Atomic Structure

Ionisation Energies

Ionisation Energies

First ionisation energy

The energy that is required to remove one electron from each of a mole of a free gaseous atom of an element is the first energy of that element.

The electronic configuration and the number of protons in the nucleus of an atoms dictates the amount of energy needed to remove an electron from that atom.

There are a number of factors regarding first ionisation energy:

  • Energy is needed to remove electrons from atoms so that their attraction to the nucleus can be overcome.
  • Nuclear charge: the more protons in the nucleus the harder it is to remove the electrons. The name given to the number of protons is the nuclear charge.
  • Shielding: other electrons in the atom cancel out some of the effect of the nuclear charge. Basically, one electron in the inner shell and inner sub-shell cancels out one unit charge of the nucleus. This is called shielding.
  • Effective nuclear charge: electrons located in the shells furthest from the nucleus feel only a residual positive charge from this cancelling out. This residual charge is known as the effective nuclear charge.
  • Electron repulsion: the ease with which electrons can be removed is affected by the repulsion of the outermost electrons.

In general, the first ionisation energy becomes greater over a period of time due to the fact that the nuclear charge also increases but the shielding does not change.

From group II to group III of the periodic table there is a decrease in ionisation energies. This is because the p-orbital electrons are removed in group III which means the outer shell s-electrons are shielded. Therefore there is a decrease in the effective nuclear charge.

These concepts can be used to explain the trend in first ionisation energies for the first seven elements of the periodic table. However, they do not show why there is a drop between group V and group VI. The reason behind this is that the electron which is removed in a group VI atom is paired. Therefore there is a greater repulsion between electrons making the electron easier to remove.

If you descend a group the effective nuclear charge does not change but there is an increase in the inner shell number. Instability is caused by repulsion between the inner shells and the outer electrons. Subsequently, the electrons are pushed further away from the nucleus thereby making it easier for them to be removed.


Successive ionisation energies

The second ionisation energy of an atom is the energy that is needed for the removal of one electron from each of a mole of free gaseous unipositive ions.

Subsequent ionisation energies can be defined in the same way.

The formula for the nth ionisation energy is:

M(n-1)+(g) ? Mn+(g) + e

Successive electrons become progressively harder to be removed from an atom. There are two explanations for this:

  • As the number of electrons removed from an atom decreases so the remaining number left in the atom decrease. Therefore, the repulsion between the remaining electrons also decreases. The number of protons, however, is unchanged. This means that the electrons left are more stable and so harder to remove.
  • However, the difference in successive ionisation energies depends on the atom’s electronic configuration. This difference can be qualitatively predicted by looking at the effective nuclear charge of the electron that is to be removed as well as its shielding by the inner shell and inner sub-shell electrons. The largest difference occurs when an electron is removed from an inner shell. This is because there is a significant drop in shielding, a big increase in effective nuclear charge, and a big rise in ionisation energy.

By reversing these principles you can predict the electronic structure of a species through analysis of successive ionisation energy data.