Question

What is the mass number of a calcium atom if it has $20$ neutrons?

Answer 1

The electronic configuration of calcium can be used to determine its electrons or protons (as calcium is a neutral atom). The mass number can therefore be calculated by finding out the atomic number of calcium.

Complete answer:
An atom consists of electrons, protons and neutrons. The electrons are present in specific energy levels known as orbitals. The protons and neutrons are together called the nuclear particles as they are present inside the nucleus of an atom. These three subatomic particles make up the mass of an atom.
The atomic mass of an atom is a measure of the amount of matter contained by it.
Any element can be represented by its chemical symbol $X$ with a superscript and subscript written along with it. The atomic mass number $A$ is written in superscript and the atomic number $Z$ is written in subscript. Both these numbers are written on the left side of the chemical symbol of the element.
The symbol can be written as: ${_Z^A}X$
The atomic number of an element is equal to the total number of protons present inside it. The mass number is equal to the sum of the total number of neutrons and protons.
Calcium has an electronic configuration $[Ar]4{s^2}$ and contains twenty protons inside its nucleus. It also contains twenty neutrons (as mentioned in the question).
Thus the atomic mass number of Ca can be calculated as follows:
$A = Z + {\text{(number of neutrons)}}$
$A{\text{(for Ca)}} = 20 + 20 = 40$
Hence, the atomic mass number of Calcium is $40$ .

Note:
The atomic number $Z$ is equal to the number of protons or electrons in an element as the number of protons is equal to the number of electrons in an atom. But the atomic number is not defined using electrons as the number of electrons is subject to change during ion formation but the protons remain unchanged.