Question

Calcium (atomic number 20) has electron configuration 1s2,2s2,2p6,3s2,3p6,4s2. The atomic number of scandium is 21. Which orbital does the 21st electron enter?

A) 3d
B) 4p
C) 4d
D) 4f

Answer 1

Calcium has this eletron configuration(1s2 2s2 2p6 3s2 3p6 4s2) and has 20 eletrons, then scandium with one more eletron will have the same configuration but with the 21st eletron in the next level.
1s2 2s2 2p6 3s2 3p6 4s2 3d¹

The next level is 3d.
Answer: 3d.