Where does the 0.5 in chlorine's atomic mass come from?

Answer 1

There are different isotopes of the element.

Isotopes are atoms of an element that have different amounts of protons, so the isotopes with more protons weigh more because there is more mass in the atom.

Some isotopes are more common than others, and the Atomic Mass is a weighted AVERAGE of all the isotopes of that element. On most elements there is a large difference between the most common and lesser common isotopes, but chlorine has 2 isotopes that are very close. So the average ends up between the two numbers.

The actual weight of 1 atom will never be something.5 because protons and neutrons weigh 1amu and electrons weigh 0. But in most cases the sample of a chemical you use will be a mixture of the isotopes, so the .5 weight will be correct.

Answer 2

The atomic mass of chlorine is a weighted average of the masses of all its naturally occurring isotopes, taking into account their abundance. Chlorine has two main isotopes, chlorine-35 (with an atomic mass of approximately 34.97) and chlorine-37 (with an atomic mass of approximately 36.97). The 0.5 difference in the atomic mass comes from the relative abundance of these isotopes in nature.