Unit 2 Notes
Periodic Table and Chemistry Math
Periodic Table
The periodic table is cyclic. Family: is a vertical group, row
Every element in the same family has the same number of valence electrons Period: horizontal group
The number of the period tells the number of energy levels the elements in that period have.
*Group 1 – Alkali Metals *Group 2 – Alkali Earth Metals
Have 1 valence electron Have 2 valence electrons
Highly reactive, stored in kerosene Very reactive
Not found as free elements in nature Not found as free elements
Silvery in appearance, soft in nature
Harder, denser, stronger
*Group 3-12 – Transition Metals *Group 17 – Halogens
Less reactive Most reactive non metals
Good conductors of electricity 7 valence electrons
High luster diatomic
*Group 18 – Noble Gases *Metalloids
Not reactive, inert Semiconductors
Have 8 valence electrons Have properties of metal
and non metals
*Non metals: to the right of the jiggity jagetty line *Metals: to the left of the jiggity jagetty line
Trends of the Periodic Table
You need to know the definitions and trends of 1. Ionization energy
2. Electronegativity 3. Atomic radii
Ionization Energy (IE1): the energy required to remove one electron
from a neutral atom
the energy required to form a CATION *Ionization energy increases across each period
*Ionization energy decreases as move down group
the nucleus has a positive charge, the more protons the more pull of the positive charge on the electrons, making it more difficult to remove e-. Essential Skill Question:
Put the following elements (Ca, Ge, Sr, Si) in order from the element with the least ionization energy to the element with the greatest ionization energy.
Answer: Sr, Ca, Ge, Si
Electronegativity: ability of an atom to attract electrons Fluorine is the most electronegative element *Electronegativity increases across each period *Electronegativity decreases as move down the group
Nobel Gases have the lowest electronegativity. They are stable, inert. Essential Skill Question:
Put the following elements (Ca, Ge, Sr, Si) in order from the least electronegative element to the most electronegative.
Answer: Si, Ge, Ca, Sr Atomic radii: Measures atomic size
Atomic radii: ½ the distance between the nuclei of identical atoms that are bonded together.
*Atomic radii decreases across each period
the nucleus has a positive charge, the more protons the more pull of the positive charge on the electrons, decrease the atomic radius.
*Atomic radii increases down each group Increasing energy levels
Essential Skill Question:
Put the following elements (Ca, Ge, Sr, Si) in order from the element with the smallest atomic radii to the element with the greatest atomic radii.
Molecular Formulas
Atoms bond together to form a molecule. Molecule: 2 or more atoms bonded together
Molecular formula: tells the types and number of each atom in a molecule Ex: H2O = water
The subscript tells you how many atoms of each element you have Ex: H2O has 2 atoms of hydrogen and 1 atom of oxygen
The coefficient tells you the number of molecules that you have Ex: 3H2O has 3 molecules of water
In order to determine the total number of atoms of each element, distribute the coefficient through the molecular formula.
Ex: 3H2O
3 molecules of water have 6 atoms of hydrogen 3 atoms of oxygen
Molecular Mass
Can determine the mass of a molecule = Molecular Mass The units for molecular mass are atomic mass units (amu) Add the atomic mass of the individual atoms
If more than one atom, multiply the atomic mass by the subscript. *Example: H2O
2 atoms of hydrogen 2(1.01 amu) = 2.02 amu 1 atom of oxygen 1(16.00 amu) = 16.00 amu
1 molecule of water has the molecular mass of 18.02 amu If more than one molecule, distribute the coefficient first to determine the
number of atoms of each element, then add the atomic masses. *Example: 4CO2
4 atoms of carbon: 4(12.01amu) = 48.04 amu 8 atoms of oxygen: 8(16.00 amu) = 128 amu 4 molecules of carbon dioxide have the
molecular mass of 176.04 amu
Significant Figures “Sig Figs”
Significant Figure: any digit in a measurement that is known with certainty, plus one final digit which is estimated (or uncertain). Measuring with Sig Figs
When doing science many times a quantity is being measured. Examples: Measuring liquid in a beaker or graduated cylinder.
Measuring mass using an electronic scale or triple beam balance. Distance with a ruler.
