Atomic Structure

Atomic Structure

Defining the Atom

• The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word

“atomos”)

– He believed that atoms were indivisible and

indestructible

– His ideas did agree with later scientific theory, but did not explain

Dalton’s Atomic Theory (experiment based!)

3) Atoms of different elements combine in simple whole-number ratios to form chemical

compounds

4) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

1) All elements are composed of tiny indivisible particles called atoms

2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element.

John Dalton (1766 – 1844)

Sizing up the Atom

▪

▪If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long

▪Despite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes

Structure of the Nuclear Atom

• One change to Dalton’s atomic theory is that

atoms are divisible into subatomic particles:

Discovery of the Electron

In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged

particle: the electron

Conclusions from the Study of

the Electron:

a) Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically

charged electrons.

b) Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

c) Electrons have so little mass that atoms must contain other particles that account for most of the mass

Conclusions from the Study of

the Electron:

▪ Eugene Goldstein in 1886 observed what is now called the “proton” -

particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)

The Rutherford Atomic Model

• Based on his experimental evidence:

– The atom is mostly empty space

– All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”

– The nucleus is composed of protons and neutrons (they make the

nucleus!)

– The electrons distributed around the nucleus, and occupy most of the volume

Rutheford’s Gold Foil Experiment

13

Distinguishing among atoms

-Elements are different because they contain different numbers of

protons.

Atomic Number- # protons in atom. This identifies the element. Remember, atoms are neutral, so # positive particles (protons) must equal negative particle (electrons)

Therefore, # protons = # electrons

Mass Number- # protons + # neutrons.

How to Find:

# Electrons: Atomic # # Protons: Atomic #

Practice

• Carbon

• Sodium

• Practice Finding # protons, neutrons, electrons

• Fabulous Foldable!!

• Include definitions and an example for the

following terms:

– Proton

– Neutron

– Electron

– Nucleus

Isotopes

• Isotopes- Atoms that have the same number of protons but different number of neutrons.

• They also have different atomic masses/mass numbers

• However, isotopes are chemically alike because they have identical numbers of protons and electrons (which are the particles responsible for chemical behavior)

Atomic Mass

• In nature, most elements occur as a mixture of two or more isotopes.

• Each isotope has a fixed mass and natural abundance (the percentage that isotope is found in nature)

• Atomic mass- weighted average mass of atoms in a naturally occurring sample of the element. This reflects the mass and the relative abundance of the isotopes as they occur in nature.

• Atomic mass is expressed in amu or atomic mass units

• 1 atomic mass unit = 1.66053886 × 10-27 kilograms

Calculating Average Atomic Mass

CHLORINE: atomic mass = 35.45 on p. table

Chlorine-35 accounts for 75% naturally occurring atoms (75% natural abundance) with mass of 34.9

Chlorine-37 accounts for 25% naturally occurring atoms (25% natural abundance) with a mass of 36.9

(.75 x 34.9) + (.25 x 36.9) = 35.4

Calculating Atomic Mass for Carbon

• Carbon has 2 stable isotopes:

• Carbon-12 with a natural abundance of 98.89% and mass of 12.00amu • Carbon-13 with a natural abundance of 1.11% and mass of 13.003amu • Mass of C on p. table: 12.01am

• Calculate Atomic Mass of Carbon:

• (12.000amu x .9889) + (13.003amu x .0111) = 12.01amu

Practice

• The element copper has

naturally occurring isotopes Copper 63 and Copper 65. The relative abundances and atomic masses are 69.2% for mass = 62.93

amu, and 30.8% for mass = 64.93 amu.

• Calculate average atomic mass for copper.

• Check answer on periodic table

• Mass # = 63.55

Practice

• Calculate the atomic mass of bromine. The two isotopes of

bromine have atomic masses and relative abundances of 78.92

amu (50.69%) and 80.92 amu (49.31%)

• Check answer on periodic table!

• Br = 79.90

Average Atomic Mass Stations!

• You will start at one of the three stations

around the room.

• Write down the

question on a piece of paper, then solve!

• Show all your work

• The smallest particle of an element that

retains the properties of that element is a(n)

• A. atom

• B. electron

• C. proton

• The subatomic particles of an atom are

• A. Protons and neutrons

• B. Protons and electrons

• C. Electrons and Neutrons

• What two particles are in the nucleus?

• A. protons and electrons

• B. neutrons and electrons

• What is the charge of an electron?

• A. Negative

• B. Positive

• What was Rutheford’s model of the atom?

• A. A dense sphere with no charge

• B. A tiny sphere with negative charged particles throughout

• Dalton thought the atom was a

• A. tiny, dense sphere with no charge

• B. A tiny sphere with negatively charged particles throughout

• J.J. Thompson discovered the electron with the use of the

• A. Gold Foil experiment

• B. Cathode Ray Tube

All atoms are

• A. positively charged, the number of protons exceeding the number of electrons

• B. negatively charged, with the number of electrons exceeding the number of protons

• C. neutral, with the number of protons equaling the number of electrons

A theory is

• A. proposed explanation for an observation

• B. well tested explanation for a broad set of observations

• C. summary of the results of many observations

Which of the following is NOT a

physical property

• A. mass

• B. color

• C. Ability to rust

When a substance forms a vapor, what

physical state was it before?

• A. gas

• B. liquid

• C. solid

What state of matter has no definite

shape or volume?

• A. solid

• B. Liquid

• C. Gas

Which state has a definite volume but

no definite shape?

• A. gas

• B. Solid

• C. Liquid

Which state of matter has both a

definite shape and definite volume?

• A. solid

• B. Liquid

• C. Gas

What is another word for a

homogeneous solution?

• A. Solutions

• B. Homozygous mixture

• C. Mixture

Which of the following is an example

of a homogeneous mixture?

• A. Oil and water

• B. Rubbing alcohol

• C. Italian salad dressing

Which is true for compounds?

• A. They can be physically separated

• B. They have compounds that vary

• C. They are substances (aka pure substances)

Which of the following is a chemical

property?

• A. hardness

• B. Color

• C. melting point

When metal rusts, it is evidence of a

• A. Physical change

• B. Chemical change

• C. homogeneous solution

What is the difference between a

mixture and a substance (aka pure

substance)

• A. substance are compounds, mixtures are not

• B. Mixtures are groups of elements and compounds arent

• C. Sample of the same substance have different properties

Quick Review

• In scientific method:

• Another word for independent variable is manipulated variable.

• Another word for dependent variable is

The variable that is observed during an

experiment is called what type of

variable?

• A. independent

• B. manipulated

• C. control

All of these are examples are physical

changes EXCEPT:

• A. bending

• B. cutting

• C. rusting

During a chemical reaction

• A. matter is neither created nor destroyed

• B. some matter is destroyed

• C. some matter is created

The number 2012 in scientific notation

is

• A. 2.012 x 10^3

• B. 2.012 x 10^-3

• C. 20.12 x 10^-3

In the measurement 0.709ml, which

digit is the least precise?

• A. 7

• B. 0

• C. 9

In the measurement, .6745g, which

number is the estimated digit?

• A. 6

• B. 4

• C. 5

When a measurement is close to the

correct value, it has good

• A. Precision

• B. Accuracy

• C. Usefulness

The closeness of your measurement to

those of your lab partners indicates

• A. accuracy

• B. precision

• C. reproducibility

How many significant figures are in

3008?

• A. 2

• B. 4

• C. 1

How many sig figs are in 20.0090?

• A. 2

• B. 6

• C. 5

Round 1050 to two significant figures