Discuss and compare the trend in ionisation enthalpy of the elements of group-$1$ with those of group-$17$ elements.

(N/A) The ionisation enthalpy decreases regularly as we move from top to bottom in any group.
This trend is explained by the following factors:
$(i)$ Atomic size: On moving from top to bottom,the atomic size increases due to the addition of a new principal energy shell at each succeeding element. This increases the distance between the valence electrons and the nucleus,reducing the force of attraction,thus decreasing the ionisation enthalpy.
$(ii)$ Shielding effect: With the addition of new shells,the shielding effect increases,which decreases the effective nuclear charge felt by the valence electrons,leading to a decrease in ionisation enthalpy.
$(iii)$ Nuclear charge: Although the nuclear charge increases with atomic number,the combined effect of the increase in atomic size and the screening effect more than compensates for it. Consequently,valence electrons are less firmly held by the nucleus,and ionisation enthalpy decreases down the group.
Comparison: Group-$1$ elements (alkali metals) have the lowest ionisation enthalpies in their respective periods because they have the largest atomic size. Group-$17$ elements (halogens) have the highest ionisation enthalpies in their respective periods due to their small atomic size and high effective nuclear charge.

Group-$1$ elements and $1^{st}$ ionisation enthalpies $(kJ \ mol^{-1})$	Group-$17$ elements and $1^{st}$ ionisation enthalpies $(kJ \ mol^{-1})$
$H$: $1312$	$F$: $1681$
$Li$: $520$	$Cl$: $1255$
$Na$: $496$	$Br$: $1142$
$K$: $419$	$I$: $1009$
$Rb$: $403$	$At$: $917$
$Cs$: $374$	-