# Why is atomic mass a decimal?

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. $1.00794 \cdot g \cdot m o {l}^{-} 1$.