Problem: The total number of electrons in the 3d orbitals of Cr3+ isa. 1.b. 2.c. 3.d. 4.e. 5.

Neutral Cr: (transition metal) 

Atomic number → 24 → 24 e^-

Recall:

s           _                1 orbital   →  max of 2 electrons

p        _ _ _             3 orbitals  → max of 6 electrons

d       _ _ _ _ _        5 orbitals → max of 10 electrons

Fill up the orbitals with electrons:

Aufbau Principle → totally fill in the lowest energy orbital first 
Hund’s Rule → electron orbitals that are degenerate (same energy) are first half-filled before they are totally filled          

Electron configuration (lowest energy level to the highest energy level: s < p < d < f):

• n = energy level → row/period number

• Cr: 1s²2s²2p⁶3s²3p⁶4s²3d⁴
• But, d orbitals → half filled or fully filled → exception

• Ground state electron configuration:
  • Cr: [Ar]3d⁵ or 1s²2s²2p⁶3s²3p⁶4s¹3d⁵