Chemical formulas indicate the relative number of atoms of each kind element in a chemical compound (ionic and molecular)
Ionic compound the number of atoms in a formula units
Capital letter each element NaCl
CH
3
Subscript # atoms
Cl
Molecular compound the number of atoms in a molecule

Ionic Compounds
Monatomic ions - ions formed from a single atom Na + or S 2usually tell by column on periodic table, some elements have more than one oxidation number or charge
Binary compounds - only 2 elements in the compound Na
2
S
Polyatomic ions - ions formed from more than one type of atom covalently bonded together OH PO
4
3NH
4
+

Quiz yourself
How many oxygen atoms in the following?
–CaCO
3
–Al
2
(SO
4
)
3
How many ions in the following?
–CaCl
2
–NaOH
–Al
2
(SO
4
)
3

Rules for writing formulas for ionic compounds
1. when making the formula the cation
(positive ion) always goes 1 st then the anion (negative ion)
2. the compound is neutral + = - charge
3. subscripts added to make charges cancel
4. When adding subscript to polyatomic ion it is put in ( )’s
5. Formula unit is always the simplest ratio of ions must ÷ subscripts

The positive side of the formula must = the negative side of the element
Examples on board
1. Sodium chloride Na + Cl -
NaCl
( no charges written in the formula)
2. Calcium cholride Ca 2+ Cl -
CaCl
2
3. Potassium sulfide K + S 2-
K
2
S

4. Aluminum chloride Al 3+ Cl -
AlCl
3
5. Barium phosphate Ba 2+ PO
4
3-
Ba
3
(PO
4
)
2
Cross over method - the charges become the subscripts of the other ion
Beware subscripts must be simplest ratios

Naming ionic compounds (stock system)
1. Binary compound - 2 elements
•when naming the compound the name of the first element(+ion) stays the same
•the last element ends in “ide” example NaCl sodium chloride

Formula: ZnS
Compound Name: Zinc Sulfide

2. Polyatomic ions - use their own name
( on ion sheets).
Example NH
4
Cl ammonium chloride
Ba
3
(PO
4
) barium phosphate
Na OH sodium hydroxide http://www.chem4kids.com/files/atom_compounds.html

Stock System
3. Elements with more than one oxidation number (charge) -put the oxidation number in Roman numerals in ( )’s
Example Pb(NO
3
)
2 lead (II) nitrate
Usually transition metals have more then one oxidation number also lead and tin
Check ion sheet

Molecular Compounds - made of neutral atoms sharing electrons & form neutral molecules
2 types of formulas molecular formula - represents a molecule
C
6
H
6 empirical formulas - simplest ratio of atoms (formula units are always empirical formulas)
CH

H diatomic molecule - 2 atoms of the same element covalently bonded together
2
N
2
O
2
F
Cl
I
2
Br
2
2
2
There are 7 diatomic elements and they form the shape of a 7 on the periodic table except for H which is always an oddball
Only diatomic when pure element not necessarily when forming a compound

Use prefixes to name molecular compounds prefix subscript mono di tri no subscript (1 atom)
2
3 tetra penta hexa hepta octa nona
4
5
6
7
8
9 & deca 10

Rules for molecular compounds
1. The less electronegative element is given 1st and only given a prefix if it has a subscript >1 (never start a name with mono)
2. Second element- add a prefix and add
“ide”
(drop prefix ending if first letter in the name of the element is a vowel)
P
4
O
10 tetraphosphorous decoxide

Oxidation Numbers
• In polar covalent bonds, the electrons are located closer to one atom than the other. This is reflected in a partial charge,

+ and

-, for each atom.
•
The oxidation state or oxidation number of an atom is an indication of the amount of charge each atom carries. The one with the greater EN gets both e -

Example:
We know that in HCl, H has a lower
EN and is

+ and Cl has a greater EN is

-.
We say H is in the +1 oxidation state and Cl is in the
-
1 oxidation state.

Acids endings change
“-ate” to “-ic” H
2
SO
4
“-ite” to “-ous” H
2
SO
3 sulfuric sulfurous
Important acids to know:
HCl hydrochloric
HNO
3
H
3
PO
4
HC
2
H
3
O
2
H
2
CO
3 nitric phosphoric acetic (also CH carbonic
3
COOH)