Which one among the following has valency of 3?
- (a)Carbon
- (b)Oxygen
- (c)Nitrogen
- (d)Sodium
Correct — C, Nitrogen. Nitrogen (atomic number 7) has the electronic configuration 2,5 — five electrons in its outer shell, three short of a stable octet. It completes the octet by sharing three electrons (three covalent bonds), as in ammonia (NH3) or the N2 molecule, so its combining capacity — its valency — is 3.
- (a)Carbon — Carbon (config 2,4) has four outer electrons and forms four bonds, so its valency is 4 (e.g. CH4, CO2) — not 3.
- (b)Oxygen — Oxygen (config 2,6) needs only two electrons to complete its octet, giving a valency of 2 (e.g. H2O) — not 3.
- (d)Sodium — Sodium (config 2,8,1) has a single outer electron which it loses to form Na+, giving a valency of 1 (e.g. NaCl) — not 3.
Valency is the combining capacity of an atom — the number of electrons it gains, loses or shares to reach the nearest noble-gas (octet) configuration. For an atom with up to four valence electrons the valency equals the number of valence electrons; for those with more than four it equals 8 minus the number of valence electrons.
Read each element's outer-shell electron count from its group. Carbon has 4 valence electrons (valency 4), nitrogen 5 (valency 3), oxygen 6 (valency 2) and sodium 1 (valency 1). Only nitrogen gives 3.
- Valency = number of electrons an atom shares, gains or loses to complete its octet.
- Nitrogen (Z=7, config 2,5) forms three bonds — valency 3 (NH3, N2).
- For non-metals near the octet, valency = 8 - (valence electrons): N gives 3, O gives 2, F gives 1.
- Carbon has valency 4, oxygen 2 and sodium 1.
Only nitrogen has a valency of 3 — hence (c).
- Confusing valency with the number of valence electrons — they differ for N, O and F, where valency = 8 - valence electrons.
- Assuming valency simply equals the group number of the element.
NDA and UPSC ask for the valency of a named element — derive it from the electronic configuration, remembering valency = 8 - valence electrons for non-metals close to the octet.
No directly related past PYQ was found.
- practice — not a real PYQ
What is the valency of oxygen?
- (a)1
- (b)2
- (c)3
- (d)4
Answer(b) 2 — oxygen (config 2,6) needs two electrons to complete its octet, as in H2O.
- practice — not a real PYQ
Which one of the following elements has a valency of 4?
- (a)Sodium
- (b)Magnesium
- (c)Carbon
- (d)Chlorine
Answer(c) Carbon — configuration 2,4, it shares four electrons to form four bonds as in CH4.