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For how many of the following pairs of consecutive elements does the second element have the lower first ionisation enthalpy: (Be, B), (N, O), (Mg…

Single correct option

For how many of the following pairs of consecutive elements does the second element have the lower first ionisation enthalpy: (Be, B), (N, O), (Mg, Al), (P, S), (C, N), (Zn, Ga)?

A. 5
B. 3
C. 4
D. 6

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Answer: (A) 5

\to

B: the

2p

electron of B is higher in energy and is shielded by

2s

, so

\mathrm{IE_1}

drops. N

\to

O: half-filled stability of N and pair repulsion in O, so it drops. Mg

\to

Al: same

3p

versus

3s

argument, drops. P

\to

S: half-filled

3p^3

again, drops. C

\to

N: increases normally. Zn

\to

Ga: Ga's

4p

electron is easier to remove than Zn's filled

4s^2

(

906

579\ \mathrm{kJ\,mol^{-1}}

), drops. Five pairs.

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