Question

Which of the following electronic configuration of an atom has the lowest ionisation enthalpy? [CBSE AIPMT 2007]

• A. $1s^2,2s^{2}2p^5$
• B. $1s^2,2s^{2}2p^3$
• C. $1s^2,2s^{2}2p^5,3s^1$
• D. $1s^2,2s^{2}2p^6$

Answer 1

The correct option is C

$1s^2,2s^{2}2p^5,3s^1$

The electronic configuration $1s^2,2s^2,2p^5,3s^1$ shows lowest ionisation energy because configuration is unstable due to the presence cf electron in s-orbital. Hence, less energy is required to remove the electron.