CDS (I) 2024 General Knowledge · Question 79 of 120

Correct — D, K₂S. Potassium is the only alkali metal in the list and the most electropositive element present, and its ion is the one least able to distort the electron cloud of its partner. NCERT sets out the test in Fajans' rules — the smaller the cation and the larger the anion, the greater the covalent character of an ionic bond, and the greater the charge on the cation, the greater that covalent character. The potassium ion is large and carries only a single positive charge, so on both counts it has the weakest pull of any cation offered here, and the bond stays close to a straightforward transfer of electrons. Potassium sulphide accordingly behaves as a typical ionic salt. The other three are not ionic in the same sense at all: silicon dioxide is a giant covalent network, nitrogen trichloride is a molecule made of two non-metals whose electronegativities are almost identical, and the beryllium compound is the standard textbook case of a nominally ionic formula that is in practice heavily covalent.

• (a)BeF₃ — This is the one that needs care. Fluorine is the most electronegative element, so on the electronegativity gap alone a beryllium fluoride would score above potassium sulphide. Fajans' rules override that here: the beryllium ion is the smallest common doubly charged cation, its polarising power is exceptionally high, and beryllium halides are the recognised exception among group-2 halides for their covalent character. Note also that beryllium is divalent, so its fluoride is BeF₂; the paper's subscript is a slip.
• (b)SiO₂ — Silica is a giant covalent network in which every silicon is joined to four oxygens by shared pairs. The bonds are polar, but the solid is held together by a continuous covalent framework, not by a lattice of ions.
• (c)NCl₃ — Nitrogen and chlorine are both non-metals with almost the same electronegativity, so the shared pair is barely displaced. This is the least ionic of the four, not the most.

No bond is purely ionic or purely covalent — NCERT says as much, noting that even the bond between two hydrogen atoms has some ionic character, and that ionic bonds in turn have partial covalent character. Two tools decide where a given bond sits. The first is the difference in electronegativity between the two atoms: the larger the gap, the more the shared pair is pulled towards one atom and the more ionic the bond. The second is Fajans' rules, which look at the ions themselves — a small or highly charged cation polarises the anion's electron cloud, pulling charge back into the bonding region and making the bond more covalent, and a large, soft anion is easier to polarise.

The two tools do not always point the same way, and this question is a case where they diverge, so it is worth being honest about that rather than pretending the answer is obvious. Judged only on the electronegativity gap, a beryllium fluoride would come out ahead of potassium sulphide. What settles the matter for the examiner is the cation. Beryllium sits at the top of group 2 with an ion so small and so highly charged that its compounds are anomalously covalent — this is the standard beryllium anomaly of the s-block chapter. Potassium is at the opposite extreme, a large singly charged ion with feeble polarising power, and potassium sulphide is a plain alkali-metal salt. The practical rule to carry into the exam is simple: an alkali metal joined to a group 16 or 17 element is as ionic as school chemistry gets, and everything made of two non-metals is covalent. One more thing about the printed paper is worth flagging, purely as a matter of reading it accurately. Beryllium forms two bonds, so the fluoride is BeF₂; the formula BeF₃ as printed does not correspond to a real beryllium fluoride, and it does not change the answer.

• Fajans' rules — the smaller the cation and the larger the anion, the greater the covalent character; the greater the charge on the cation, the greater the covalent character.
• A larger electronegativity difference between the bonded atoms means greater ionic character.
• The potassium ion is large and singly charged, giving it the weakest polarising power among the cations offered.
• Beryllium's ion is the smallest common doubly charged cation, which is why beryllium halides are notably covalent.
• Silicon dioxide is a giant covalent network solid and nitrogen trichloride is a molecule of two non-metals.

• Ranking on the electronegativity gap alone and forgetting that a small, highly charged cation drags a bond back towards covalent.
• Treating silicon dioxide as ionic because it contains oxygen and a high-melting solid; it is a covalent network.
• Reading BeF₃ as a real formula — beryllium is divalent, and its fluoride is BeF₂.

Asked as a one-line comparison in which three formulae involve no metal at all, so the discrimination is really between a beryllium compound and an alkali-metal salt.

• practice — not a real PYQ

  According to Fajans' rules, covalent character in an ionic bond increases when

  • (a)the cation is large and singly charged
  • (b)the cation is small and highly charged
  • (c)the anion is small and hard
  • (d)both ions have a noble-gas configuration

  Answer(b) the cation is small and highly charged — such a cation polarises the anion strongly and pulls charge into the bonding region.

• practice — not a real PYQ

  Which one of the following is a giant covalent network solid?

  • (a)Sodium chloride
  • (b)Silicon dioxide
  • (c)Potassium sulphide
  • (d)Carbon dioxide

  Answer(b) Silicon dioxide — every silicon is covalently bonded to four oxygens in a continuous framework, unlike the ionic lattices in (a) and (c) or the simple molecules of (d).