive properties)
ii. Using Physical Change to Separate—Physical properties can be used to separate substance. Salt from water by distillation. This takes advantage of the fact that water boils at a much lower temperature than salt. We can boil off the water and leave the salt behind.
iii. Distillation—distillation takes advantage of the different in boiling point of substances to separate them.
c. Chemical Properties and Changes—a characteristic of a substance that indicates whether it can undergo a certain chemical change. Flammable—able to combust in oxygen. Photoreactive—able to react in the presence of light. Able to react with water. Able to react with oxygen.
d. Detecting Chemical
ii. Clues of change include: odor, color change, heat, light, sound, gas formation, solid formation.
iii. Using chemical change to separate—metals can be separated from their ores by chemical reaction. Gold and silver from rock.
Practice Problems: p. 463 #1 and #2
Answer #1. 120g-66g=54g of water; #2 463.2g = 196.2 g + 267 g AlCl3
e. Weathering—chemical or physical change? Natural processes wear away rock. A combination of physical and chemical change make weathering happen.
i. Physical—when water seeps into rock then freezes, the frozen water expands and splits the rock. Streams can carry rock particles that wear away the surfaces on which they flow. Glaciers can do the same thing.
ii. Chemical—Limestone, CaCO3, is a chemical that reacts with acid. If rain is slightly acidic, limestone will react chemically. It will produce CO2, and the resulting Ca will dissolve in water and flow away.
f. Conservation of Mass—wood is combustible in oxygen. What is left is a pile of ash. The ash seems small and light compared to the heavy wood that started to burn, but mass is not destroyed. The law of conservation of mass says that mass of all substances that are present before a reaction equals the mass of the substances that remain after a
reaction. Where does the rest of the wood mass go after it reacts with oxygen?
Schmidt notes for Chapter 16 : Solids, Liquids, and Gases 1. Kinetic Theory
a. States of Matter—How are the different states of matter different?
i. Kinetic Theory—explains how the particles in matter behave. Three assumptions
1. All matter is composed of tiny particles (atoms, molecules, and ions)
2. These particles are in constant, random motions
3. These particles collide with one another and the walls of any containers that might contain them.
Let’s use balloons to visualize this. ii. Thermal Energy
1. Ice cube: atoms are held in place. The attractions between particles give the solid a definite shape and volume. The thermal energy is so low that the particles vibrate in place.
2. TE=KE + PE
3. If the temp. is lowered the particles will have less TE
iii. Average Kinetic Energy—temp. is average KE. On average, molecule of frozen water at 0C will move slower than particles of liquid water at 100C. Particles have some KE all the way down to absolute zero where scientists theorize that no more KE can be removed (-273 deg C).
iv. Solid State—most solid materials have a specific type of geometric arrangement in which they form when cooled. Ice crystals (see p. 477). The type of geometric arrangement determines some of their chemical and physical properties.
1. Definite volume, definite shape.
v. Liquid State—if the ice heats, colliding particles transfer energy to neighboring particles. If the collisions continue to happen, the particle of ice gain enough KE to overcome attractive forces between them. The KE/T where this happens is the melting point. The amount of heat that it takes to increase the particles to overcome their attractive force is called the heat of fusion (latent heat of fusion).
vi. Liquids flow—particles have overcome some of their attraction to each other, they can slip past each other.
1. Definite volume, no definite shape (assume the shape of their container)
vii. Gas State—liquid particles are constantly moving. Most of the time they maintain some attraction to each other. Occasionally, one particle will have enough KE to escape the attractive force of another entirely. This is called
vaporization. This happens in one of two ways
1. Evaporation—below the liquid’s boiling point
particles have enough KE to aggressively escape their attraction to other particles.
3. No fixed shape, No fixed volume (volume depends on temperature) viii. Boiling Point—the temperature at which the pressure of the vapor at the
surface (vapor pressure) of the liquid is equal to the external pressure (atmospheric pressure) acting on the surface of the liquid.
