MPSC 2021 Prelims Paper-I · Question 65 of 100

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.

• (2)Ca atom transfer its one valence electron to the same oxygen atom — 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.
• (3)Oxygen atom transfer its two valence electrons to the same calcium atom — 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.
• (4)Oxygen atom transfer its one valence electron to the same calcium atom — 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.

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.

Chemical bonding is a standing item in the science section of MPSC papers because it can be tested with no calculation and no laboratory knowledge — only the electron arrangements of the first twenty elements and the octet rule. The Commission's usual devices are all on display in this question: it asks for the direction of transfer, it asks for the number of electrons moved, and it builds the four options by permuting those two variables so that a candidate who is vague about either will find two options equally plausible. The remedy is not to memorise particular compounds but to be able to write the electron arrangement of any of the first twenty elements from its atomic number, and then to read off the valency and the direction of transfer directly. It is worth adding that the stem carries one of this paper's many printing slips — 'stable octate' for 'stable octet' — and that the options read 'Ca atom transfer its two valence electrons'. Neither affects the chemistry, and neither should be corrected in the mind into something with a different meaning; the safe habit in an examination hall is to read a misprint as the nearest sensible technical term and answer the question that was plainly intended.

• Calcium has atomic number 20 and the electron arrangement 2, 8, 8, 2, so it has two valence electrons; oxygen has atomic number 8 and the arrangement 2, 6, so it needs two more electrons to complete its outer shell.
• In forming calcium oxide the calcium atom transfers both of its valence electrons to one oxygen atom, becoming Ca2+ with the argon arrangement, while the oxygen becomes the oxide ion O2- with the neon arrangement; the compound is held together by electrostatic attraction between the two ions.
• In every ionic compound the electrons move from the metal, which has a low ionisation energy, to the non-metal, which has a high electronegativity — never in the reverse direction.
• An ionic solid is not made of molecules; the ions form a giant three-dimensional lattice, and calcium oxide adopts the rock-salt arrangement in which each calcium ion is surrounded by six oxide ions and each oxide ion by six calcium ions.
• Ionic compounds are typically hard, brittle, high-melting crystalline solids that do not conduct electricity in the solid state but do conduct when molten or dissolved in water, because only then can the ions move.

• Reversing the direction of electron transfer, and letting the non-metal donate to the metal, when electronegativity settles the direction in every ionic compound
• Transferring the wrong number of electrons, when the number is fixed by what the outer shells of the two atoms require
• Assuming that an ionic compound exists as discrete molecules, when it is a giant lattice and its formula gives only the ratio of the ions
• Being thrown by a printing error such as 'octate' for 'octet', rather than reading it as the nearest sensible technical term and answering the question intended

Bonding questions in MPSC papers come in a few recognisable shapes: which type of bond is present in a named compound, how many electrons are transferred or shared in its formation, why one compound conducts electricity in solution and another does not, and which of four named substances is ionic. The Commission is fond of pairing an element from group 1 or 2 with one from group 16 or 17, because the arithmetic of the transfer is then clean and the answer follows from the electron arrangements alone. Expect also the property questions that ride on the same idea — high melting point, brittleness, solubility in water, conduction only when molten — and the applied questions on calcium compounds, since quicklime, slaked lime and limestone belong equally to the chemistry and to the industry portions of the syllabus. Writing out the electron arrangements of the first twenty elements once, and being able to reproduce them, answers a large share of this whole family.

No directly related past PYQ was found.

• practice — not a real PYQ

  In the formation of magnesium chloride from magnesium and chlorine, how many electrons are transferred and in which direction ?

  • (a)One electron from magnesium to one chlorine atom
  • (b)Two electrons from magnesium, one to each of two chlorine atoms
  • (c)Two electrons from one chlorine atom to magnesium
  • (d)Two electrons shared equally between magnesium and chlorine

  Answer(b) Two electrons from magnesium, one to each of two chlorine atoms — magnesium has the arrangement 2, 8, 2 and must lose both valence electrons to reach the neon arrangement, while each chlorine atom, 2, 8, 7, needs only one electron to complete its octet. One magnesium therefore satisfies two chlorine atoms, giving Mg2+ and two Cl- ions and the formula MgCl2. The transfer runs from the metal to the non-metal, and because the electrons are handed over rather than shared, the bonding is ionic and not covalent.

• practice — not a real PYQ

  Which one of the following properties is characteristic of ionic compounds such as calcium oxide ?

  • (a)They are generally gases at room temperature
  • (b)They conduct electricity in the solid state but not when molten
  • (c)They have high melting points and conduct electricity when molten or dissolved in water
  • (d)They are always soluble in petrol and other organic solvents

  Answer(c) They have high melting points and conduct electricity when molten or dissolved in water — the ions in an ionic solid are locked in a giant lattice by strong electrostatic forces, so a great deal of energy is needed to break it down, and the ions cannot move while they are fixed in place. Melting the solid or dissolving it in water frees the ions, and it is their movement that carries the current. Ionic compounds are therefore non-conducting as solids, generally soluble in water rather than in organic solvents, and crystalline rather than gaseous at ordinary temperatures.