Answer:
Ionisation enthalpy
It is defined as the "minimum
amount of energy required to remove the most loosely bound electron from an
isolated gaseous atom"
\[M(g)-{{M}^{+}}(g){{e}^{-}}{{I}_{1}}\]
= First ionisation enthalpy
Similarly, second and third electrons
are also removed by providing successive ionisation enthalpy.
Factors on which Ionisation
Enthalpy Depends:
(i) Size of the atom: The
larger the atomic size, smaller is the value of ionisation enthalpy. In a
larger atom, the outer electrons are far away from the nucleus and thus force
of attraction with which they are attracted by the nucleus is less and hence
can be easily removed.
Ionisation enthalpy \[\propto
\]\[\frac{1}{Atomic\text{ }size}\]
(ii) Screening effect:
Higher the screening effect, the lesser is the value of ionisation enthalpy as
the screening effect reduces the force of attraction towards nucleus and hence
the outer electrons can be easily removed.
Ionisation enthalpy \[\propto
\frac{1}{Screeing\,effect}\]
(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.
Ionisation enthalpy \[\propto \]
Nuclear charge
(iv) Half-filled and fully
filled orbitals: The atoms having 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 case of such an atom.
Ionisation enthalpy \[\propto \]
Stable electronic configuration
(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 down the group due to increase in atomic size. On the other
hand, 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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