How many unpaired electrons are there in ${\mathrm{Ni}}^{2+}$ ion if the atomic number of $\mathrm{Ni}$ is 28 .

 2 unpaired electrons are there in ${\mathrm{Ni}}^{2+}$ ion if the atomic number of $\mathrm{Ni}$ is 28.

Nickel has two unpaired electrons in the 3d sublevel. When nickel ionizes to the +2 state it loses the two electrons in the 4s, so there are still two unpaired electrons in  ${\mathrm{Ni}}^{2+}$.

An unpaired electron is an electron that does not form part of an electron pair and occupies an atom's orbital single. Each of an atom's three atomic orbitals, designated by the quantum numbers n, l, and m, has the capacity to hold a pair of two electrons with opposing spins.