To record a measurement properly it must have the correct number of sig figs. Measure the quantity to the last known “mark”, then estimate one additional
number.
How many Sig Figs does a number have?
When given a number, you can determine how many sig figs it has. Knowing the definition of sig figs will tell you more precisely what
that number means.
Example: measured quantity is 1.20cm
The zero is significant so it tells you the person measured to the tenths place then determined that the object ended exactly on the “2” not a little past or a little before (so they estimated the zero) Rules for determining significant figures
1. All numbers, non-zero digits, are significant Examples:
2.83 has 3 sig figs 8.274 has 4 sig figs
2. Zero sandwiches: Zeros in between non-zero digits are significant Examples:
1.004 has 4 sig figs 302 has 3 sig figs 3. Look for a decimal point?
NO decimal then done
YES decimal then zeros after the number are significant Examples:
.0032 has 2 sig figs 0.02101 has 4 sig figs 100 has 1 sig fig 100. has 3 sig figs 3.20 has 3 sig figs 3020 has 3 sig figs
ATLANTIC-PACIFIC RULE:
-Decimal point PRESENT. Come from the PACIFIC side. Ignore all zeros until you get to a non zero number. ALL remaining digits are significant.
EX: 0.007150 g = 4 sig figs 100 g = 3 sig figs
-Decimal point ABSENT. Come from the ATLANTIC side. Ignore all zeros until you get to a nonzero number. ALL the remaining digits are significant.
EX: 100 mL = 1 sig fig 100 cm = 1 sig fig (well, kind of)
Using Sig Figs When Calculating Multiplying and Dividing
Round off your answer to the same amount of sig figs as the measurement having the fewest sig figs.
Example: 3.2 x 8.63 = 27.616 3.2 has 2 sig figs 8.63 has 3 sig figs
Therefore the answer should be recorded as 28 The answer has 2 sig figs
Scientific Notation
This is a way of writing really big numbers: 42300000000000 and really small numbers: .00000000123 How to write:
1. Write the number with the correct amount of sig figs, with only one number to the left of the decimal.
_ . _ _ has 3 sig figs _ . _ has 2 sig figs 2. Write: x 10
3. The exponent for 10 is the number of places you moved the decimal If move the decimal to the left the exponent is positive (+),
a big number
Example: 4230000000000000 is written as 4.23 x 1015
If move the decimal to the right the exponent is negative (-), a small number.
Example: .00000000123 is written as 1.23 x 10-9
Calculators
Do NOT type in x 10
Use the EE or EXP button, these mean x 10
Honors Chemistry Notes Unit 2
Electron Configuration
Writing the electron configuration for an element tells the exact location of that elements electrons
Energy levels (shells) are made up of subshells There are 4 different types of subshells: s, p, d, f
Each subshell has orbitals that create as specific shape Each orbital holds 2 electrons
The s has 1 sphere shaped orbital The p has 3 dumbell shaped orbitals 1 orbital with 2 electrons 3 orbitals each hold 2 electrons
6 electrons total
The d has 5 flower shaped orbitals The f has 7 “crazy” shaped orbitals 5 orbitals each hold 2 electrons 7 orbitals each hold 2 electrons
10 electrons total 14 electrons total
Which orbitals are found at each energy level? Look at the periodic table.
The energy level is the same as the number of the period.
Except: d block = period, n-1 f block = period, n-2 Valence electrons are always found in the s and p orbitals.
An element is more stable with 2 electrons in the 4s orbital than those 2 electrons in the 3d orbitals. Even though that 4s orbital is further from the nucleus.
*** Note: This topic is more complex that what is presented here. This is an introduction.***
How to write electron configurations “Take a walk on the periodic table”
The electron configuration for Oxygen is The electron configuration for Chlorine
1s22s22p4 1s22s23s23p5
The electron configuration for Iron (Fe) 1s22s22p63s23p64s23d6
To shorten the amount of writing you can use noble gas notation Look at the noble gas that is prior to the element you are trying to write an
electron configuration for Write that noble gas is square brackets.
This indicates that the energy levels are full until that noble gas. Then continue writing the electron configuration after the noble gas The noble gas notation for oxygen is
Orbital Notation
Rules
1. Each box represents an orbital containing two electrons, each electron is represented by an arrow. The first arrow points up and the second arrow points down.
