The oxidation states of all elements in any allotropic form is _________.

Question

UnlockIAS · Accepted Answer

Correct — option (3), Zero. The very first rule for assigning oxidation numbers says that an atom of an element in the free or uncombined state has an oxidation number of zero. An allotrope is nothing but a free element wearing a different structural arrangement: diamond and graphite are both pure carbon, dioxygen and ozone are both pure oxygen, white and red phosphorus are both pure phosphorus, rhombic and monoclinic sulphur are both pure sulphur. In every one of these the only bonds present are between atoms of the same element. Oxidation number is a bookkeeping device that asks which atom would take the shared electrons if the bond were broken unequally; between two identical atoms there is no reason to give the pair to one rather than the other, so the electrons are split evenly and nothing is gained or lost. Each atom is therefore assigned zero — every carbon in diamond and in graphite, every oxygen in O2 and in O3, every phosphorus in P4, every sulphur in S8. This is also consistent with the rule that the oxidation numbers in a neutral molecule must add up to zero: since all the atoms are identical and must carry the same value, that value can only be zero. The allotropic form makes no difference at all, which is exactly the point the question is testing. Note that the stem as printed reads 'The oxidation states ... is', a number disagreement in the English column; the sense is not in doubt.

Why the other options are wrong — (a) An oxidation number of +2 belongs to an element that has been combined with something more electronegative than itself — the alkaline earth metals magnesium and calcium in their compounds, iron in the ferrous state, or, unusually, oxygen in oxygen difluoride, where fluorine is the only element electronegative enough to make oxygen positive. All of these require a second element to be present. An allotrope contains only one element, so there is nothing for it to be oxidised by, and no positive value can arise. Any positive option in this question fails for the same structural reason before its particular value is even considered.  (b) The value +1 is the standard oxidation number of hydrogen when it is bonded to a non-metal and of the alkali metals in their compounds — sodium in sodium chloride, hydrogen in water and in hydrochloric acid. It always describes an atom that has lost the tug of war for a shared pair to a more electronegative partner. In an allotrope there is no partner: hydrogen in H2 is not +1 but zero, because both atoms are hydrogen. The number describes the atom's situation in a compound, not any fixed property of the element itself, and that is the distinction the question is built on.  (d) A value of −1 belongs to an atom that has won the shared pair from a less electronegative partner — chlorine in sodium chloride, hydrogen in a metal hydride such as sodium hydride, or oxygen in a peroxide such as hydrogen peroxide, where the O-O bond is shared evenly and each oxygen carries only −1 rather than the usual −2. Every one of those cases is a compound. In an allotrope the only bonds are between identical atoms, so no atom can be reduced relative to another, and the value stays at zero however elaborate the structure — including the S8 ring and the P4 tetrahedron.

Concept: Allotropy is the existence of an element in two or more distinct forms in the same physical state, differing in how the atoms are joined rather than in what they are. Carbon is the richest example: diamond, in which every carbon is bonded tetrahedrally to four others in a rigid three-dimensional network, giving extreme hardness and no electrical conduction; graphite, in which the atoms lie in flat hexagonal sheets held to each other only weakly, giving softness, lubricating properties and electrical conduction; and the fullerenes, the closed cage molecules of which buckminsterfullerene, C60, is the best known. Oxygen exists as ordinary dioxygen, O2, and as ozone, O3. Phosphorus has white, red and black forms of sharply different reactivity. Sulphur has rhombic and monoclinic crystalline forms, both built of S8 rings. Because the difference between allotropes is structural and not compositional, no chemical bookkeeping quantity that depends on which elements are combined can distinguish them.

State PCS 2026 Q69 — Science & Technology PYQ with Answer

State PCS 2026 Q69 — Science & Technology PYQ with Answer