Question

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> The correct order of decreasing first ionisation energy is:

AIIMS 2019

• A. $C>Be>B>Li$
• B. $C>B>Be>Li$
• C. $Be>Li>B>C$
• D. $B>C>Be>Li$

Answer 1

The correct option is A $C>Be>B>Li$

In general, as we move from left to right across the period, the ionisation enthalpy increases as nuclear charge increases.
But removing an electron from outer shell of $B$ will make them to attain a fully filled stable electronic configuration $(B^{+}-1s^{2}2s^2)$ so that the first ionisation energy of $B$ is lower than $Be$. Thus, the correct order of decreasing first ionisation energy is,
$C>Be>B>Li$
Hence, option (b) is the correct answer.