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

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Ionisation Enthalpy: It is the change in the molar enthalpy when an electron is removed from a gaseous phase atom in its ground state.

Factors on which Ionisation Enthalpy Depends:
(i) Size of the atom: The larger the atomic size, the smaller the value of ionisation enthalpy. In a larger atom, the outer electrons are far away from the nucleus, so the force of attraction with which they are attracted by the nucleus is less and hence can be easily removed.
(ii) Screening effect: The higher the screening effect, the lesser the value of ionisation enthalpy, as the screening effect reduces the force of attraction towards the nucleus and hence the outer electrons can be easily removed.
(iii) Nuclear charge: As the nuclear charge increases among atoms having same number of energy shells, the ionisation enthalpy increases because the force of attraction towards nucleus increases.
(iv) Half-filled and fully-filled orbitals: The atoms with half-filled and fully-filled orbitals are comparatively more stable; hence, more energy is required to remove the electron from such atoms. The ionisation enthalpy is rather higher than the expected value in the case of such an atom.
(v) Shape of orbital: The s-orbital is more close to nucleus than the p-orbital of the same orbit. Thus, it is easier to remove electron from a p-orbital in comparison to s-orbital. In general, the ionisation enthalpy follows the following order:
(s>p> d>f) orbitals of the same orbit.
Variation of ionisation enthalpy in the periodic table
In general, the ionisation energy decreases in the group due to an increase in atomic size. Whereas, the ionisation energy increases across the period from left to right, again due to decrease in atomic size from left to right.

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