Chemistry - Ionisation energy
The successive ionisation energies of an atom support the concept of
stationary states or electron shells, as proposed by Bohr.
Ionisation energy, the energy required to remove an electron from an atom
or an ion (in the gas phase), is often very useful in explaining attributes
of atoms. It is generally a good reflection of the strength of attraction
between the outermost electrons in the atom (or ion) and the nucleus.
Also useful is the concept of core charge, which is generally considered
to be an indication of the effective positive force acting from the
The core charge is the effective attractive force acting on the electrons
in the current outer shell and may be represented as: the atomic number
less all the electrons except those in the outermost shell.
core charge of 12Mg (2,8,2) = 12 - 10 = +2
core charge of 12Mg+ (2,8) = 12 - 2 = +10
Consider the respective ionisation energies of sodium, shown here with
the associated ionisation equations:
A careful examination of these ionisation energy values (all in the units
of MJ mol-1) should enable you to deduce that:
* there is a big jump in value between the first and second ionisation
energies - a tenfold increase
* there is a gradual increase in ionisation energy from the second
through to the ninth, corresponding to the removal of a total of eight
* there is a big jump in ionisation energy between the ninth and tenth
These observations are consistent with neutral sodium atoms having an
electron arrangement of
The big jump between the first and second values occurs because the second
electron is removed from a shell (energy level) closer to the nucleus, i.e.
the L or second shell.
The gradual increase from the second to the ninth occurs because the core
charge is acting on progressively fewer electrons in the same shell.
The big jump from the ninth to the tenth ionisation energies occurs
because the tenth electron is removed from the K or first shell.
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