Define Ionisation enthalpy. Discuss the factor affecting ionisation enthalpy and its trends in the periodic table?

Hint: Ionisation energy can be defined as the minimum energy required to remove the most loosely bound electron from the gaseous isolated atom. An atom can have ionisation enthalpy \[1\] , ionisation enthalpy \[2\] and so on. But the \[I{E_1}\]ionisation enthalpy \[1\] ) is always greater than \[I{E_2}\].

Complete answer:
So, as we know from above, ionisation enthalpy is the amount of energy required to remove the electron from the valence shell in the gaseous atom. So, definitely, ionisation enthalpy depends on several factors.
So, now let’s discuss about those factor one by one:
Nuclear charge- so , as the nuclear charge increases , the ionisation enthalpy increases , because nuclear force of attraction binds the electrons more tightly and thus, the more energy is required to remove the electrons from the valence shell.
Therefore, Nuclear charge \[\propto \] IE
Size- the most important factor that affects ionisation enthalpy is size, as the size increases, the ionisation enthalpy decreases.
i.e. , Size \[\propto \dfrac{1}{{{\text{IE}}}}\]
This is because, the larger the size of an atom, the farther it is from the nuclear charge or nuclear force of attraction, and thus easier to remove the electrons from the atom.
Penetration effect: as the penetration effect of the electrons increases, the ionisation energy also increases.
The penetration effect of the orbitals in decreasing order is :
S \[ > \] p \[ > \] d \[ > \] f
Thus, Penetration effect \[\propto \] IE.

Note:
Ionisation enthalpy varies in the periodic tables also and follows a definite trend except few exceptions. So as we move down in the group, the ionisation enthalpy increases as the size of the atom decreases. But when we move from left to right in the row, the ionisation enthalpy increases, because the size of the element decreases.