What is the electron configuration of Fe²⁺? (Iron's neutral configuration is [Ar]3d⁶4s².)
A[Ar]3d⁴4s²
B[Ar]3d⁶
C[Ar]3d⁵4s¹
D[Ar]3d⁴4s²
When transition metals form ions, electrons are removed from the highest principal quantum number first — the 4s electrons are lost before the 3d. So Fe²⁺ = [Ar]3d⁶, not [Ar]3d⁴4s². This is counterintuitive because 4s fills before 3d in neutral atoms (Aufbau), but energetics shift upon ionization so 4s becomes higher in energy than 3d.
Question 2 True / False
According to Hund's rule, two electrons in the same subshell generally pair up in the same orbital before occupying empty orbitals.
TTrue
FFalse
Answer: False
Hund's rule states the opposite: electrons fill empty degenerate orbitals one at a time with parallel spins before any pairing occurs. Pairing is energetically costly due to electron-electron repulsion. Carbon (1s²2s²2p²), for example, has its two 2p electrons in separate p orbitals, not the same one.
Question 3 Short Answer
Why does chromium have the electron configuration [Ar]3d⁵4s¹ rather than the expected [Ar]3d⁴4s²?
Think about your answer, then reveal below.
Model answer: A half-filled d subshell (3d⁵) has extra stability due to exchange energy — electrons with the same spin in different orbitals lower the total energy through favorable exchange interactions. This stabilization is large enough that one electron migrates from 4s to 3d, giving a half-filled d subshell at the cost of a singly occupied 4s.
This is one of several Aufbau exceptions where the stability of a half-filled or fully filled d subshell (e.g., Cu = [Ar]3d¹⁰4s¹) outweighs the expected energy ordering. These exceptions matter for predicting the reactivity and magnetic properties of transition metals.