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

a. 3d   
The electrons in an atom fill the shells in order from least energy to higher energy. The orbitals from lowest to highest are
 1s - 1 orbital, 2 electrons
 2s - 1 orbital, 2 electrons
 2p - 3 orbitals, 6 electrons
 3s - 1 orbital, 2 electrons
 3p - 3 orbitals, 6 electrons.
 4s - 1 orbital, 2 electrons. Note the energy for the 4s orbital is just barely
lower than the 3d orbitals. So it fills up before the 3d orbitals.
 3d - 5 orbitals, 10 electrons.
 
 Since the 20th electron fills the 4s shell, the 21st electron goes into the next higher energy orbital which is one of the 3d orbitals.

The answer is not c