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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: 10th Aug 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).
$\Rightarrow$Number of neutrons=$40 - 20 = 20$
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.