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JEE Main levelSingle correct optionDifficulty 2 of 3pt129

The correct increasing order of first ionisation enthalpy for $Li$ , $Be$ , $B$ and $C$ is

A $Li < B < C < Be$
B $Li < Be < B < C$
C $Li < B < Be < C$
D $B < Li < Be < C$

Answer: C

Observed values are Li 520, B 801, Be 899 and C

1086 kJ mo l^{- 1}

. Boron lies below beryllium because its outer electron is in a

2 p

orbital rather than the filled, more penetrating

2 s

orbital.

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