Atomic mass is equivalent to the number of?

Answer 1

The Atomic Mass is the mass of an atom of that element in AMUs. (Atomic Mass Units)
The atomic number, however, is equivalent to the number of protons in an atom of that element.

The mass number of an atom is the total number of protons and neutrons in the nucleus of an atom. This is therefore always a whole number.

The relative atomic mass of an element is the weighted average of the masses of the isotopes relative to 1/12 of the mass of a carbon-12 atom.

It is a weighted average as it takes into account the relative abundances of the different isotopes (atoms of the same element but with different numbers of neutrons) of an element. This number is found in the Periodic Table.

For example chlorine has two isotopes, 35Cl and 37Cl, in the approximate ratio of 3 atoms of 35Cl to 1 atom of 37Cl.

The number of protons and neutrons in a 35Cl atom must add up to 35, the mass number. The relative atomic mass of chlorine takes into account both isotopes and is therefore 35.5.