Question

If 4s orbital donates its electron to 3d orbital because there was one room empty. Then why does it not do the same for other elements?

Answer 1

According to Hund's rule, electron orbitals are most stable when they are fully filled or half filled. So the most stable configuration for the 3d subshell is $3d^{10}or3d^5$. In the case of chromium, after the $4s^{2}3d^4$ configuration is attained, an electron from the 4s orbital jumps to 3d subshell because $3d^5$ is a much more stable configuration than $3d^4$. That's why final configuration for chromium is $4s^{1}3d^5$. This is possible because of the fact that the two energy levels being considered here are very close and the electron shift is easy. In case of other elements in the period, it goes against the Aufbau rule and with other elements in the group, the energy difference between ns and (n-1)d orbitals increases and so that transition is not easily possible.