Question

The species which has $5^{th}$ ionisation potential equal to 340 eV is:

• A. $B^{+}$
• B. $C^{+}$
• C. $B$
• D. $C$

Answer 1

The correct option is C $B$

For the fifth ionisation potential, we have to remove 4 electrons such that the resulting species is hydrogen-like, with just 1 electron. Both $B$ & $C^{+}$ will be hydrogen-like species after the removal of four electrons.

Given that the I.E. = 340 eV
According to the Bohr's model of hydrogen-like atom, energy is given as:
$E=13.6\times \frac{Z^2}{n^2}$
$\Rightarrow 340=13.6\times \frac{Z^2}{1^2}$
So $Z^2=25$ and Z = 5
This value corresponds to B atom.