Electron configuration transition metals

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 (column) number	3	4	5	6	7	8	9	10	11	12
atomic number element symbol	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn
electron configuration	3d¹4s²	3d²4s²	3d³4s²	3d⁵4s¹	3d⁵4s²	3d⁶4s²	3d⁷4s²	3d⁸4s²	3d¹⁰4s¹	3d¹⁰4s²

The valence configuration for first series transition metals (Groups 3 - 12) is usually 3dⁿ 4s².

Exceptions: The electron configurations for chromium (3d⁵ 4s¹) and copper (3d¹⁰ 4s¹).
This is because 3d and 4s orbitals are very close in energy, and the energy of 3d orbitals drops going across the row.
For both chromium and copper the configuration having more electrons in in 3d orbitals is of lower energy.

For chromium this is because the difference in 3d and 4s orbital energies is similar to the pairing energy (Electron pairs are of higher energy).
The 3d⁵ 4s¹ configuration is of lower energy because this configuration has the maximum number of unpaired electrons for a d-subshell.

At copper (near the end of the transition series) 3d orbital energy has dropped so that 3d orbitals are of lower energy than 4s orbitals
This means the 3d¹⁰ 4s¹ configuration is of lower energy because it has more electrons in 3d orbitals.

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 number of d electrons equals the total number of valence electrons 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