How many neutrons are there in an atom of $_{86}^{222}Ra$?

Hint:An atom consists of proton, electron and neutron. Number of protons and electrons is equal to the atomic number of an element. Also, the sum of atomic number and number of neutrons is mass number. So, the number of neutrons can be calculated by simply subtracting the atomic number from the mass number.

Complete step-by-step answer:The skeletal representation of an atom of element can be represented as $_{A}^{Z}X$.
 Here $Z$= mass number of element and $A$= Atomic number
Also, we know that mass number = atomic number + number of neutrons
$\therefore \,\,Z=\,A\,+\,number\,of\,neutrons$
From the above equation we can derive the formula for number of neutrons as
Number of neutrons=$Z\,-\,A$or we can say that,
Number of neutrons= Mass number – Atomic number
In the above question, an atom of $_{86}^{222}Rn$is given and we are asked to calculate the number of neutrons.
For the given atom of Radon$\left( Rn \right)$we can see that, $Z=\,222$and$A\,=\,86$
Therefore, Number of neutrons can be calculated by using the above formula;
Number of neutrons = $Z-A$
$\therefore $ Number of neutrons in an atom of $_{86}^{222}Rn$ = $222\,-\,86\,=\,136$neutrons

Hence, we can conclude that an atom of Radon $_{86}^{222}Rn$ has 136 neutrons in it.

Note:It is not necessary that the number of neutrons is the same as the number of protons or electrons. It can be in some cases and cannot be in the majority of cases. In some atoms of nitrogen, oxygen, etc the number of electrons, protons and neutrons are all the same.