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# The element has the atomic number of 20 and the mass number of 40, how many neutrons does it have?

Last updated date: 26th Feb 2024
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Hint: We know that atoms are the smallest unit of any matter. There are three subatomic particles of atoms namely, proton, electron and neutron. The nucleus is the centre of atoms. Protons and neutrons are present in the nucleus and electrons are present in orbits surrounding the nucleus.

Let’s first discuss the atomic number in detail. Atomic number is the identification of every element. None of the two elements have the same atomic number. Atomic number of elements is the number of protons present in the nucleus. In ground state, the number of protons and electrons is equal. For example, the number of protons and electrons present in a sodium atom is 11. But, in sodium ion $\left( {{\text{N}}{{\text{a}}^ + }} \right)$ the number of protons is 11 but the number of electrons is 10 (one electron lost).
Now, we will discuss the mass number in detail. The summation of the number of protons and neutrons of an atom gives the mass number. It is symbolically represented by ‘A’.
Mass number= Number of protons +Number of neutrons
Here, the atomic number of the element is given as 20,that means, the number of protons present in the atom is 20 and the mass number is given as 40.
Now, we have to apply the above formula to calculate the number of neutrons.
Number of neutrons=Mass number-Number of protons
$\Rightarrow$Number of neutrons=$40 - 20 = 20$

Hence, the number of neutrons of the element is 20.

Note: It is to be noted that an element is represented chemically by using both the atomic number and mass number. The chemical representation of an element X is ${}_{\text{Z}}^{\text{A}}{\text{X}}$, where A is mass number and Z is atomic number.