1. The amount of energy required to get the particles to escape the attractive forces of a liquid is the “heat of vaporization”.
ix. Gases Fill their Container—bec. Attractive forces between particles are
overcome, the particles are free to move anywhere they wish. By diffusion they fill the container that they are in. Particles continue to collide with each other, other particles in the room, walls of the container
1. Spraying body spray.
x. Heating curve of a liquid—see the boiling curve on page. 480.
xi. Plasma state—good for killing aliens, plasma consists of positively and
negatively charged particles. The particles have so much energy, that when they collide, the energy of the collision causes electrons of the atoms to be stripped away (we will learn more about electrons later)
b. Thermal Expansion—ever notice the lines that the workers put in the concrete of sidewalks. Expansion joints! If you don’t use them, the concrete will crack (sometimes it cracks anyway).
i. Expansion of Matter—as something heats up, the collisions cause the particles to separate. Thermal expansion is the increase in the size of a substance when the temperature increases. Contraction is the name for the opposite process. ii. expansion and contraction occurs in solids, liquids and gases
iii. Expansion in Liquids—thermometers show this…because the tube is narrow, the expansion of a liquid in a thermometer can be seen, even if the liquid expands only a little bit.
iv. Expansion in Gases—hot air balloons show this…heat can cause the air in balloons to expand greatly. The big expansion causes the density of the air to increase. Less dense air goes up while low density air goes down. The balloon will rise as long as the less dense air is trapped and stays hot (less dense). v. The strange behavior of water—normally things expand when their temp rises.
Highly positive and highly negative areas of water molecules cause ice (solid water) to form a complex solid crystal that contains a lot of space. That means that a solid ice crystal is actually less dense than the liquid (that’s why ice floats). Our planet would not work properly if this phenomena did not occur. c. Solid or a Liquid?—weird stuff.
1. Example: glass and plastic—they are made of long structures that get jumbled and twisted.
ii. Liquid crystals—for some reason these substances do not lose their orderly structure entirely when they go into the liquid phase. They are sensitive to temperature and electric fields. Used in LCD devices.
Self Check p. 483 answers: 1. In text 2. Solid: vibrate in place; liquid: slide past; gas: move freely; 3. Particles gain enough KE to break some attractive force and slip out of their orderly crystalline
arrangement.; 4. Particles overcome their downward pressure from the atmosphere and escape from the liquid; 5. Where is the atmospheric pressure higher? On the top of a mountain or at sea level? What would that do to the boiling point? 6. From -15C to 0C the solid water absorbs energy, the energy goes into breaking attractive forces (heat of fusion) and the particles begin to have higher TE even though their temperature has not risen. When all of the particles are in the liquid phase, the temperature begins to rise. Overall TE increases because the KE is going up. This continues until boiling point. At that time, the temp stops rising because the extra energy goes into breaking the remaining attractive forces between the molecules. This energy is called the heat of vaporization. Once all of the molecules are free, the vapor can continue to increase temperature by increasing the KE.; 7. Draw a temperature heat graph like the one on page 480 but change the melting point and boiling point plateaus.
2. Properties of Fluids
a. How do ships float?—Most ships today are built of materials that are far too dense to float (steel). Whenever anything goes in water it experiences two opposing forces: downward force of gravity (weight) and upward force of buoyancy. Buoyancy is the ability of a fluid—a liquid or a gas—to exert an upward force on an object immersed in it. IF the buoyant force is equal to the object’s weight, the object will float. If the buoyant force is less than the weight of the object, the object will sink.
i. Archimedes’ Principle—the buoyant force on an object is equal to the weight of the fluid displaced by the object. Example, a block of wood will push water out of the way as it begins to sing—but only until the weight of the water displaced equals the block’s weight. When the weight of the water displaced—the buoyant force—becomes equal to the weight of the block, it floats.
ii. Density—Archimedes’ principle is related to buoyancy. Example—a same size steel block as the wood block above will displace the same volume of water and therefore the same weight of water. The buoyant force of both are the same. The still will sink because the weight of the block exceeds the weight of the water. This is because steel is far more dense than wood.
1. The shape of the steel can be changed to increase the volume of the mass. In the shape of a boat hull, the overall density of the steel boat and air contained within the design decreases the density.
one end of a balloon, the balloon expands out on the other end. Example: squeezing toothpaste, squeezing mustard, hydraulic lifts, car brakes.
i. Applying the principle—pressure applied on a small cylinder is transmitted throughout the fluid to the large cylinder.
ii. Examples on page 487 explain how the force is transmitted and changes depending on the surface area of the piston.
c. Bernoulli’s Principle—as the velocity of a fluid increases, the pressure exerted by the fluid decreases. Ex. Blowing across paper. Used in aircraft wings and fluid-transporting piping systems, and hose-end sprayers.
d. Fluid Flow—tendency to resist flow is called viscosity. Fluids vary in their tendency to flow. When a bottle is tipped, the falling particles have a tendency to pull on stationary particles. If this pull is high, then the stationary particles will experience a quick change in energy…the product will flow more…low viscosity. If the pull between particles is weak, then the moving particles will have less affect on the stationary particles. This will cause flow to be very slow…high viscosity.
