Structure 3.1.9 – Formation of Variable Oxidation States in Transition Elements

What Are Variable Oxidation States?

Transition elements exhibit variable oxidation states, meaning they can form more than one stable ion by losing different numbers of electrons. This behavior contrasts with main group elements, which usually have one fixed oxidation state (e.g., Group 1 = +1, Group 2 = +2).

Cause: Similar Energies of 4s and 3d Orbitals

• Transition elements have valence electrons in both the 4s and 3d sublevels.
• The energy difference between 4s and 3d orbitals is small.
• This allows removal of electrons from both sublevels to form ions with different oxidation states.

Ionization Energy and Oxidation States

• Successive ionization energies (to remove electrons) in transition metals are relatively close in value.

• This allows formation of multiple stable oxidation states.
• Lower oxidation states (e.g., +2, +3) are more stable for later transition metals.
• Higher oxidation states (e.g., +5, +6, +7) are more common in early transition metals and in covalent compounds.

Trends in Oxidation States

• Scandium (Sc) only forms +3, so it’s often excluded from “typical” transition metals.
• Titanium to manganese exhibit a wide range of oxidation states (+2 to +7).
• Iron to copper show fewer oxidation states and tend to stabilize at +2 or +3.

Table of Common Oxidation States

Key Notes

• +2 oxidation state is common across all transition elements.
• Maximum oxidation state is the total number of 4s + 3d electrons (e.g., Mn = +7).
• Later transition metals tend to stabilize in lower oxidation states.

Electron Configurations of Transition Metal Ions

1. Rules of Electron Configuration of Transition Elements

• Transition metals have partially filled 3d sublevels.
• According to the Aufbau principle, electrons fill the lowest energy orbitals first.
• The 4s orbital fills before the 3d orbital because it is lower in energy initially.
• Electron configurations are commonly written using the argon core notation: [Ar] represents the 1s to 3p orbitals for first-row transition elements.
• Electronic configurations may be written with 3d before or after 4s — both are accepted.

2. Exceptions

• Chromium: [Ar] 3d⁵ 4s¹ (more stable than 3d⁴ 4s²)
• Copper: [Ar] 3d¹⁰ 4s¹ (more stable than 3d⁹ 4s²)
• This stability comes from half-filled and fully filled d sublevels.

3. Ion Formation in Transition Elements

• Electrons are removed from the 4s orbital before the 3d.
• Once filled, 4s becomes slightly higher in energy due to repulsion.
• The most common oxidation state is +2, corresponding to the loss of both 4s electrons.

4. Electron Configurations of Ions

• To determine ion configurations, remove electrons in this order: 4s → 3d.