Question

Which of the following has maximum ionisation potential?

• A. $K$
• B. $Na$
• C. $Be$
• D. $Mg$

Answer 1

Answer: (C)

$Be$

As we go down in a group, the ionisation energy decreases.
As we move along the period the ionisation energy increases.
This fact is based on effective nuclear charge.
More the effective nuclear charge, more will be ionisation energy.
Effective nuclear charge is inversely proportional to size of atom and directly proportional to nuclear charge.
So, due to smallest size Be has maximum ionisation energy.
Therefore, order of ionisation energy is as follows:
$K<Na<Mg<Be$