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Question
While filling up of electrons in the atomic orbitals, the 4s orbital is filled before the 3d orbital but reverse happens during the ionisation of the atom. Explain why?
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Solution
Electrons are filled to the n + l rule. If an orbital has lower n + l value, then the electron will enter that orbital.
For 3d, n + l = 3 + 2 = 5
4s, n + l = 4 + 0 = 4
So, the electron will first enter 4s and then 3d while filling. But, 4s electrons are held loose by the nucleus and are outside of 3d, so removing a 4s electron becomes easier than removing a 3d electron.
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