Self Check p. 489 Answers 1. Weight is pushing down and the buoyant force is pushing up. 2. Archimedes’ principle says that the buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object. The overall density of an air-filled ship is less than that of water. 3. When you squeeze one end of the mustard container, the pressure is transmitted throughout the mustard (fluid), forcing mustard out the top. 4. The fast-moving winds in tornadoes create a low pressure area above the roof. The pressure under the roof is greater than the pressure above the roof, pushing the roof off. 5. The air in the balloon is compressed. Thus, its weight exceeds the buoyant force of the surrounding air. Helium is less dense than air, and the balloon would float. 6. Not assigned (the buoyant force is 1.2N) 7. Not assigned (F=514N)
3. Behavior of Gases
a. Pressure—pressure is caused by the constant collision of particles. In balloons and bicycle tires, the more air that you pump in, the more collisions are possible. The more collisions, the higher the pressure.
i. UNITS: in SI the unit is called pascal (Pa). This is the amount of force divided by the amount of area. 1 pascal is equal to 1 N/m2. A pascal is a very small unit, so most applications that we see use the kPa (1000 Pa). Standard atmospheric pressure on a usual day will be about 101.3 kPa. This means that the earth exerts a pressure of 101300 N for every square meter.
b. Boyle’s Law—For gases, pressure and volume relationship with constant temperature.
1. Example: weather balloons—neoprene weather balloons will expand anywhere from 30 to 200 times their size as they rise into the low pressure regions of high altitude.
2. If pressure increases, then volume will decrease, for gases.
ii. Boyle’s Law in Action—P1V1=P2V2
c. The Pressure-Temperature Relationship—at constant volume, an increase in temperature results in an increase in pressure.
d. Charles’s Law—relates volume and temperature. At constant pressure, an increase in temperature will increase the volume of a gas. Likewise, if temperature decrease, then the volume of a gas will decrease.
i. Using Charles’s Law—V1/T1=V2/T2
Schmidt’s notes for Chapter 17: Properties of Atoms and the Periodic Table 1. Structure of the Atom
a. Scientific shorthand—Some chemical symbols have one capital letter and one lower case letter. Some chemical symbols have only one capital letter with no lower case symbol. For some elements, the symbol is the first letter of the name plus another letter from the name. Some symbols are derived from Latin names (ex. In Latin, the work for silver is Argentum). Some elements are named for an elements properties or in honor of scientists . Some elements are named according to rules set by an international
committee. Symbols are used worldwide by scientist to conveniently write chemical formulas (H2O vs. dihydrogen monoxide)
b. Atomic components—an atom is not the fundamental particle that matter is made of, it is the smallest part of matter that actually still acts like the element that it is associated with (retains the property of the element).
i. Atoms are made up of smaller particles called protons, neutrons and electrons ii. Protons and neutrons are in a very small positively charged center of the atom
called a nucleus
iii. Protons have +1 charge
iv. Neutrons are neutral, no charge v. Electrons have a -1 charge
vi. Atoms of different elements vary in the amount of protons that they have. c. Quarks—even smaller particles—It turns out that protons and neutrons are made of
smaller particles. So far, scientists have confirmed existence of six uniquely different quarks. Apparently if three types of quark get together with the strong nuclear force, they form a proton. If a different three get together with the strong nuclear force, a neutron is formed.
a. Note: Quarks are classified by flavors
Up, down, charm, strange, top, bottom
ii. Finding quarks—these are found in machines known as particle accelerators. The accelerators speed up particles to incredible speeds then crash them together. The Tevatron in Batavia, Ill and the Large Hadron Collider on the border of France and Switzerland.
1. Scientists use various detectors and, like a detective piecing together a case, use inference to identify the subatomic particles by marks and trails left behind on detectors.
iii. The Sixth Quark—took a long time and lots of scientists to find it
d. Models—tools for scientists—models help us talk about something that cannot be easily seen. The best models account for the most information that is known about matter and the behavior of atoms.
the 1800’s John Dalton was able to prove the atoms exist. Models develop from that time (see p. 510)
ii. Electron cloud Model—An electron cloud is an area around the nucleus of an atom where the electron is most likely to be found. The electron cloud is 100,000 times larger in diameter than the nucleus. Each electron is much smaller than a proton (just less than 2000 times smaller)
1. Because electrons are so small and moving so quickly, it is impossible to find the exact location. Like a bike tire spinning, you know that the spokes are there, but you cannot see an individual spoke while the tire is spinning quickly.
