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
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