Chemistry - Ionisation energy
The ionisation energy of an element is the energy required to remove an
electron from an atom where the process is understood to occur in the gas
The stronger the attraction of the outer-shell electrons to the nucleus,
the greater the amount of energy required to remove an electron from the
The first ionisation energy of Na is 496 kJ mol. For Al, further along
period 3, it is 578 kJ mol.
It is possible to relate this increase in ionisation energy across the
period from 11Na to 13Al to the fact that Na has a core charge of +1 and Al
has a core charge of +3.
In the case of elements in the same group, e.g. Na and K in group I, the
ionisation energy decreases going down the group - potassium has a lower
first ionisation energy than sodium. Sodium and potassium atoms both have
the same core charge, +1, i.e. they have the same effective attractive
force acting from the nucleus on the outer-shell electrons. However, since
K atoms have their outermost electron in the fourth shell, compared to
sodium atoms where the outermost electron is in the third shell, the
attractive force acting from the nucleus is less effective (over the
greater distance to the fourth shell) and less energy is required to remove
the outer-shell electron.
So the trends in ionisation energy are:
* ionisation energy increases across a period
* ionisation energy decreases going down a group
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