What does atomic mass measure?

Answer 1

Atomic mass is count of the protons and neutrons in the nucleus of an atom. The most common form of hydrogen has one proton and no neutrons, so its atomic mass is 1. The most common form of helium has two protons and two neutrons, so its atomic mass is 4.

When you see fractional atomic masses (such as 45.27), that's because there are isotopes of that element that occur naturally in large enough quantities to shift the average away from a whole number. For instance, if the most common form of some element has an atomic mass of 100 but there is an isotope with an atomic mass of 101 that shows up in every 1 in four atoms, the resulting atomic mass will be 100.25.