Question

explain why Cr has configuration 3d5 4s1 and not 3d4 4s2

Answer 1

Hund's rule states that when electrons start filling up subshells (like the 3d or 4s subshell) they do so in such a way that electrons of the same spin must solely occupy the orbitals within the subshell first. Then electrons of opposite spin will start filling up the remaining space in the orbitals. The orbitals are filled in according to the increasing energy levels such as:  1s<2s<2p<3s<3p<4s<3d<4p  etc.

However, we also know, half filled is more stable than partially filled orbitals. Also, 3d5 is more stable and less energy than 4s2 configuration. In case of Chromium, after the 4s2 3d4 configuration is attained, an electron from the 4s orbital jumps to 3d subshell because 3d5 is a much more stable configuration than 3d4. That's why final configuration for Chromium is 4s1 3d5 and not 3d4 4s2.

Answer 2

Answer:

Electronic configuration of Cr is [Ar]3d5 4s1, instead of the expected [Ar]3d4 4s2. This is so because half filled d orbitals have extra stability. So in case of Cr, one electron from the 4s orbital goes to the 3d orbital to make it half filled, and Cr attains extra stable state.

Explanation:

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