Self Check p. 511 answers: 1. C, Al, H, O, Na; 2. Proton, +1, nucleus; neutron, 0, nucleus; electron, -1, electron cloud; 3. Quark, by accelerating protons and making them collide with so much force that they break apart; 4. This model says electrons are most likely to be found in a cloud surrounding the nucleus; it is 100,000 times large than the diameter of the nucleus.; 5. The blades of a fan that is rotating appear to be a smooth metal surface around a center hub. The probability area for electrons is an atom also appears to be a smooth area of solid appearance. The fan is a limited model because it can be turned off and the blades can be seen easily.
2. Masses of the Atom
a. Atomic mass—protons and neutrons are far more massive than electrons. This means that most of the mass of an atom resides in the nucleus. Protons and neutrons have about the same mass. Protons and neutrons are almost 1800 times larger than an electron. The atomic mass unit is defined as 1/12 of a carbon 6 atom. Protons are about 1 amu.
i. Protons identify the element—ex. Every carbon has 6 protons. If it has seven, it isn’t carbon. The number of protons in an atom is equal to a number called the
atomic number. If you are give the name of the element or the number of protons or the atomic number, you can figure out the other two.
1. You will need a periodic chart for this.
ii. Mass number—of an atom is the sum of the number of protons and the number of neutrons. If you know the mass number and the atomic number, you can calculate the number of neutrons.
Number of neutrons = mass number – atomic number
Atoms of the same element that have different mass numbers (different numbers of protons) have slightly different properties. For example, carbon 12 and carbon 14 (carbon 12 is very abundant on the planet, carbon 14 is not as abundant and is radioactive.
Note: the mass number indicated on the periodic chart is the weighted average of the masses of all of the isotopes of that element.
have the same number of protons with different number of neutrons. One of these has 5 neutrons the other has 6 neutrons. They are both still boron because they both have 5 protons (atomic number of 5), but they have different masses because they each have a different number of neutrons. They respond in nature in all the same ways that boron usually responds, and there are small differences that are important.
i. Identifying isotopes---the average atomic mass of an element is the weighted-average mass of the mixture of its isotopes. For example, foru out of five atoms of boron are boron-11, and one out of five is boron-10. Ex calc. using weighted average:
4/5 (11amu) + 1/5 (10 amu) = 10.8 amu
Self check p. 515 answers 1. Mass number 35 AMU, atomic number = 17. ; 2. Isotopes have the same number of protons and different numbers of neutrons. 3. Elements have several isotopes with different numbers of neutrons, and thus different masses. The average atomic mass is the weighted average of the masses of the element’s isotopes. 4. Mass number – atomic number = number of neutrons = 40 – 19 = 21 AMU. 5. The average of 35.45 lies closer to the 35 mass number than to the 37 mass number. 6. 3; 7. Use the information on table 2 to figure this out….I know that you can do it!
3. The Periodic Table
a. Organizing the elements—the term periodic means that certain trends and properties of the elements periodically repeat themselves along the periodic table. Mendeleev first noticed patterns in the properties of discovered elements. He noticed that patterns in properties of lighter mass elements repeated in the properties of heavier elements. Today the periodic table of elements are arranged by increasing atomic number and by changes in physical and chemical properties (associated with atomic number…that is, number of protons).
i. Mendeleev’s predictions—because of the periodic trends, Mendeleev was able to make predictions that elements would be discovered that had not yet been discovered.
ii. Improving the Periodic Table—Henry Moseley, figured out that the trends were more regular when arranged by atomic number rather than mass.
b. The atom and the periodic table—The vertical columns on the periodic table are called groups or families. They are sometimes number 1-18. Elements in the same group have very similar properties. The structure of the atoms reveals the explanation for this phenomenon.
in the outer energy level are the last to fill. These electrons determine the properties of the element. Elements in the same group have the same number of electrons in the outer energy level. This explains why they have similar properties.
ii. Energy levels—energy levels are named using number from one to seven. The maximum number of electron that can be contain in each of the four levels is shown in Fig 10 p. 520.
iii. Rows on the table—the last element in each row has a completely full outer energy level. Ex. Hydrogen has only one electron. That electron occupies the only energy level available to the hydrogen atom. That energy level can only hold two electrons maximum. Well, the helium element will be neutral when two electrons fill the out energy level. Helium has two electrons—the maximum number of electrons that the energy level can hold.
