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 3orbitals 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