The valency of phosphorus is
- (a)2, 3
- (b)3, 4
- (c)4, 5
- (d)3, 5
Correct — D, 3, 5. Phosphorus has atomic number 15, so its electron arrangement is 2, 8, 5 and it carries five electrons in the outermost shell. Valency is the combining capacity of an atom, worked out as either the number of electrons it must gain to complete the shell or the number it can put to use in bonding. Needing three more electrons to reach eight gives a valency of three, which is what phosphorus shows in phosphine, PH₃, in phosphorus trichloride, PCl₃, and in phosphorus trioxide, P₄O₆. But phosphorus is in the third period and has vacant 3d orbitals, so it can also expand its shell and use all five outer electrons, giving a valency of five in phosphorus pentachloride, PCl₅, in phosphorus pentoxide, P₄O₁₀, and in phosphoric acid, H₃PO₄. Both values are real and both are in common compounds, so the answer is the pair 3 and 5.
- (a)2, 3 — A valency of two belongs to the oxygen group — oxygen and sulphur, which need two electrons to complete the octet. Phosphorus, with five outer electrons, needs three.
- (b)3, 4 — Half right. Three is correct, but four is carbon's characteristic valency and comes from four outer electrons; phosphorus's second valency is five, not four.
- (c)4, 5 — The reverse error. Five is correct, but four is not a valency phosphorus shows in its common compounds — there is no PH₄ or PCl₄ among the standard ones, while PH₃ and PCl₃ are everyday laboratory substances.
Valency is the combining capacity of an atom and is fixed by the electrons in its outermost shell. An atom with fewer than four outer electrons usually loses them, and its valency equals that number; an atom with more than four usually gains, and its valency equals eight minus that number. Elements of the third period and beyond have empty d orbitals available and can therefore show more than one valency by expanding their octet — which is why phosphorus shows three and five, and sulphur shows two, four and six.
Two of the four options contain the number three and two contain the number five, so the item is decided by holding both at once rather than by picking a single value. The route is: atomic number 15 gives 2, 8, 5; five outer electrons means three needed to complete the octet, giving valency three; the availability of 3d orbitals allows all five to be used, giving valency five. A useful check is to name a compound for each value, because a valency you cannot attach to a compound is a valency you have half-remembered. Phosphine and phosphorus trichloride carry the three; phosphorus pentachloride and phosphoric acid carry the five. Nitrogen, directly above phosphorus, is the instructive contrast: it has the same five outer electrons but no d orbitals in the second shell, so it cannot reach a valency of five in the same way.
- Phosphorus has atomic number 15 and the electron arrangement 2, 8, 5 — five electrons in the outermost shell.
- It shows a valency of 3 in PH₃, PCl₃ and P₄O₆, and a valency of 5 in PCl₅, P₄O₁₀ and H₃PO₄.
- The second valency is possible because third-period elements have vacant 3d orbitals and can expand the octet.
- Nitrogen, in the same group but the second period, has no available d orbitals and does not form NCl₅.
- Valency is the combining capacity fixed by the outermost shell; it is not the same as oxidation number, which carries a sign.
Attach a compound to each value. A valency you cannot name a compound for is one you have half-remembered.
- Reading valency as the number of outer electrons themselves — that would give five only, missing the three.
- Assuming nitrogen behaves like phosphorus because they share a group; nitrogen cannot expand its octet.
- Confusing valency with oxidation number, which carries a sign and can be negative.
As this direct valency item, as a formula-writing item requiring the correct valency, or as a variable-valency comparison across elements.
The valency of an element depends upon the
- (a) total number of protons in an atom
- (b) mass number of an atom
- (c) total number of neutrons in an atom
- (d) total number of electrons in the outer most shell of an atom
Answer(d) total number of electrons in the outer most shell of an atom
The principle this item applies. Once valency is understood as a property of the outermost shell, phosphorus's five outer electrons give the two combining capacities directly — three to complete the octet, or five when all of them are used.
- practice — not a real PYQ
The valency of an element is determined by
- (a)the number of protons in its nucleus
- (b)its mass number
- (c)the number of electrons in its outermost shell
- (d)the number of neutrons in its nucleus
Answer(c) the number of electrons in its outermost shell — valency is the combining capacity, and it is the outer electrons that take part in bonding.
- practice — not a real PYQ
In the compound PCl₅, the valency shown by phosphorus is
- (a)2
- (b)3
- (c)4
- (d)5
Answer(d) 5 — phosphorus is bonded to five chlorine atoms, each of valency one, so it must be using all five of its outer electrons.