Why Transition Metals Show Variable Oxidation States
Chemistry · D And F Block Elements · NEET
Transition metals show many oxidation states because their outer (n-1)d and ns electrons have almost the same energy. So the metal can lose the ns electrons AND a changing number of (n-1)d electrons, one step at a time. Memory hook: "d and s are close, so many states are possible."
The (n-1)d and ns orbitals sit very close in energy, so a transition metal loses its ns electrons first (+2) and then a changing number of (n-1)d electrons one at a time, producing many oxidation states.
Your doubts, answered
Why do transition metals show variable (many) oxidation states but sodium shows only +1?
In sodium the next electron to remove sits in a deep, low-energy inner shell, so it needs a huge amount of energy. That is why Na only gives +1. In a transition metal the (n-1)d and ns orbitals lie very close in energy. The energy needed to remove one more d electron is small. So the atom can lose the 2 ns electrons and then a variable number of d electrons. This gives a range of oxidation states like +2, +3, +4 and so on.
Why do the oxidation states of transition metals often differ by just 1 (like +2, +3, +4)?
This is the key point that separates them from main-group (p-block) elements. In p-block, states usually jump by 2 (like +2 and +4 in tin) because electrons come off in pairs from s and p. In transition metals the d electrons are removed ONE at a time because the d orbitals are close in energy. So the states rise in single steps: +2, +3, +4, +5. NEET loves this contrast.
Why does manganese show the most oxidation states (+2 up to +7)?
Manganese is [Ar] 3d5 4s2. It has 2 s electrons plus 5 d electrons, giving 7 electrons that can take part in bonding. Because these d and s electrons are close in energy, Mn can lose from 2 up to all 7 of them. So it shows +2, +3, +4, +5, +6 and +7. The maximum state +7 matches its group number. Elements near the middle of the series show the most states; those at the ends (Sc, Zn) show fewer.
Which oxidation state is most stable, and why is +2 so common?
For most transition metals the +2 state is very common because the 2 ns electrons are lost first and easily. Extra stability comes from half-filled (d5) or fully-filled (d10) sub-shells. For example Mn2+ is 3d5 (half-filled) and is very stable, and Zn2+ is 3d10. This is why Mn2+ forms so readily and why the Mn3+/Mn2+ change (d4 to d5) has a high positive E° value.
Do variable oxidation states connect to any other property NEET asks about?
Yes. The ability to change oxidation state easily is the reason transition metals and their compounds act as good catalysts. The metal can gain or lose electrons during a reaction (for example Fe in the Haber process, V2O5 in making H2SO4) and then return to its start state. So variable oxidation state directly explains catalytic activity, and NEET often tests them together.
⚠️ The NEET trap ✗ Students think transition metals catalyse reactions mainly because they are heavy, dense metals. ✓ They are good catalysts because they show variable oxidation states (can give or take electrons) and can form complexes, not because of their density. 🧠 Catalyst power comes from CHANGING oxidation states, not from being heavy.
Real NEET questions
NEET 2024
The E° value for the Mn3+/Mn2+ couple is more positive than that of Cr3+/Cr2+ or Fe3+/Fe2+ due to the change of
A · d5 to d2 configuration
B · d4 to d5 configuration ✓
C · d3 to d5 configuration
D · d5 to d4 configuration
Solution: The reduction is Mn3+ + e- -> Mn2+. Mn3+ is 3d4 and Mn2+ is 3d5. So the change is d4 to d5. The product Mn2+ has the very stable half-filled 3d5 configuration, so this reduction happens easily. This extra stability makes E° for Mn3+/Mn2+ strongly positive compared with Cr3+/Cr2+ and Fe3+/Fe2+. This is a direct example of why half-filled stability controls which oxidation states are favoured.
NEET 2020
Identify the incorrect statement.
A · Interstitial compounds are formed when small atoms like H, C or N are trapped inside the crystal lattices of metals.
B · The oxidation states of chromium in CrO4^2- and Cr2O7^2- are not the same. ✓
C · Cr2+ (d4) is a stronger reducing agent than Fe2+ (d6) in water.
D · Transition metals and their compounds show catalytic activity due to their ability to adopt multiple oxidation states and to form complexes.
Solution: In both CrO4^2- and Cr2O7^2- chromium is in the +6 state, so saying they are 'not the same' is wrong, making (b) the incorrect statement. Notice option (d): it states the real reason transition metals are catalysts is their variable (multiple) oxidation states and complex formation. That statement is correct, which is exactly the concept of this page.
NEET 2017
The reason for greater range of oxidation states in actinoids is attributed to
A · the radioactive nature of actinoids.
B · actinoid contraction.
C · 5f, 6d and 7s levels having comparable energies. ✓
D · 4f and 5d levels being close in energies.
Solution: In actinoids the 5f, 6d and 7s energy levels lie very close together. Because these orbitals have comparable (nearly equal) energies, electrons from all of them can be lost to varying extents. This is the same 'close energy levels' idea that gives d-block metals their variable states, and it lets actinoids show an even wider range of oxidation states than lanthanoids.
Solved D And F Block Elements NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the simple reason transition metals show variable oxidation states?
Their (n-1)d and ns orbitals have almost equal energy. So after losing the ns electrons, the metal can also lose a changing number of d electrons using only a little extra energy, giving several oxidation states.
Why do their oxidation states differ by 1 and not by 2 like p-block?
Because d electrons are removed one at a time due to their close energy levels. So states rise in single steps (+2, +3, +4). In p-block, s and p electrons often leave in pairs, so states jump by 2.
What is the maximum oxidation state a transition metal can show?
The maximum usually equals the total number of ns plus (n-1)d electrons available, up to the group number. For example Mn ([Ar]3d5 4s2) can reach +7, which is its group number.
Why is the +2 state so common in the first transition series?
The two 4s electrons are lost first and easily, giving the +2 state. Extra stability of half-filled (d5) or full (d10) sub-shells, like in Mn2+ and Zn2+, makes +2 even more favourable.
How is variable oxidation state linked to catalytic activity for NEET?
A catalyst must accept and release electrons during a reaction. Because transition metals can easily change oxidation state, they do this well and then return to the original state. This is why they are common catalysts, such as Fe and V2O5.
