Atomic Mass of different isotopes?

Hello gregorygoulenko!

So, according to what I understand, the relative atomic mass of chlorine is 35.5 rather than a whole number; this is because chlorine contains two different isotopes, and chlorine 35 with chlorine 37. The periodic table shows the mass numbers and abundances of the isotopes of copper in a naturally-occurring sample. 

So in other words, in every 100 chlorine atoms, 75 atoms have a mass number of 35, and 25 atoms have a mass number of 37.

About the mass number for the isotope chlorine 37: Its nucleus contains 17 protons and 20 neutrons for a total of 37 nucleons; chlorine-37 accounts for 24.23% of natural chlorine, chlorine 35 accounting for 75.77%, giving chlorine atoms in bulk an apparent atomic weight of 35.453(2) g/mol. 

Did you realise whenever you're given a question,  you'll notice that the answer is closer to 35 than of 37, this reason is due to chlorine-35 isotope is much more abundant than the chlorine-37 isotope.

If you were to calculate, the formula's requirement would be:

"Ar = total mass of atoms ÷ total number of atoms."

-Hopefully you understood, 

~Galaxy.