1. Electron dot diagrams—by using dots, scientists can represent how many electrons are in the outermost energy level. Group 1 will all have one dot, because their outer energy level typically has only one
electron. Meanwhile, the Helium group will have two electrons in the outer energy level. These are also called Lewis diagrams.
2. Same Group—Similar Properties—Because the dot diagrams of the elements in group 17 (halogens) have one open space for an additional electron. Because the group 1 (alkali metals) elements have one electron available in the outer energy level, these two groups typically react together to form a combined substance (a compound) where electrons are either shared or exchanged.
c. Regions on the periodic table—horizontal rows on the periodic table are called periods. Elements atomic number increase by one as you advance through a period. The periodic table has regions as labeled in figure 14 p. 523—metals, metalloids, and nonmetals.
i. A Growing Family—some elements are being synthesized in the laboratory. The discovery of new elements continues to be confirmed. These new elements have been very short lived.
d. Elements in the universe—hydrogen and helium are believed to be the building blocks of all elements in the universe. The fusion of these elements in the extreme conditions from supernova (exploding stars) has lead to the synthesis of all of the elements—heavy elements like iron are flung out into the universe. Promethium, technetium, and elements with an atomic number above 92 are rare or are not found on Earth. Some of these elements are found only in trace amounts in Earth’s crust as a result of uranium decay. Others have been only found in stars.
Schmidt’s Notes for Chapter 18: Radioactivity and Nuclear Reactions 1. Radioactivity
a. The nucleus—we are constantly bombarded with particles emitted by unstable atoms. The particles come from the nucleus of unstable atoms. Review structure of the nucleus of atoms. The total amount of charge in a nucleus is determined by the number of protons (atomic number). Electrons swarm around the nucleus.
i. Protons and Neutrons in the Nucleus—protons and neutrons are packed tightly in the nucleus. The nucleus is tiny compared to the overall size of an atom. The amount of empty space that makes matter is difficult to comprehend.
b. The strong force—Protons and neutrons are held together by a force called the strong force. The strong force is one of the four basic forces in nature. It is 10,000 stronger than the electric force that causes charges to repel and be attracted. However, the strong force is very attractive a close distances only while the electric force is powerful at relatively far distance. Protons that are far apart are repelled by the electric force from each other (two positives repel). Protons that are very close together, within range of the strong force, may be able to stick together because the powerful strong force can overcome the electric force that causes repulsion.
i. Attraction and Repulsion—many atoms have protons and neutrons together in the nucleus of the atom. These nuclei are held together less tightly. If a nucleus has only a few protons and neutrons, the strong force does a good job holding the nucleus together.
ii. Forces in a Large Nucleus—For very large nuclei, protons and neutrons may have considerable distance between each other and their neighbors. Because only the closest protons and neutrons attract each other in a large nucleus, the strong force holding them together is about the same in a small nucleus. However, the protons in the nucleus also experience a stronger repulsive electric force from other protons in the large nucleus that are outside the range of the strong force. Because the repulsive electric force in a large nucleus increases while the strong force remains about the same, the nucleus has a net force held less tightly. A large nucleus may be less stable than a small nucleus. c. Radioactivity—If a nucleus is large enough that the strong force does not hold it
together, the nucleus can decay and give off some of the matter from the nucleus. This nuclear decay is called radioactivity. Large nuclei are unstable. Atoms with atomic number higher than 83 are radioactive. Some nuclei with atoms less than 83 are also radioactive. Atoms with atomic number higher than 92 do not exist naturally on Earth. They have been synthesize in laboratories…called synthetic elements. They decay soon after they are created.
i. Isotopes—have the same number of protons and electrons and different number of neutrons. C-12, C-13, C-14. (review)
are stable if the ratio is about 3:2. However, any nuclei that vary from this ratio are unstable…whether they are light or heavy. In other words, nuclei with too many neutrons can be unstable and nuclei with too few neutrons can be unstable.
iii. Nucleus numbers—the total of the atomic number and the number of neutrons is called the mass number.
