According to the n plus l rule, which orbital is filled first?

  • A. 3d
  • B. 4s
  • C. 4p
  • D. 4d

Explanation

For 4s the sum is 4 plus 0, that is 4, while for 3d it is 3 plus 2, that is 5, so the lower sum fills first and 4s is occupied before 3d. When two orbitals have the same sum, the one with the lower n fills first. This ordering explains why potassium and calcium come before the transition metals.

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