Exceptions

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

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

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

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 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. 

For copper (near the end of the transition series) the 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.