Question

Why is atomic mass a decimal?

Answer 1

You mean why atomic mass is non-integral? It is because of the existence of isotopes.

Explanation:

Let's take the hydrogen atom....the most abundant element in the universe. All hydrogen atoms contain the ONE nuclear proton, i.e. `Z=1`, and this is what defines the atom as hydrogen, i.e. `""^1H`. A few hydrogen NUCLEI contain a neutron in addition to the defining proton, to give the deuterium isotope, i.e. `""^2H`, and a smaller few hydrogen NUCLEI contain TWO neutrons, to give the tritium isotope, i.e. `""^3H`. The weighted average of the isotopic masses gives rise to the atomic mass quoted on the Periodic Table, i.e. `1.00794*g*mol^-1`.

Most higher elements have an envelope of isotopes, and their weighted average is the atomic mass printed on the Periodic Table.