Question

The first ionisation energies of six $3^{rd}$ period elements are 496, 578, 738, 789, 1000 and 1012 ${\text{kJ mol}}^{-1}$. The elements to which these energies correspond respectively are:

• A. $Na,Mg,Al,Si,P$ and $S$
• B. $Na,Al,Mg,Si,S$ and $P$
• C. $Mg,Na,Si,Al,P$ and $S$
• D. $Na,Si,Mg,S,Al$ and $P$

Answer 1

The correct option is B $Na,Al,Mg,Si,S$ and $P$

Ionisation energy generally increases across a period (left to right) due to the increase in $Z_{eff}$. But some elements do not follow the trend.

$Al$ has a lower value of ionisation energy (IE) than magnesium $\left(Mg\right)$ because, $3s$ electrons of $Mg$ are more penetrated as compared to $3p$ electrons of $Al$ and also $Mg$ has completely filled electronic configuration.

Sulphur$\left(S\right)$ has a lower ionisation energy value than phosphorous$\left(P\right)$ due to the relative stability of the half filled ${}^{\prime }{p}^{\prime }$ orbitals in phosphorous. Also, the electron-electron repulsion is more in sulphur than in phosphorous.