Atomic number + number of neutrons = mass number
Symbols used to write these numbers are useful for discussing nuclear reactions. Example.
This isotope of carbon is also called carbon-12 to keep things simple. This isotope is stable. Meanwhile, carbon-14 isotope is unstable…radioactive.
iv. The discovery of radioactivity—in 1896—Henri Becquerel left uranium salt in the same drawer as a photographic plate. When he went to develop the plate later, he noticed that the image of the uranium salt was exposed onto the plate. Two years later, Marie and Pierre Curie discovered two new elements—polonium and radium. Radium had to be extracted from a mineral called pitchblende. After about 3 years of work they managed to extract 0.1 g of radium.
Self check p. 540 answers 1. Short range, attractive, between protons, neutrons, and protons and neutrons. 2. The strong force is exerted by nearest neighbors, so it is always the same no matter the size of the nucleus. 3. Large nuclei are unstable because they have so many protons that the strong force is unable to overcome the total electrical repulsion of the protons from each other. 4. Becquerel found out that uranium salt emitted radiation. Marie and Pierre Curie discovered polonium and radium. 5. 82/132= 0.62. This might be radioactive because it differs from the 2/3 (0.67) radio that we saw in our text. 6. 86/136= 0.63. 7. Total mass of the rod is 32.76g; silicon-28 percent is 30.21g/32.76g= 92.25; silicon-29 = 1.53g/32.76g = 4.7%; silicon-30 percent is 1.02g/32.76g = 3.1%.
2. Nuclear Decay
a. Nuclear radiation—when an unstable nucleus decays particles and energy called radiation are emitted from it. There are three types of radiation: alpha, beta, and gamma. Alpha and beta are particles that have mass. Gamma radiation is an electromagnetic wave.
i. Alpha particles—an alpha particle is made of two protons and two neutrons. This is also a helium-4 nucleus. This has a charge of +2 and an atomic mass of 4. Its symbol is:
alpha particles pass through matter they pull electrons away from the matter. Because of their relatively high mass and high charge, these particles do not penetrate matter very much. They can be stopped by a sheet of paper.
ii. Damage from alpha particles—these can be dangerous if released inside the human body. The ionizing affect as these particles pass through sensitive tissues can damage DNA. These can lead to illness and disease (cancer).
iii. Smoke detectors—used alpha particles emitted by Americium-241 to complete an electrical circuit. When smoke gets into the circuit, the ionizing radiation current is broken and an alarm goes off.
iv. Transmutation—If an atom emits and alpha particle, it has few protons, so it is a different element. Transmutation is the process of changing one element to another through nuclear decay. In alpha decay, the element atomic number is reduced by two and the mass number is reduced by 4.
b. Beta particles—happen when part of a neutron decays. The neutron loses part of itself and sends out an electron. The rest of the neutron stays in the atom as a proton. A beta particle is an electron emitted from a nucleus. This leads to a transmutation also because now the atom will have an additional proton. That atom will not have lost a significant amount of mass. A beta particle symbol:
Example:
i. Beta particles are smaller (less massive) and have a -1 charge. Because they have less mass and a lower charge, they can penetrate other matter more deeply. Their charge also causes the ionizing effect that damages living tissue. Because these particles penetrate more deeply, a thicker barrier is needed to stop the particles—aluminum foil is enough.
c. Gamma rays—the most penetrating form of radiation—because the gamma rays have no mass and no charge. Gamma rays are electromagnetic waves with the highest frequencies and the shortest wavelengths in the electromagnetic spectrum. They travel at the speed of light. Thick blocks of dense material such as lead or concrete can block gamma radiation. Over the same distance, gamma rays produce fewer ions than alpha particles or beta particles because they have no electrical charge. They are high energy so they are still quite damaging.
d. Radioactive half-life—A lump of a radioactive element will lose its radioactivity. In other words, the radioactive isotopes in the element, decay to stable elements. Some
measure of the amount of time that it takes for half the nuclei in a sample to decay. Figure 10 on page 544 shows this well. Every 12. 3 years, half of the sample of H-3 decays into the daughter element He-3 (beta decay). Half-lives vary depending on the element. They vary widely. See page 544.
e. Radioactive dating—half-life is the way that geologist, biologist, and archaeologists determine the age of a sample of something. By measuring the amount of radioactive element and the amount of daughter isotope present, the number of half-lives needed to attain the measured amount of material can be determined.
