Periodic Classification and Periodic Properties 2 Question 46
46. Explain the following :
“The first ionisation energy of carbon atom is greater than that of boron atom whereas, the reverse is true for the second ionisation energy.”
(1989, 2M)
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Solution:
- The first ionisation energy of carbon is greater than the same of boron as predicted from periodic trend. However, for 2nd
$$ \mathrm{B}^{+}=1 \mathrm{~s}^{2} \frac{11}{2 s^{2}} ; \text { more stable than } \mathrm{C}^{+}=1 s^{2} \quad 2 s^{2} \quad \begin{array}{ll|l|l|} \hline 1 & & \ \hline \end{array} $$
ionisation trend is reversed due to stability of completely filled $2 s$-orbital of $\mathrm{B}^{+}$:
