Question

How to find the number of neutrons in an atom? Is there any formula?

Answer 1

Neutrons are the neutral particles which are present inside the nucleus of an atom. The neutrons are only particles which constitute the mass of an atom.

Complete step by step answer:
In the modern form of periodic table the elements are arranged in the increasing order of the atomic numbers. An atom is usually represented as the symbol followed by its mass number and the atomic number.
The atomic number is the number of protons contained in the nucleus of an atom. It is also equal to the number of electrons for a neutral atom in ground state. For example a sodium atom with atomic number $11$ contains $11$ protons and $11$ electrons.
The mass number as told earlier is responsible for the mass of an atom. The mass number is the sum total to the number of protons and neutrons. The protons and the neutrons in an atom are collectively called the nucleons.
The atomic number is denoted by $Z$ and the mass number is denoted by $M$. Thus an element $X$ is represented as $X_Z^M$ .
The formula for the determining the number of neutrons is thus obtained by subtracting the mass number and the atomic number
${N_{nu}} = M - Z$ where ${N_{nu}}$ is the number of neutrons.
Thus the number of neutrons is determined using the mass number and the atomic number. For the sodium atom the mass number is $23$.
Thus the number of neutrons in sodium atom =$23 - 11 = 12$.

Note:
The hydrogen atom is a lone element in the universe which has no neutron. It has only one proton and one electron. But its higher isotopes i.e. deuterium or tritium contain one and two neutrons respectively.