Ionic Radii
When an atom gains an electron the radius increases.
When an atom loses an electron the radius decreases.
When an atom loses an electron the number of protons would be bigger than the number of electrons and therefore each electron would have a larger nuclear pull making the radius smaller.
When an atom gains an electron the number of protons would be smaller than that of the electrons, and therefore each electron would have a smaller nuclear pull making the radius bigger.
The change in size between the atom and the ions can be seen in the diagram above. Anions become bigger while cations become smaller.
Changes across a period
Atomic radius decreases
Going across a period the number of protons increases while the shielding remains the same. This means that each electron in the outer shell is being pulled towards the centre by more protons, making the radius smaller.
Electron affinity increases
As one progresses across a period it becomes easier for an atom to gain an electron, since the radius is smaller and more protons are available.
Ionisation energy increases
The energy required to lose an electron become bigger since the radius is smaller and the number of protons increases, making it more difficult to lose an electron.
As one can see from this graph there is a decrease in energy between Magnesium and Aluminium, and Phosphorus and Sulfur.
This is due to the fact that from Magnesium to Aluminium there is a full s orbital, resulting in a little bit of extra shielding and thus less pull from the nucleus.
When it comes to Phosphorus and Sulfur there is a change from a half-full p orbital to paired electrons in the p orbital. These paired electrons repel, making it easier to lose an electron.
Ionisation Energies
The Ionisation Energy is the energy require to remove an electron from a gaseous atom or ion at stp.
There can be multiple ionisation energies, such as:
First Ionisation Energy: Energy required to remove an electron from a gaseuos atom to form a uni-positive ion.
Second Ionisation Energy: Energy required to remove an electron from a gaseuos monopositive ion to form a di-positive ion.
Third Ionisation Energy: Energy required to remove an electron from a gaseuos dipositive ion to form a tri-positive ion.
Trends Down a Group
The Ionisation Energy of elements down the group decreases. This is due to the fact that the outer electron is further away from the nucleus and therefore the nuclear pull will be lower.
Trends Along a Period
Along a period the atom decreases in size and therefore there will be an increase in the ionisation energy as the electron will be closer to the nucleus, with a higher effective nuclear pull on each electron.
Exceptions for Ionisations Energies Along a Period
There is a decrease in energy between Group 2 and Group 3 and between Group 5 and Group 6.
- Group 2 and Group 3 difference
On going from Magnesium to Aluminium there is a full s orbital, resulting in a little bit of extra shielding and thus less pull from the nucleus.
- Group 5 and Group 6 difference
On going from Phosphorus and Sulfur there is a change from a half-full p orbital to paired electrons in the p orbital. These paired electrons repel, making it easier to lose an electron.
Changes down a group
Atomic radius increases
Doing down a group there is an increase in the number of shells, and therefore the radius would increases.
Electron affinity decreases
The bigger radius down a group would mean that the nuclear pull decreases down a group, making it more difficult to attract an electron.
Ionisation energy decreases
The bigger radius down a group would mean that the nuclear pull decreases down a group, making it easier to lose an electron from the outer shell, and thus decreasing the ionisation energy down a group.
Successive Ionisation Energies
Ionisation energy is highly dependent on the size of the atom/ion and as the ion decreases in radius, the ionisation increases.
- The first electron in sodium is lost from the 3s1. This electron is found in the shell furthest away from the nucleus and therefore it would require the least amount of energy to be removed.
- The second electron in sodium is lost from the 2p6. This electron is found in the second shell and therefore it is much closer to the nucleus. This would need a higher amount of energy to be removed.
- The 3rd till the 9th electrons are then all lost from the 2nd shell. This would mean that the shielding remain similar, same as the nuclear pull. Since the number of electrons is decreasing the effective nuclear pull on each electron will be higher, making the radius smaller and resulting in a slight increase in the ionisation energy.
- The 10th electron will be removed from the 1s2 orbital, which is found in the first shell and thus closest to the nucleus. This would then result in another big increase in the ionisation energy.
The most imporant thing to note is that when an electron is removed from a shell that is closer to the nucleus there will be a big increase in the ionisation energy.