In species  $X^{2+}$  the mass number is 20 and number of neutrons are 10 then calculate the number of electrons in species $X^{2+}$

Mass number (proton + neutron) of  $X^{2+}=20$

Number of neutrons = 10

Hence, number of protons of  $X^{2+}=10 (20-10)$

X is an atom, which means it has same number of electrons and protons. +2 charge on X indicates that it looses two electrons and forms $X^{2+}.$ Therefore, the number of electrons in $X^{2+}$ is 8.