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Variable Valence in Transition Elements and Actinoids

Chemistry · Periodic Classification Of Properties · NEET

Transition elements (d-block) and actinoids show variable valence. This means the same element can form ions with more than one charge, like Fe as Fe2+ and Fe3+. This happens because the (n-1)d and outer ns electrons (or 5f and ns for actinoids) have almost the SAME energy, so a variable number of these electrons can take part in bonding. Memory hook: "d and s are close, so many electrons can leave" - that is why one metal gives many oxidation states.
Why Variable Valence: Close Energy LevelsEnergy4s3dTransition metal3d and 4s CLOSE3s2p (full, buried)Sodium (Na)BIG gapmany electrons leave to variable valenceonly 1 electron leaves to fixed +1
In transition metals the 3d and 4s levels have almost equal energy, so a variable number of electrons can leave, giving many oxidation states. In sodium a large energy gap allows only one electron to leave, so its valence is fixed at +1.

Your doubts, answered

Why do transition elements show variable valence but sodium does not?

In sodium (Na, group 1) only ONE electron is easy to remove (3s1). The next electron sits in a full, low-energy shell that needs a huge amount of energy, so Na is only +1 (fixed valence). In transition elements the (n-1)d and ns electrons have very close energy. So the atom can lose just the ns electrons, or the ns plus some (n-1)d electrons. A different number lost each time means many oxidation states. Example: Mn can be +2, +3, +4, +6, +7.

Which electrons take part in bonding in transition elements?

Both the outer ns electrons and the inner (n-1)d electrons take part. Because their energy levels are so close, there is no big energy jump between them. So after removing the ns electrons, the atom can still remove one or more (n-1)d electrons without spending much extra energy. This is the whole reason for variable valence.

Why do actinoids show even more oxidation states than lanthanoids?

In actinoids the 5f, 6d and 7s electrons all have very close energy. So many electrons can take part in bonding. Uranium, for example, shows +3, +4, +5 and +6. In lanthanoids the 4f electrons are held more tightly and are more buried, so most lanthanoids mainly show only +3 (with a few showing +2 or +4). More electrons of similar energy = more oxidation states.

Is variable valence the same as variable oxidation state?

For NEET, treat them as the same idea here. 'Variable valence' means the element combines using different numbers of electrons, giving different oxidation states (different ion charges). So iron showing Fe2+ and Fe3+ is an example of both variable valence and variable oxidation state.

Do Zn, Cd and Hg show variable valence?

Not really. Zn, Cd and Hg have a completely full (n-1)d10 shell that stays full in their common compounds. They mostly show only +2 (Hg also shows +1 as Hg2 2+). Because their d-subshell is full and stable, they are called non-typical transition elements and do NOT show the wide variable valence that Fe, Mn or Cr show.

⚠️ The NEET trap
✗ Transition elements show variable valence only because their atoms are big.
✓ They show variable valence because the (n-1)d and ns electrons have almost equal energy, so a variable number of electrons can take part in bonding.
🧠 NEET tests the REASON, not the size. Always answer 'close energy of (n-1)d and ns electrons' - size alone is a trap.

Solved Periodic Classification Of Properties NEET PYQs

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Frequently asked

Give two examples of variable valence in transition elements.

Iron shows +2 (Fe2+) and +3 (Fe3+). Manganese shows +2, +3, +4, +6 and +7. Copper shows +1 (Cu+) and +2 (Cu2+).

Why is the lowest oxidation state usually +2 for many transition metals?

The two outer ns electrons are the easiest to remove. Losing just these two gives the common +2 state. Removing extra (n-1)d electrons then gives the higher states.

Do all transition elements show variable valence?

Most do, but not Zn, Cd and Hg. Their d-subshell is full (d10) and stays full, so they mainly show only +2 and are called non-typical transition elements.

What is the highest oxidation state of manganese and why?

Manganese shows +7 (as in KMnO4). It can lose 2 ns and 5 (n-1)d electrons (total 7) because these have close energy, giving the maximum +7 state.

Related concepts:
General properties of d-block (colour, variable valence, catalysts)Why Zn, Cd, Hg are not typical transition elementsLanthanoids vs actinoids: differences and configuration3d transition series: why 4s fills before 3dDobereiner's Law of Triads explained
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Periodic Classification Of Properties PYQs