Formulas and Names of Compounds
I. Chemical Formula – Is an abbreviation of a compound
A. What do we know about chemical formulas?
1. 1 atom of Li and 1 atom of Fluorine you get LiF
B. What happens if you do not know how many of each atom is present in
the compound? Can you still figure out the formula?
1. Yes!!!!!!
2. How? By the atoms oxidation number?
a. Oxidation Number – positive or negative # assigned to an
element to show its combining ability in
a compound
- Indicates how many electrons an atom has gained, lost,
or shared when bonding with other atoms
Example: When Na forms an ion it loses an electron
and has a charge of +1, therefore the
oxidation # of Na is +1
When Cl becomes an ion it gains an electron
giving it a charge of -1, therefore the
oxidation # of Cl is -1.
b. Anytime you gain an electron you get a negative charge
c. Anytime you lose an electron you get a positive charge
d. How do you know the oxidation numbers of the elements?
- By their groups
- Group 1 = +1
- Group 2 = +2
- Group 13 = +3
- Group 14 = +4
- Group 15 = - 3
- Group 16 = - 2
- Group 17 = - 1
- Group 18 = 0
- Groups 3-12 (Transition Elements) the
oxidation # varies and is generally given to you
as a roman numeral. Example: Cu (II) = +2
C. Formulas for Binary Compounds
1. Binary Compound – compound made up of 2 elements
2. Once you know the oxidation #’s you can write the formula using
these 3 steps
a. Write first the symbol of the element that has the positive
oxidation #
b. Then write the symbol of the element with the negative
oxidation number
c. Add oxidation 3’s
- If they = 0, then get rid of the numbers
- If they don’t = 0, get rid of the charges and criss cross
the numbers so that oxidation # of one element
becomes the subscript of the other element.
Example: Bromine
and
Lithium
- 1st: What are the oxidation #’s: Br-1 Li +1
- 2nd: Write the atom with the positive oxidation # first
and then the atom with the negative oxidation #:
LiBr
rd
- 3 : Add subscripts so the sum equal 0: To do this find
the same least common multiple for the oxidation
numbers. In this case it is 1: LiBr
Example: Lithium and Sulfur
- Find the oxidation #’s: Li+1
S-2
- Put positive atom first and neg. second LiS
- Now we need to add the subscripts: Li2S
- 2 was the least common multiple for 1 and 2
D. Naming Binary Compounds
1. Use the following rules to name the binary compound
a. Write the name of the first element
b. Write the root of the name of the second element
c. Add the ending –ide to the root
Example: CaCl2 = Calcium Chloride
KI
= Potassium Iodide
FeO
= Iron Oxide
MgF2 = Magnesium Fluoride
E. Compounds with Polyatomic Ions:
1. Polyatomic Ion: is a positively or negatively charged, covalently
bonded group of atoms (Reference Tables or
Page 318)
2. Writing the formula:
a. Same rules apply as with binary compounds with one
additional rule: Place parenthesis around the group
representing the polyatomic ion when more than one of the
ion is needed
Example:
1st: Find the oxidation #’s Calcium = Ca+2
Nitrate = NO32nd: Place the positive atom first then the negative atom
Second
rd
3 : Determine the lowest common multiple which is 2
4th: Since there is more than one polyatomic ion in this
formula (There are 2 NO3 which is determined by
the subscript) place parenthesis around NO3 and
the subscript 2 outside the parenthesis.
5th: Answer = Ca(NO3)2
3. Naming the compound:
a. Write the name of the positive element
b. Write the name of the polyatomic ion
Example: CaCO3 =
1st: Write the positive element first Calcium
2nd: Write the polyatomic ion second Carbonate
Answer = Calcium Carbonate