1. Chemical Formulas and Counting Atoms

2. In any chemical formula, we show:
Symbols: for the elements in the compound
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
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

3. Example:

4. 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

5. 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

6. Naming Ionic Compounds
Metals combine with non-metals
Write the name of the metal first and the name of the non-metal second
Change the ending of the non-metal to “ide”
Each atom has its own combining capacity
Atoms combine so that each can fill its combining capacity

7. 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

8. 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
( = 8 electrons) CO2

9. 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

10. Steps to Naming Molecular Compounds:
Examine the formula
Name the first element. (Note: When the first element has only one, we do not write “mono”)
Name the second element that will end in “ide”
Add prefixes indicating the number of atoms

11. Example: N2O
Examine the formula: We see that we have 2 nitrogen atoms and one oxygen atoms
Name the first element: Nitrogen
Name the second atom: Oxygen – ending changed to “ide” – Oxide
Add prefixes: Dinitrogen Monoxide

12. 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