In CaO, so as to achieve the stable octate :

Question

UnlockIAS · Accepted Answer

Correct — option (1). Calcium oxide is an ionic, or electrovalent, compound, and the question is asking in which direction and in what number the electrons move when it is formed. Start from the two atoms. Calcium has atomic number 20 and the electron arrangement 2, 8, 8, 2, so it carries two electrons in its outermost shell; oxygen has atomic number 8 and the arrangement 2, 6, so it is two electrons short of a full outer shell. Both can reach a noble-gas arrangement — the 'stable octate' of the stem, which is the paper's spelling of the stable octet — by a single transaction: calcium hands over both of its valence electrons and oxygen accepts both. Calcium is left with 2, 8, 8, the arrangement of argon, and carries a double positive charge as Ca2+; oxygen becomes 2, 8, the arrangement of neon, and carries a double negative charge as the oxide ion O2-. The two oppositely charged ions are then held together by electrostatic attraction, which is what an ionic bond is, and because the charges are equal and opposite the formula is a simple one-to-one, CaO. The direction of the transfer is not arbitrary, and it is the point of the question. Electrons move from the atom that gives them up easily to the atom that attracts them strongly. Calcium is a metal of group 2 with a low ionisation energy, so removing its two loosely held outer electrons costs relatively little; oxygen is a highly electronegative non-metal with a strong affinity for electrons, so accepting two is favourable. A transfer in the opposite direction would be doubly absurd — it would require the most electron-hungry atom in the pair to surrender electrons to a metal, and it would leave neither atom with a complete octet. The number is not arbitrary either: two electrons are moved because that is exactly what calcium has to spare in its outer shell and exactly what oxygen needs to complete its own, and any smaller number leaves both atoms short. The phrase 'the same oxygen atom', which appears in the English column only — the मराठी prints ऑक्सीजनच्या अणुस, without त्याच — is doing real work as well. It rules out the two electrons being split between different partners and describes the whole valency of one calcium being satisfied by one oxygen, which is why the formula is CaO and not something else. In the solid, of course, there are no separate CaO molecules at all: the ions pack into a giant lattice of the rock-salt type, each calcium ion surrounded by six oxide ions and each oxide ion by six calcium ions, and that three-dimensional network of strong electrostatic forces is why calcium oxide is a hard solid with a very high melting point.

Why the other options are wrong — (b) The direction is right but the number is wrong, and the stem is asking specifically about achieving a stable octet, which this transfer does not achieve for either atom. If calcium gave up only one electron it would be left with 2, 8, 8, 1 — an odd electron sitting alone in the outermost shell, which is no noble-gas arrangement at all — and oxygen, having accepted one, would be 2, 7, still one short of a complete octet. Two species each carrying an unpaired electron and neither satisfied is precisely what the octet rule says will not be the resting state. The charges would not fit the compound either: a Ca+ and an O- would combine one to one, but neither ion is one that calcium and oxygen actually form. Calcium is a group 2 metal and its characteristic valency is two, oxygen is a group 16 non-metal and its characteristic valency is two, and the whole of the reaction follows from those two facts.  (c) This reverses the direction of the transfer, which is the single most instructive error available on this question. Oxygen does not have two valence electrons to give — it has six, and it needs two more. If it somehow surrendered two it would be left with 2, 4, further from an octet than it started, while calcium receiving two would become 2, 8, 8, 4, which is not a noble-gas arrangement either. The chemistry rules it out for a second reason as well. Oxygen is one of the most electronegative elements known, so it attracts shared or transferable electrons strongly and forms negative ions; calcium is an electropositive metal with a low ionisation energy, so it loses electrons and forms positive ions. In every ionic compound the metal is the donor and the non-metal the acceptor, and reading an option's arrow in the wrong direction is a habit worth checking before anything else on a bonding question.  (d) This combines both errors of the two options above: it sends the electrons the wrong way, from the non-metal to the metal, and it sends only one of them. Oxygen losing a single electron would be left with 2, 5, and calcium gaining one would be 2, 8, 8, 3, so neither atom is anywhere near a stable octet and the option fails on the stem's own criterion. It is worth noticing how the four choices are constructed, because the pattern recurs: the examiner has taken two variables — which atom donates, and whether one electron or two are moved — and printed all four combinations. When an option set is built that way, the candidate's job is simply to settle each variable on its own, and each is settled by one fact: the metal always donates, and the number transferred is the number the outer shells require, which for calcium and oxygen is two.

Concept: Chemical bonding is the study of how atoms attain the stable electron arrangement of a noble gas. In ionic or electrovalent bonding, one atom transfers electrons outright to another; in covalent bonding, atoms share pairs of electrons. Which occurs depends on the pair of elements: a metal of low ionisation energy combined with a non-metal of high electronegativity gives an ionic compound, while two non-metals of comparable electronegativity share and give a covalent one. In sodium chloride a single electron passes from sodium, 2, 8, 1, to chlorine, 2, 8, 7, giving Na+ and Cl-. In magnesium oxide and in calcium oxide two electrons pass from the group 2 metal to oxygen, giving doubly charged ions, and because the charges are larger the electrostatic attraction is stronger and the melting point higher. In calcium chloride the calcium's two electrons are shared out to two chlorine atoms, one each, giving the formula CaCl2 — which is why the wording 'to the same oxygen atom' matters in a question of this kind. Ionic compounds are, as a class, crystalline solids with high melting and boiling points, brittle, soluble in water more often than in organic solvents, and non-conducting when solid but conducting when molten or in solution, because only then are the ions free to move. Covalent compounds are typically gases, liquids or low-melting solids and do not conduct. Calcium oxide itself, known as quicklime, is manufactured by heating limestone so that calcium carbonate decomposes to calcium oxide and carbon dioxide, and it reacts vigorously with water, giving out heat, to form calcium hydroxide or slaked lime.

State PCS 2021 Q65 — Science & Technology PYQ with Answer

State PCS 2021 Q65 — Science & Technology PYQ with Answer