2. Within a block, one electron must be placed in each block before a second electron is placed there.
Quantum Numbers
Quantum numbers are an “address” by which electrons can be found. Is used to describe the last, or outermost, electron of an atom or can be used to identify the element.
Quantum numbers specify the properties of atomic orbitals and the properties of electrons in those orbitals.
There are four quantum numbers for an electron which indicate: The first quantumnumber is the period, or row.
The numbers are the same as the table; 1-7 The second quantum number is the block. (s, p, d, f)
s block is 0 p block is 1 d block is 2 f block is 3
The third quantum number is the orbital
The center orbital is generally considered 0 orbitals to the left of center are negative orbitals to the right of center are positive
s 0
p -1 0 +1
d -2 -1 0 +1 +2
f -3 -2 -1 0 +1 +2 +3
The forth quantum number is the spin Number the first electron in the orbital is + ½ the second electron is – ½ .
Percent Composition and Empirical Formula
Percent Composition: The percent composition of an element in a compound is the percent, by mass, of an element in a compound.
What are the steps to calculating percent composition? 1. Calculate the molar mass of the compound.
1 mole of H2O = (2 x 1.01) + 16.00 = 18.02 g
2. Calculate the molar mass of the element of interest in the compound by multiplying the subscript by the molar mass of the element.
H = (2 mol x 1.01 g/mol) = 2.02 g; O = (1 mol x 16.00 g/mol) = 16.00 g 3. Divide the molar mass of the element in the compound by the total molar mass of the
compound and multiply by 100.
% composition of H = 2.02g x 100% = 11.2% 18.02g
% composition of O = 16.00g x 100% = 88.8% 18.02g
Question: Find the percent composition of each element in Cu2S.
Answer: 80% Cu 20% S
Empirical Formulas – a formula that gives the simplest whole number ratio of atoms in a compound.
Steps to Determine the Empirical Formula:
1. Give the mass percent of each element, assume a 100 g sample so change each percentage to grams.
81.68% carbon = 81.68g 18.32% hydrogen = 18.32g
2. Divide the grams by the molar mass to find the moles of each element.
3. Divide each element’s number of moles by the smallest number.
4. If the resulting mole numbers are not very close to a whole number, multiply the resulting mole numbers by the same number so that the products will be whole numbers.
Must multiply 2.66 by 3 in order to get a whole number = 8 Then must do the same for carbon 1 x 3 = 3
Final Answer: C3H8
Question: Barium carbonate, a white powder used in paints, enamels, and ceramics, has the following composition: Ba, 69.58%; C, 6.090%; O, 24.32%. What is its empirical formula?
Molecular Formula – The ACTUAL formula for the molecular compound.
The empirical formula (simplest formula) may or may not be the correct molecular formula. Steps to Determine the Molecular Formula
Given: an experimental mass, 132.33 g, for a substance with a formula of C3H8.
1. Find the formula mass of C3H8:
(3 x 12.01) + (8 x 1.01) = 44.01g
2. Divide the experimental mass by the formula mass:
3. Multiply the empirical formula by the value to get the molecular formula (C3H8)3 = C9H24
Question: A substance has an experimentally determined mass of 180.18g. The empirical formula is CH2O. What is the molecular formula? Answer: C6H12O6
Now put it all together!
One of the chlorofluorocarbons (a freon), which is used in refrigerator compressors and contributes to destruction of ozone in the upper atmosphere, has a experimental mass of 132.9 g/mole and a percent composition of 53.34% Cl, 28.59% F, and 18.07% C.
1. What is the empirical formula of this freon? Answer: CClF
2. What is the molecular formula of this freon? Answer: C2Cl2F2
Using Sig Figs when
Adding and Subtracting
***This is bonus (points towards an A)***
Round off your answer to the same number of decimal places as the measurement with the fewest decimal places.
If there is no decimal point, round your answer to the leftmost uncertain digit in the quantities that are being added or subtracted.
Example: 4.28
+ .836 Answer should be recorded as 5.12
5.116 because 4.28 has the fewest decimal places (2) Example: 3200
+ 284 Answer should be recorded as 3500 3484 because when the numbers are “lined up”
