How to Count Atoms Review SNC2P – Fall 2014

The symbol of an element represents one atom of that element. e.g.: Ca = calcium S = sulfur

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

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

(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

(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