Chemical Formulas

and Counting

Atoms

In any chemical formula, we

show:1. Symbols: for the elements in the

compound2. 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