An ion $X^{2+}$ contains $18$ electrons and $20$ neutrons. Calculate the atomic number and mass number of element $X$. Name the element $X$.

(N/A) The number of electrons in the $X^{2+}$ ion is $18$.
Since a positive ion is formed by the loss of electrons from a neutral atom,the number of electrons lost is equal to the number of units of positive charge on the ion.
Therefore,the number of electrons in the neutral atom $= 18 + 2 = 20$.
For a neutral atom,the atomic number is equal to the number of protons,which is also equal to the number of electrons.
Thus,the atomic number of element $X = 20$.
The mass number of element $X$ is the sum of the number of protons and the number of neutrons,which is $20 + 20 = 40$.
The element with atomic number $20$ is calcium $(Ca)$.