Question

A certain atom has $ 26 $ protons, $ 26 $ electrons and $ 30 $ neutrons. What is its mass number?

Answer 1

Hint: The mass number of any atom is the measure of the net mass contained by its electrons. It is always the number of protons and neutrons that are present inside the dense nucleus that contribute to the mass number of an element.

Complete answer:
The total number of protons present inside the nucleus of an atom or the total number of electrons in a neutral atom are called the atomic number of the atom.
The mass number of an atom is the sum of the masses contributed by all the inter-nuclear particles as electrons have negligible contribution. Thus the total number of protons and neutrons which is the sum of the atomic number and the number of neutrons is called the atomic mass number.
Each proton and neutron nearly contributes a single unit of atomic weight which is why the number of protons and neutrons are used to calculate the mass number.
The formula can be written as follows:
 $ {\text{mass number}} = {\text{ atomic number}} + {\text{number of neutrons}} $
Inserting the values of atomic number and the number of neutrons in the above formula we get,
 $ {\text{mass number}} = {\text{ 26}} + 30 = 56 $
 $ \Rightarrow $ Thus, the mass number of the given atom is $ 56 $ .

Note:
The mass number and the atomic number can be used to identify the element as no two elements in the periodic table have the same number of atomic numbers and atomic mass numbers. The numbers of electrons can change with the process of ionization or electron gain.