For the reaction 2k(s)+br2(l)→2kbr(s) calculate how many grams of the product form when 21.4 g of br2 completely reacts. assume that there is more than enough of the other reactant.

The balanced equation for the above reaction is as follows;
2K + Br₂ --> 2KBr 
Stoichiometry of Br₂ to KBr is 1:2
we have been told that K is present which means that Br₂ is the limiting reactant. Limiting reactant is the reactant that is fully consumed in the reaction and amount of product formed depends on amount of limiting reactant present.
Number of Br moles reacted - 21.4 g / 160 g/mol = 0.134 mol
According to stoichiometry, 
1 mol of Br₂ forms 2 mol of KBr 
therefore 0.134 mol of Br₂ forms 2x0.134 = 0.268 mol of KBr
mass of KBr formed - 0.268 mol x 119 g/mol = 31.9 g of KBr
 a mass of 31.9 g of KBr is formed 

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.