Question

Identify the elements X and Y using the ionisation energy values given below:
Ionisation energy $\left(\text{kJ/mol}\right)$
$\begin{array}{ccc}& 1^{st}& 2^{nd}\\ X& 495& 4563\\ Y& 731& 1450\end{array}$

• A. $X=F;Y=Mg$
• B. $X=Mg;Y=F$
• C. $X=Na;Y=Mg$
• D. $X=Mg;Y=Na$

Answer 1

The correct option is C $X=Na;Y=Mg$

Ionisation energy is the minimum energy required to remove an electron from an isolated gaseous atom.

For alkali metals, the second ionisation energy is very high since the electron is being removed from a stable noble gas configuration.
Whereas for alkali earth metals, the energy difference between the first and second ionisation energy is not that huge.

For halogens such as fluorine, the first ionisation energy is very high due to the small size and large effective nuclear charge.