Identify the element having zero valency
- (a)Sulphur
- (b)Phosphorous
- (c)Lead
- (d)Radon
Correct — D, Radon. Valency is the combining capacity of an atom, and it is set by how far the outermost shell is from being full. Radon sits in Group 18, the noble gases, with atomic number 86 and a complete outer shell of eight electrons. It has nothing to gain, nothing to lose and nothing to share, so its combining capacity is zero — that is what zero valency means, and it is why the whole of Group 18 is written with valency zero in the periodic table. The other three elements all have incomplete outer shells and therefore real, non-zero valencies: sulphur usually two, phosphorus three, lead two. One honest footnote for a curious student: since 1962 chemists have forced compounds out of the heavier members of the group, and even radon fluoride is known, so 'inert' has given way to 'noble'. The conventional valency assigned to Group 18 in school chemistry, and the one this question is testing, remains zero.
- (a)Sulphur — Sulphur has six electrons in its outer shell and needs two more, so its common valency is two, as in hydrogen sulphide. It also shows four and six in compounds such as sulphur dioxide and sulphur trioxide, but never zero.
- (b)Phosphorous — Phosphorus has five outer electrons and a common valency of three, as in phosphine, rising to five in compounds such as phosphorus pentachloride. Note that the booklet prints the element's name as 'Phosphorous', which is properly the name of an acid and its salts; the element is phosphorus. The slip does not affect which option is right.
- (c)Lead — Lead is a metal of Group 14 and shows valencies of two and four, the divalent state being the more stable. A metal that forms oxides, sulphides and salts cannot have zero combining capacity.
An atom is chemically stable when its outermost shell is complete, and valency measures how many electrons it must gain, lose or share to reach that state. Group 18 elements — helium, neon, argon, krypton, xenon and radon — already have complete outer shells, so their valency is written as zero and they were long regarded as chemically inert. Radon is the heaviest of them, is radioactive, and forms in the decay chain that runs from uranium through radium.
Read the question as asking which element is already satisfied. Three of the four options are ordinary reactive elements whose valencies a candidate can quote, so the odd one out declares itself as soon as radon is recognised as a noble gas rather than a metal or a solid non-metal. Radon also carries a real-world hook worth remembering: because it is a radioactive gas seeping out of certain rocks and soils, it can build up in poorly ventilated basements and mines and is a recognised health hazard.
- Radon is a Group 18 noble gas with atomic number 86 and a complete outer shell, so its valency is zero.
- Sulphur commonly shows a valency of two, phosphorus three, and lead two or four.
- Group 18 was called the inert gases until compounds of xenon were prepared in 1962; the name 'noble gases' replaced it.
- Radon is radioactive and arises in the uranium decay series, which is why it accumulates in poorly ventilated basements and mines.
Only a full outer shell gives a combining capacity of zero, which points to radon.
- Reading zero valency as meaning the element does not exist as a substance, when it simply means a full outer shell.
- Assuming a noble gas can never react at all — the heavier members do form compounds under forcing conditions.
Valency turns up in NDA GAT both directly, as here, and through matching lists of elements against their highest valency, so learn the outer-shell count for the first twenty elements and for the common metals.
Which one of the following is also called Stranger Gas?
- (a) Argon
- (b) Neon
- (c) Xenon
- (d) Nitrous oxide
Answer(c) Xenon
Another member of the same zero-valency group, and the one whose compounds in 1962 ended the belief that Group 18 could not react at all.
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
States the rule that this question applies — once valency is tied to the outermost shell, a complete shell must mean zero.
- practice — not a real PYQ
The valency of an element is decided by the number of electrons in its
- (a)nucleus
- (b)innermost shell
- (c)outermost shell
- (d)second shell
Answer(c) outermost shell — valency measures how far that shell is from being complete.
- practice — not a real PYQ
Which one of the following gases is radioactive and is a member of Group 18?
- (a)Argon
- (b)Krypton
- (c)Radon
- (d)Neon
Answer(c) Radon — it arises in the uranium decay series and can collect in unventilated basements.