# Question #7e2b4

Jul 15, 2017

The mass number is the sum of protons and neutrons present in the nucleus while the atomic mass is the average weight of the most abundant isotopes present. Atomic mass is not always greater.

#### Explanation:

Atoms consist of protons and neutrons, both similar in weight, plus electrons, which barely contribute any weight to the atom.

So the mass number can be calculated by adding the number of protons to the number of neutrons.

Alternatively it can be read off the periodic table by *rounding off * the atomic mass or atomic weight, which is the decimal number below the atomic symbol.

Due to the presence of isotopes, elements can have some atoms that have a greater mass number than those that are most common. There are also some atoms in the mix that will be smaller.
In some cases the atomic mass will be greater than the mass number calculated above, because the number of stable isotopes with extra neutrons is higher for that element.

For example hydrogen, ${H}^{1}$ is the $1 s t$ element on the table and will have a mass number of $1$ by the addition of $1$ proton to $0$ neutrons, and an atomic weight of $1.008$. The extra weight occurs because the isotopes of hydrogen, ${H}^{1} + 1$ neutron and ${H}^{1} + 2$ neutrons exist and add weight (but not much because they are not abundant).

And zinc, $Z {n}^{30}$ is the $30 t h$ element on the table and will have a mass number of $65$ by the addition of $30$ protons to $35$ neutrons, and an atomic weight of $65.39$. This occurs because of the existence of heavier stable isotopes of zinc.