Read the passage carefully and answer the questions.

Nearly all the transition elements display typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre. The transition metals (with the exception of Zn, Cd and Hg) are very hard and have low volatility. Their melting and boiling points are high. They have high enthalpies of atomization. Ions of the same charge in a given series show progressive decrease in radius with increasing atomic number. However, the variation within a series is quite small. The filling of 4f before 5d orbital results in a regular decrease in atomic radii called Lanthanoid contraction which essentially compensates for the expected increase in atomic size with increasing atomic number. The net result of the lanthanoid contraction is that the second and the third d series exhibit similar radii (e.g., Zr 160 pm, Hf 159 pm) and have very similar physical and chemical properties. There is an increase in ionisation enthalpy along each series of the transition elements from left to right. The first ionisation enthalpy, in general, increases, but the magnitude of the increase in the second and third ionisation enthalpies for the successive elements, is much higher along a series. One of the notable features of a transition elements is the great variety of oxidation states these may show in their compounds.

Among Cr (Z=24), Co (Z=27), Mn (Z=25) and Fe (Z=26); which will show the maximum variety of oxidation states?

Mn

The correct answer is Option (1) → Mn

Among Cr, Mn, Fe and Co, Manganese (Mn) shows the maximum variety of oxidation states.

Reasoning:

A key property of transition elements mentioned in the passage is that they show a great variety of oxidation states. This happens because both $(n-1)d$ electrons and $ns$ electrons can participate in bonding.

Now compare their electronic configurations:

• Cr (24): $[Ar] 3d^{5} 4s^{1}$
• Mn (25): $[Ar] 3d^{5} 4s^{2}$
• Fe (26): $[Ar] 3d^{6} 4s^{2}$
• Co (27): $[Ar] 3d^{7} 4s^{2}$

Manganese (Mn) has a half-filled $3d^{5}$ configuration plus $4s^{2}$, giving it 7 valence electrons available for bonding.

Because of this, Mn can lose different numbers of electrons and form many oxidation states.

Typical oxidation states:

• Cr: $+2, +3, +6$
• Mn: $+2, +3, +4, +5, +6, +7$ $\leftarrow$ Maximum range
• Fe: $+2, +3$ (mainly)
• Co: $+2, +3$ (mainly)

Thus, Manganese exhibits the widest range of oxidation states, which matches the statement in the passage that transition elements can show large variability.