Which one of the following is the correct order of the valencies of elements Ne, Si, N and Mg?
- A.Ne < Mg < N < Si
- B.Si < N < Mg < Ne
- C.Ne < N < Si < Mg
- D.Mg < Ne < N < Si
Correct answer
Explanation
The correct answer is A, Ne < Mg < N < Si. Their valencies are neon 0, magnesium 2, nitrogen 3 and silicon 4.
Valency is the combining capacity of an atom, the number of electrons it loses, gains or shares to reach a stable octet. Neon (2, 8) already has a full outer shell, so its valency is 0. Magnesium (2, 8, 2) gives away its two outer electrons, so its valency is 2. Nitrogen (2, 5) needs three more electrons, so its valency is 3, as in ammonia, NH3. Silicon (2, 8, 4) shares its four outer electrons, so its valency is 4, as in silicon dioxide, SiO2. Arranged from lowest to highest, the order is 0, 2, 3, 4.
B is wrong because it is the exact reverse, from highest to lowest.
C is wrong because it places magnesium, valency 2, above silicon, valency 4.
D is wrong because it puts magnesium below neon, though neon's valency of 0 is the lowest.
Exam tip: valency = outer electrons if 1–4, or 8 minus outer electrons if 5–8; noble gases, 0.
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