NDA (II) 2016 General Ability Test · Question 94 of 150

Correct — B, the code 2 – 1 – 4 – 3. Sulfur sits in group 16 and can put all six of its outer electrons to work, as it does in sulphur hexafluoride and in sulphuric acid, so its highest valency is six and A pairs with 2. Phosphorus is in group 15 with five outer electrons and reaches five in phosphorus pentachloride and phosphorus pentoxide, so B pairs with 1. Lead is in group 14 and reaches four, as in lead dioxide and lead tetrachloride, so C pairs with 4. Silver is left with two, the maximum it attains in unusual compounds such as silver difluoride, so D pairs with 3. Reading the four together gives 2 – 1 – 4 – 3.

• (a)2 4 1 3 — Gets sulfur and silver right but exchanges phosphorus and lead. It gives phosphorus a highest valency of four and lead a highest valency of five, when the group positions require the opposite — five for phosphorus in group 15 and four for lead in group 14.
• (c)3 1 4 2 — Correct in the middle but swaps the two ends. It assigns two to sulfur and six to silver, which is exactly backwards; sulfur is the element that reaches six, and no ordinary silver compound goes anywhere near it.
• (d)3 4 1 2 — Every pair is wrong. It shifts each element one place along the list, giving sulfur two, phosphorus four, lead five and silver six, so a single check on sulfur alone is enough to reject it.

Valency is the combining capacity of an element, and the maximum valency an element can show is governed by how many electrons it has available in its outermost shell. For the main-group elements this follows the group number directly: group 14 elements such as carbon, silicon, tin and lead reach four; group 15 elements such as nitrogen and phosphorus reach five; group 16 elements such as oxygen and sulfur reach six. Transition and post-transition metals are less tidy, and silver, which almost always behaves as a one-valent metal in familiar compounds such as silver nitrate and silver chloride, is credited here with a maximum of two.

A matching item never has to be solved four times over. Fix the two pairs you are surest about and use the code to eliminate. Sulfur equals six is the safest anchor because sulphur hexafluoride and sulphuric acid are both familiar, and that single decision removes options (c) and (d) at once. Phosphorus equals five, remembered from phosphorus pentachloride, then separates (a) from (b), and the answer falls out without any thought about lead or silver. Two points of honesty about the item as printed. The booklet writes 'Phosphorous' for the element, which is the adjective form — the element is phosphorus, and this very paper spells it correctly in the safety-match question later on, so the slip is the compositor's rather than the chemistry's. The same paper also writes 'Sulfur' here and 'Sulphur' in an earlier question; both spellings are legitimate, the first being the international convention and the second the older British form, but printing both in one booklet is an inconsistency worth noticing so it does not unsettle you in the hall. Finally, silver's entry deserves a word: chemists know silver(II) compounds such as silver difluoride, and even rarer silver(III) fluorides exist, but at school level the maximum taught for silver is two, which is what the key uses.

• Maximum valency for main-group elements follows the group: four for group 14, five for group 15, six for group 16.
• Sulfur reaches six in sulphur hexafluoride and in sulphuric acid, where it is in the plus six state.
• Phosphorus reaches five in phosphorus pentachloride and phosphorus pentoxide, and lead reaches four in lead dioxide and lead tetrachloride.
• Silver behaves as one-valent in its common compounds such as silver nitrate and silver chloride, and reaches two only in unusual compounds such as silver difluoride.

• Reading a matching item element by element instead of anchoring on the one or two pairs you are certain of and eliminating.
• Confusing the common valency of an element with its highest valency; silver is one-valent in almost every compound you meet but is credited with two here.

NDA sets one or two matching items per paper, and they are always solvable from two confident pairs plus elimination, so never spend time deciding all four.

• practice — not a real PYQ

  The highest valency shown by sulfur is

  • (a)two
  • (b)four
  • (c)six
  • (d)eight

  Answer(c) six — sulfur is in group 16 and reaches six in compounds such as sulphur hexafluoride and sulphuric acid.

• practice — not a real PYQ

  In which one of the following compounds does phosphorus show its highest valency?

  • (a)PH₃
  • (b)PCl₃
  • (c)PCl₅
  • (d)P₄

  Answer(c) PCl₅ — phosphorus is bonded to five chlorine atoms, using all five of its outer electrons.