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General ScienceEasyAsked in: CDS · CDS-II 2020, GK

The valency of phosphorus is

  1. A.2, 3
  2. B.3, 4
  3. C.4, 5
  4. D.3, 5

Correct answer

D. 3, 5

Explanation

The correct answer is D, 3, 5. Phosphorus has five electrons in its outer shell and shows two common valencies, 3 and 5.

Phosphorus, atomic number 15, has the electronic configuration 2, 8, 5. It can share or gain three electrons to complete its octet, giving a valency of 3, as in phosphine (PH3) and phosphorus trichloride (PCl3). Because its third shell has vacant d orbitals, it can also use all five outer electrons in bonding, giving a valency of 5, as in phosphorus pentachloride (PCl5) and phosphorus pentoxide (P4O10). Nitrogen, in the same group 15, cannot expand its octet and so does not form NCl5.

Option A is wrong because phosphorus does not show a valency of 2 in its common compounds. Option B is wrong because 4 is not one of its usual valencies. Option C is wrong because it leaves out 3, the valency seen in phosphine and PCl3, and includes 4.

Exam tip: phosphorus 2, 8, 5 gives valency 3 (PCl3) and 5 (PCl5); nitrogen, with no d orbitals, has no NCl5.

Practice Questions

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Q1.General ScienceAsked in: CDS · CDS-II 2026, GKMedium

Which one of the following statements is NOT correct ?

  1. A.Single celled eukaryotes belong to Kingdom Protista
  2. B.Yeast, a fungus is multicellular
  3. C.The fungi constitute a unique kingdom of heterotrophs
  4. D.Ascomycetes, a saprophytic fungi is multicellular or rarely unicellular
Show answer
Correct answer: B. Yeast, a fungus is multicellular

This statement is false, because yeast is one of the few fungi that are unicellular. In Whittaker's five-kingdom system, fungi form a separate kingdom.

Q2.General ScienceAsked in: CDS · CDS-II 2017, GKMedium

Desalination of seawater is done by using reverse osmosis. The pressure applied to the solution is

  1. A.larger than osmotic pressure
  2. B.smaller than osmotic pressure
  3. C.equal to osmotic pressure
  4. D.equal to atmospheric pressure
Show answer
Correct answer: A. larger than osmotic pressure

Only a pressure greater than the osmotic pressure of seawater can push pure water backwards through the membrane. In ordinary osmosis, water moves through a semipermeable membrane from the dilute side to the concentrated side.

Q4.General ScienceAsked in: CDS · CDS-II 2017, GKEasy

The foul smell of urine of a healthy man having healthy food, when spilled on floor, is mainly due to the bacterial decomposition of

  1. A.urea into sulphur dioxide
  2. B.sugar into carbon dioxide
  3. C.lipids into methane
  4. D.urea into ammonia
Show answer
Correct answer: D. urea into ammonia

Bacteria on the floor break down the urea in urine and release ammonia, which has a sharp, pungent smell. Urea, CO(NH2)2, is the main nitrogenous waste of humans.

Q5.General ScienceAsked in: CDS · CDS-II 2020, GKEasy

Which one of the following is used for storing biological tissues?

  1. A.Liquid nitrogen
  2. B.Liquid helium
  3. C.Liquid argon
  4. D.Liquid bromine
Show answer
Correct answer: A. Liquid nitrogen

It is very cold, chemically inert and cheap, which makes it the standard coolant for preserving cells and tissues. Liquid nitrogen boils at about −196 °C. At such low temperatures the chemical and enzyme activity inside cells almost stops, so tissues can be kept unchanged for years.