Which of the following has the highest first ionisation enthalpy?

A $Rb$
B $Li$
C $K$
D $Na$

Answer: B

Within group 1, size increases down the group, so the outer electron is held less tightly and the first ionisation enthalpy decreases. The smallest atom,

Li

(

520 kJ mo l^{- 1}

), has the highest value.

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More questions on Ionisation enthalpy

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