Q: I understand that atomic mass is the sum of the protons and neutrons in the nucleus of an atom. If chlorine, with atomic mass of 35.5, does that mean that it contains half a proton or neutron?

A::The number of protons of a specific element is always the same. For chlorine it is 17. But the number of neutrons can differ, and it is still chlorine. Naturally occuring chlorine contains 18 or 20 neutrons, resulting in two ISOTOPES, one with mass 35 (17 + 18) and one with mass 37. If you mix 75% of the 35 with 25% of the 37, the AVERAGE mass is 35 x 0.75 + 37 x 0.25 = 35.5. Still confused? If you have a bag with 10 apples, half of them weighing 90 grams and the other half weighing 110 grams, what would your friend conclude was the AVERAGE weight of each apple if he weighed the whole bag?