Exchange Energy : Cause of Extra Stability of Half-filled & Completely Filled Orbitals :In case of 3d4 4s2 configuration in Cr :
The electron no.1 can exchange its position with electrons nos. 2,3 & 4 i.e., in 3 ways.
The electron no.2 can exchange 2 ways with electrons 3 & 4 (with electron 1 has already been considered).
The electron no.3 can exchange only in 1 way (as exchanges with electrons 1 & 2 has already been calculated).
Hence there are 3+2+1=6 ways of exchange are possible in 3d4 arrangement.
Similarly, in 3d5 configuration, the total possible ways of exchange is 4+3+2+1=10.
Therefore, 3d5 configuration is more stable than 3d4 configuration.