Question

What is the mass number of an element?

Answer 1

Mass number: the total number of neutrons and protons in an atom..

Explanation:

The reason it is called Mass because all the mass of the atom is due to neutrons and protons present in the nucleus of an atom.
for example lets take "Carbon".. Carbon has 6 protons and 6 neutrons, so eventually it's mass number is 12.

https://www.khanacademy.org/science/chemistry/atomic-structure-and-properties/introduction-to-the-atom/v/atomic-number-mass-number-and-isotopes

Hope this helps. :)