i. Carbon dating—is used to estimate the ages of plant and animal remains. Carbon-14 has a half life of 5730 years. Plants use carbon dioxide from the air when they make their food. The CO2 in the air has a stable amount of carbon-14. The carbon-14 become part of their body. While the plant or animal is living, the ratio of carbon-12 to carbon-14 in the body remains constant because fresh carbon-14 is taken in and replacing old C-14 and C-12. When the organism dies, it stops taking in C-14 and the C-14 decays without being replaced. As a result, the C-14 to C-12 ratio in the dead organism decreases over time. By measuring this ration, the age of the organism can be estimated long after its death. C-14 can only be used for organisms that have lived over the last 50,000 years because the C-14 will decay to amount too small to detect.
ii. Uranium dating—can be used to estimate the age of rocks. Uranium has two radioactive isotopes that decay into different forms of lead. By measuring the amount of the daughter lead isotopes, the age of the rock can be deduced.
Self check p. 545 answers 1. The atomic number increases by one and the mass number doesn’t change. 2.
;;;3. Alpha—by paper, beta—by aluminum foil, gamma—by blocks of lead or concrete;;;4. They contain the same number. After three half lives, each sample contains one eighth of the original number of nuclei, which was the same for both sample. ;;; 5. 12.5%;;; 6. 32 days = 4 half-lives = 1/16 of the original number of nuclei left. 1/16 x 5g = 0.3 g left.
3. Detecting Radioactivity
a. Radiation detectors—you can’t see of feel radiation, so detectors are used to “see” it. i. Cloud chambers—radiation cause the cloud to condense along the path of the
particles
ii. Bubble chambers—particles show up as paths of bubbles in a superheated liquid.
iii. Electroscopes—charged plates in an electroscope move in the presence of ionizing radiation.
c. Background radiation—radiation is everywhere. Sources include radon gas. Radon comes from the decay of uranium-238. It emits alpha particles. Seeps into houses from basements from surrounds soil and rocks. Cosmic rays are another source—as you go up in the atmosphere, more of the particles are present. Radiation in you
body--Self check p. 550 answers …Not assigned. 4. Nuclear Reactions
a. Nuclear fission—The process of splitting a nucleus into several smaller nuclei. Fission means “to divide”. Only large nuclei such are uranium and plutonium can undergo nuclear fission. The products usually include several individual neutrons in addition to the smaller nuclei. The total mass of the products is slightly less than the original nucleus and the neutron. The small amount of missing mass is converted into a tremendous amount of energy.
i. Mass and energy—Einstein theorized the mass and energy were
interconvertible E=mc2. If 1 gram of mass is converted into energy, 100 trillion joules of energy are released.
ii. Chain reactions—happen when neutrons emitted from a fission reaction causes a series of repeated fission reactions. Uncontrolled reactions can release enormous amounts of energy instantly. A reaction can be controlled by a substance that absorbs neutrons. A critical mass of material is required for a fission chain reaction to occur.
b. Nuclear fusion—splitting one nucleus of uranium-235 produces 30 million times more energy than chemically reacting one molecule of dynamite. Nuclear fusion—two nuclei with low masses are combined to form one nucleus of larger mass. Fusion fuses; fission, splits.
i. Temperature and fusion—temperatures of millions of degrees Celsius are needed to get particles moving fast enough to overcome the repulsive electric force that two protons experience. When the electric force is overcome, the protons can get close enough together to fuse. These types of temperatures are found in the center of stars.
ii. Nuclear fusion and the sun—four hydrogen nuclei are converted into one helium nucleus. As the sun ages, the hydrogen nuclei are used up as they are converted into helium. So far, 1% of the Sun’s mass has been converted to energy. The Sun can keep this up for 5 billion years.
c. Using nuclear reactions in medicine—radioactive isotopes can be incorporated into molecules that can be traced. When radioactive molecules are put into a molecule it is called a tracer. These are used in agriculture to measure the uptake of fertilizers and nutrients.
iodine-131, can be detected outside the body. The intensity of the radiation can help diagnose how well the thyroid is working.
ii. Treating cancer with radioactivity—ionizing radiation can damage a cancer cell and cause it to die or stop functions properly. Gold-198 or Iridium-192 can be placed near a tumor to keep it from growing. Co-60 emits gamma rays that can be focused on a tumor. The fast growing cancer cells are more susceptible to the radiation than slower growing tissues near the tumor. However, residual radiation can damage nearby cells and cause severe side effects.