How to Count Atoms Review

SNC2P – Fall 2014

1. The symbol of an element represents one atom of that element.

e.g.:

Ca = calcium

S = sulfur

2. A subscript is a number written at the lower right corner behind the symbol of an element. If there is more than one atom of the element in the molecule, then a subscript is used to indicate the number of atoms.

e.g. N2 = 2 nitrogen atoms

O2 = 2 oxygen atoms

3. A subscript outside a bracket multiples all the elements inside the bracket.

e.g. Ba3(PO4)3 - 3 Barium atoms

- 2 phosphorous atoms- 8 oxygen atoms

Ca(OH)2 - 1 calcium atom

- 2 oxygen atoms- 2 hydrogen atoms

4. (a) A coefficient is a number written in front of a chemical symbol and indicates the number of atoms of that element.

e.g. 3C - 3 carbon atoms

Cl2 - 4 chlorine atoms

4. (b) A coefficient is a number written in front of a chemical formula and indicates the number of molecules of that compound.

e.g. 2H2O - 4 hydrogen atoms

- 2 oxygen atoms

3Fe2SO4 - 6 iron atoms

- 3 sulfur atoms- 12 oxygen atoms

4Cu(NO3)2 - 4 copper atoms

- 8 nitrogen atoms- 24 oxygen atoms