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First-row configuration; variable oxidation states

Short answer

Ground configurations: Sc 3d1 4s2, Ti 3d2 4s2, V 3d3 4s2, Cr 3d5 4s1, Mn 3d5 4s2, Fe 3d6 4s2, Co 3d7 4s2, Ni 3d8 4s2, Cu 3d10 4s1, Zn 3d10 4s2. Variable oxidation states arise because 4s and 3d electrons have similar energies, so both can be lost in bonding, giving multiple stable ions.

Write the ground state electronic configuration of the first row transition metals. How would you explain the variable oxidation states of transition metals?

Ground state configurations (3d series)

Sc  [Ar] 3d¹ 4s²     Fe  [Ar] 3d⁶ 4s²
Ti  [Ar] 3d² 4s²     Co  [Ar] 3d⁷ 4s²
V   [Ar] 3d³ 4s²     Ni  [Ar] 3d⁸ 4s²
Cr  [Ar] 3d⁵ 4s¹     Cu  [Ar] 3d¹⁰ 4s¹
Mn  [Ar] 3d⁵ 4s²     Zn  [Ar] 3d¹⁰ 4s²

Cr and Cu are exceptions: a half-filled (d⁵) or completely filled (d¹⁰) 3d subshell is extra stable, so one 4s electron moves into 3d.

Variable oxidation states

Variable oxidation states arise because the 4s and 3d electrons have very similar energies, so both 4s and some or all 3d electrons can be used in bonding. The states differ by one unit (Fe²⁺, Fe³⁺), unlike the p-block where they differ by two.

Ti  +2 +3 +4          Fe  +2 +3
V   +2 +3 +4 +5       Co  +2 +3
Cr  +2 +3 +6          Ni  +2
Mn  +2 +3 +4 +6 +7    Cu  +1 +2
  • Minimum oxidation state is usually +2 (loss of the two 4s electrons); the maximum equals the total number of 4s + 3d electrons, reached at Mn (+7).
  • Stability of higher states falls after Mn because the nuclear charge holds the 3d electrons more tightly.
  • Half-filled d⁵ (Mn²⁺, Fe³⁺) and filled d¹⁰ (Zn²⁺) are specially stable.

✓ Answer checked by Claude Opus 5.5 (high effort)

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