Why do transition metals exhibit variable oxidation states?

Transition metals exhibit variable oxidation states primarily because the energy difference between their outermost $ns$ orbitals and the inner $(n-1)d$ orbitals is very small. This allows electrons from both these subshells to participate in chemical bonding. Unlike s-block elements where only $ns$ electrons are involved, or p-block elements where the energy gap between $ns$ and $np$ can be larger, transition metals can readily lose a varying number of electrons, leading to a wide range of stable oxidation states.

How does lanthanoid contraction affect the ionisation enthalpy of 5d transition elements?

Lanthanoid contraction refers to the greater-than-expected decrease in atomic and ionic radii across the lanthanoid series. This is due to the poor shielding effect of the 4f electrons. This contraction carries over to the 5d transition series, causing elements like Hf to have atomic radii and ionisation enthalpies very similar to their 4d counterparts (e.g., Zr). Consequently, the 5d elements often have higher ionisation enthalpies than expected, sometimes even higher than 4d elements, due to the increased effective nuclear charge.

Why is the third ionisation enthalpy of Manganese ($Mn$) exceptionally high?

Manganese has an electronic configuration of $[Ar]3d^54s^2$. When it loses its two $4s$ electrons, it forms $Mn^{2+}$ with a configuration of $[Ar]3d^5$. This $3d^5$ configuration is half-filled and thus exceptionally stable. Removing a third electron from this highly stable half-filled $3d^5$ subshell requires a very large amount of energy, making its third ionisation enthalpy ($IE_3$) significantly high. This stability explains why $Mn^{2+}$ is a very common and stable ion.

What is disproportionation, and can you give an example involving transition metals?

Disproportionation is a redox reaction where a single element in an intermediate oxidation state simultaneously undergoes both oxidation (increase in oxidation state) and reduction (decrease in oxidation state). A classic example in transition metal chemistry is the disproportionation of $Cu^+$ ions in aqueous solution. $Cu^+$ is unstable in water and reacts as follows: $2Cu^+(aq) ightarrow Cu^{2+}(aq) + Cu(s)$. Here, one $Cu^+$ ion is oxidized to $Cu^{2+}$ (oxidation state changes from $+1$ to $+2$), and another $Cu^+$ ion is reduced to $Cu$ metal (oxidation state changes from $+1$ to $0$). This occurs because $Cu^{2+}$ and $Cu$ metal are more stable than $Cu^+$ in aqueous environments.

Why does Zinc (Zn) only show a +2 oxidation state, unlike other transition metals?

Zinc has an electronic configuration of $[Ar]3d^{10}4s^2$. When it loses its two $4s$ electrons, it forms $Zn^{2+}$ with a configuration of $[Ar]3d^{10}$. This $3d^{10}$ configuration is fully-filled and extremely stable. Removing any further electrons would require breaking this highly stable, fully-filled d-subshell, which demands an exceptionally high amount of energy (very high $IE_3$). Therefore, the $+2$ oxidation state, where the $3d^{10}$ configuration is preserved, is overwhelmingly stable and the only common oxidation state for zinc.

Ionisation Enthalpy, Oxidation States

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Ionisation enthalpy, often referred to as ionisation energy, is the minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom or ion in its ground state. For transition elements, this property exhibits complex trends due to the interplay of nuclear charge, shielding effect, and the involvement of both (n-1)d and ns electrons. Oxidation states, on the…

Quick Summary

Ionisation enthalpy is the energy required to remove an electron from a gaseous atom, with successive enthalpies increasing. For transition metals, $IE_1$ shows irregular trends across a period due to the interplay of increasing nuclear charge, d-electron shielding, and the stability of half-filled ( $d^5$ ) or fully-filled ( $d^{10}$ ) configurations (e.

g., Cr, Cu, Mn, Zn). Down a group, the lanthanoid contraction causes 5d elements to have unexpectedly high IE values, often comparable to 4d elements.

Oxidation states represent the hypothetical charge an atom would have in a compound. Transition metals are characterized by exhibiting variable oxidation states, a property arising from the small energy difference between their $(n-1)d$ and $ns$ orbitals, allowing both sets of electrons to participate in bonding.

The range of oxidation states typically increases up to Manganese (max $+7$ ) and then decreases. The stability of specific oxidation states is influenced by electronic configuration (e.g., $d^5$ , $d^{10}$ ), the nature of ligands, and the environment (e.

g., aqueous solution). Some intermediate oxidation states can undergo disproportionation, where an element is simultaneously oxidized and reduced.

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Key Concepts

Factors Affecting Ionisation Enthalpy in Transition Metals

The ionisation enthalpy of transition metals is a complex interplay of several factors. Firstly, the…

Origin and Stability of Variable Oxidation States

The variable oxidation states of transition metals stem from the very small energy difference between their…

Maximum Oxidation States and Trends

The maximum oxidation state exhibited by transition metals generally increases from Group 3 to Group 7 in the…

Ionisation Enthalpy (IE): — Energy to remove an electron.

$IE_1 < IE_2 < IE_3 dots$ — (Successive IEs always increase).

3d Series $IE_1$ Trend: — Generally increases, but irregular.

- **Cr ( $3d^54s^1$ ), Cu ( $3d^{10}4s^1$ ):** Higher $IE_1$ due to stable d-configs after $4s^1$ removal. - **Mn ( $3d^54s^2$ ):** Exceptionally high $IE_3$ ( $Mn^{2+}$ is $3d^5$ ). - **Zn ( $3d^{10}4s^2$ ):** Exceptionally high $IE_3$ ( $Zn^{2+}$ is $3d^{10}$ ).

Down a Group IE: — 5d > 4d due to Lanthanoid Contraction.

Oxidation States (OS): — Hypothetical charge.

Transition Metals: — Variable OS due to small energy difference between

(n-1)d

and

ns

orbitals.

Max OS Trend: — Increases up to Mn (+7), then decreases.

Stable OS: —

d^0, d^5, d^{10}

configurations are highly stable (e.g.,

Sc^{3+}

(

d^0

Mn^{2+}

(

d^5

Zn^{2+}

(

d^{10}

)).

Higher OS Stability: — Favored by highly electronegative ligands (O, F).

Disproportionation: — Element in intermediate OS simultaneously oxidizes and reduces (e.g.,

2Cu^+ \rightarrow Cu^{2+} + Cu

In Oxidation, Variable Charges Make Fun Compounds, Never Consistently Zero.

Ionisation Enthalpy: Remember the general increase but focus on Cr, Cu (higher

IE_1

) and Mn, Zn (higher

IE_3

Oxidation States: Variable for transition metals.

Charges: Max OS increases up to Mn (+7).

Make: Mn has the widest range.

Fun: Fe commonly +2, +3.

Compounds: Cu shows +1, +2; +1 disproportionates.

Never: Ni mostly +2.

Consistently: Co mostly +2, +3.

Zero: Zn only +2 (stable

d^{10}

). Sc only +3 (stable

d^0

This mnemonic helps recall the key elements and their characteristic oxidation state patterns and IE exceptions.