What is the rate constant of a first-order reaction that takes 5.50 minutes for the reactant concentration to drop to half of its initial value?

For a first order reaction, the half life is inversely proportional to the rate constant. 
The formula is
half life = ln(2)/k = 0.693/k
where k is the rate constant

t = 5.50 minutes

k = ln(2)/5.50 = 0.126 min^-1

Your rate constant is 0.126 min^-1.

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.