Electron configuration

The region of the periodic table in which the transition metals are found is known as d-block because proceeding across each row each successive element has an additional d-electron. The lowest energy electron configurations for the first row transition metals is shown below.

Group number	3	4	5	6
atomic number element symbol	21 Sc	22 Ti	23 V	24 Cr
electron config	3d¹4s²	3d²4s²	3d³4s²	3d⁵4s¹

7	8	9	10	11	12
25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn
3d⁵4s²	3d⁶4s²	3d⁷4s²	3d⁸4s²	3d¹⁰4s¹	3d¹⁰4s²

For the transition metal atoms, the total number of valence electrons equals the number of the column (group) in the periodic table (counting from the left).

For transition metal ions having charge ≥ +2,

the 4s orbitals are not occupied
the number of electrons in the 3d subshell equals the total number of valence electrons (the Group number minus the charge on the ion).

This is because

orbitals in the 3d and 4s subshells are of similar energy.
In transition metal atoms the 4s subshell is of lower energy than the 3d subshell.
In transition metal ions of charge ≥ +2, 3d is of lower energy than 4s.
In transition metal ions of charge ≥ +2, all valence electrons in the d-subshell.

Therefore

Ni (Group 10) has 10 valence electrons and Ni²⁺ is d⁸
Fe (Group 8) has 8 valence electrons and Fe³⁺ is d⁵
Ti (Group 4) has 4 valence electrons and Ti³⁺ is d¹