For the transition metal with the electron configuration $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^6$ , how many unpaired electrons are present in its +2 ion in the ground state?

Let take Fe, the electronic configuration

$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^6$

For its +2 ion,

$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^4$

So, for the transition metal with the electron configuration $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^6$ , number of  unpaired electrons are present in its +2 ion in the ground state is 4.