Define ionisation enthalpy. Discuss the factors affecting ionisation enthalpy of the elements and its trends in the periodic table.

(N/A) Ionisation enthalpy: The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to convert it into a gaseous cation is called its ionisation enthalpy. It is represented by $\Delta_{i}H$.
Factors affecting ionisation enthalpy of the elements: It depends upon the following factors:
$(i)$ Nuclear charge: As nuclear charge increases,ionisation enthalpy increases. This is because,with an increase in nuclear charge,the electrons of the outer shell are more tightly bonded by the nucleus,and thus a greater amount of energy is required to remove that electron from the atom.
For example,the ionisation enthalpy increases as we move from left to right in a period due to an increase in nuclear charge.

Element of $2^{nd}$ period	$Li$	$Be$	$B$	$C$	$N$	$O$	$F$	$Ne$
Nuclear charge	$+3$	$+4$	$+5$	$+6$	$+7$	$+8$	$+9$	$+10$
$1^{st}$ ionisation enthalpy $(kJ \ mol^{-1})$	$520$	$899$	$801$	$1086$	$1402$	$1314$	$1681$	$2080$

$(ii)$ Atomic size or radius: As the size of the atom increases,ionisation enthalpy decreases. The distance between the outer electron and the nucleus increases with an increase in atomic radius,and the attractive force on the outer electron decreases. As a result,outer electrons are held less tightly,and hence less energy is required to remove that electron. Thus,ionisation enthalpy decreases on moving from top to bottom in a group.

Element (alkali metals)	$Li$	$Na$	$K$	$Rb$	$Cs$
First ionisation enthalpies $(kJ \ mol^{-1})$	$520$	$496$	$419$	$403$	$374$

$(iii)$ Penetration effect of the electrons: As the penetration effect of the electrons increases,the ionisation enthalpy increases. In multi-electron atoms,the $s$-orbital electrons' probability is maximum near the nucleus,and this probability decreases for $p$,$d$,and $f$ orbitals.