Chemical Formulas
and Counting
Atoms
In any chemical formula, we show:
1. Symbols: for the elements in the compound
2. Subscripts: to represent the number of atoms
of each element (written to the bottom right
of the element)—this will tell us the
proportion in which the elements combine.
Ex. H2
3. Coefficient: is the number in front of the
compound and tells us the number of
molecules we have – if there is only 1, we do
not need to write it Ex. 4 Al2CO3
Example:
Rules to Counting Atoms:
1.
The symbol of an element represents one atom of
that element.
e.g. K = one atom of potassium
2.
If there is more than one atom of the element, then
a subscript is used to indicate the number of atom.
e.g. N2 = two atoms of nitrogen
3.
A subscript outside a bracket multiples all the
elements inside the brackets.
e.g. Ca3(PO4)2 = two atoms of Phosphorus
and eight atoms of Oxygen
Rules to Counting Atoms cont.
4.
a) A coefficient is a number written in front of a chemical
symbol that indicates the number of atoms of that element.
e.g. 4 S = four atoms of sulphur
b) A coefficient is a number written in front of a chemical
formula, indicating the number of molecules in a compound.
NOTE: A coefficient multiplies the number of atoms of each
element in the formula.
e.g.
2 CO2 = 2 carbon atoms and 4 oxygen atoms
4 MgSO4 = 4 magnesium atoms, 4 sulphur
atoms and 16 oxygen atoms
3 Cu(NO3)2 = 3 copper atoms, 6 nitrogen
atoms and 18 oxygen atoms
Naming Ionic Compounds
1. Metals combine with non-metals
2. Write the name of the metal first and the
name of the non-metal second
3. Change the ending of the non-metal to “ide”
4. Each atom has its own combining capacity
5. Atoms combine so that each can fill its
combining capacity
Combining Capacity
• The ability to combine with other elements. These
numbers predict the chemical formulas of the
compounds
• Example of Ionic Compound:
• Calcium has an ionic charge of 2+ (it loses 2 electrons)
• Chlorine has an ionic charge of 1- (it gains 1 electrons)
• Therefore, when they combine, one calcium atom pairs
with two chlorine atoms as each chlorine atoms accepts
one electron: CaCl2
Combining Capacity
• Example of a molecular compound
• The combining capacity of Carbon is 4 (it needs 4
electrons to be full)
• The combining capacity of Oxygen is 2 (it needs 2
electrons to be full)
• Therefore, when they combine, one carbon atom
bonds with two oxygen atoms
(4 + 2 + 2 = 8 electrons) CO2
Naming Molecular
Compounds
• When naming molecular compounds, we use
Greek prefixes to indicate how many atoms of
each element are present in a compound.
Number of Atoms
Prefix
1
mono
2
di
3
tri
4
tetra
5
penta
Steps to Naming
Molecular Compounds:
1. Examine the formula
2. Name the first element. (Note: When the first
element has only one, we do not write
“mono”)
3. Name the second element that will end in
“ide”
4. Add prefixes indicating the number of atoms
Example: N2O
1. Examine the formula: We see that we have 2
nitrogen atoms and one oxygen atoms
2. Name the first element: Nitrogen
3. Name the second atom: Oxygen – ending
changed to “ide” – Oxide
4. Add prefixes: Dinitrogen Monoxide
Common Molecular Compounds
pg 226 in text
Common Name
Chemical Name
Formula
Use
Natural Gas
methane
CH4
Fuel, plastic
Wood alcohol
methanol
CH3OH
antifreeze
Table sugar
sucrose
C12H22O11
sweetener
Rubbing alcohol
isopropanol
C3H8O
antiseptic