Both the first and the second ionization processes remove electrons from the 4s orbital of these atoms, with the exception of Cr. In general, the 4s electrons are poorly shielded by the 3d electrons, so Zeff(4s) increases from left to right and I2 also increases from left to right. While the I1 process removes the sole 4s electron for Cr, the I2 process must remove a 3d electron. For Cr this subshell is half-filled and is rather stable as such. Thus, the higher value of I2 for Cr relative to Mn is a consequence of the special stability of half-filled 3d subshell and lower energy of 5d with respect to 4s.