Question

First ionisation enthalpy of two elements X and Y are 500 kJ/mol and 375 kJ/mol respectively. Comment about their relative position in a group as well as in a period.

Answer 1

Atoms hold their valence electrons with different amounts of energy.

If enough energy is supplied to one outer electron to remove it from its atom, this amount of energy is called the first ionization energy or first ionization enthalpy.

If two elements with first Ionization enthalpy as 500 kJ/mol and 375 kJ/mol exist in the same group, then relatively speaking element Y will be present after element X.

This is because first ionization enthalpy decreases down the group, hence down the group element X will appear first and then element Y.

On the other hand, across a period, first ionization enthalpy increases. Thus across a period, element Y will appear before X.

To put it more easily;

Across a period:

Y   X

Down the group:

X

Y