Ionisation & Electronegativity

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Trends in Ionisation Energy

Trends are seen in ionisation energy across periods and down groups as the strength of the attraction between the nucleus and the electron being removed changes.

1st ionisation energy

The 1st ionisation energy is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms in their ground state. An example is shown for sodium below:
- Na_(g) → Na⁺_(g) + e^- ΔH = +496kJ mol^-1
This is an endothermic process.

Group trend

Ionisation energies decrease (become less endothermic) going down a group as the electron being removed experiences the same effective nuclear charge but is in a higher principal energy level, which is further away from the nucleus.
The effective nuclear charge stays constant as although the nuclear charge increases so does the number of shielding electrons.

Group trend

Periodic trend

Ionisation energies generally increase (become more endothermic) across a period as the effective nuclear charge increases. Since the valence electrons experience a stronger nuclear attraction, they are more difficult to remove.

Dips in ionisation energy

Dips in the trend of increasing ionisation energy across a period are observed between group 2 to group 3 and group 15 and group 16.

Group 2 to 3

The first ionisation of magnesium (group 2) corresponds to an electron being removed from a 3s orbital, whereas the first electron in aluminium (group 3) is removed from a 3p orbital.
3p is at higher energy than 3s and so it requires less energy to remove.
Therefore the 1st ionisation energy of aluminium is lower than the 1st ionisation energy of magnesium.

Group 15 to 16

The first ionisation of sulphur (group 16) corresponds to an electron being removed from a doubly occupied p orbital, whereas the first electron in phosphorus (group 15) is removed from a singly occupied p orbital.
There is repulsion between the electrons in the doubly occupied p orbital and so less energy is required to remove the first electron.
Therefore the 1st ionisation energy of sulphur is less than the 1st ionisation energy of phosphorus.

Group 15 to 16

Trends in Electron Affinity

Trends are seen in electron affinity across periods and down groups as the strength of the attraction between the nucleus and the incoming electron changes.

1^st electron affinity

The 1^st electron affinity is the energy change that occurs when one mole of electrons is gained by one mole of gaseous atoms to form one mole of gaseous anions. An example of chlorine is shown below:
- Cl_(g) + e^- → Cl^-_(g) ΔH = -349kJ mol^-1
1^st electron affinity is typically an exothermic process due to the favourable interaction between the positive nucleus and incoming electron.

Group trend

Electron affinity decreases (becomes less exothermic) on descending a group as the incoming electron is further away from the nucleus as atomic radius increases.

Periodic trend

Electron affinity increases (becomes more exothermic) on going across a period as atoms at the end of the period have incomplete energy levels and higher effective nuclear charges.

Trends in Electronegativity

Trends are seen in electronegativity across periods and down groups as the strength of the attraction of the bonding electron pair by the nucleus changes.

Electronegativity

Electronegativity is the ability of an atom to attract electrons in a covalent bond.

Group trend

Electronegativity decreases on going down a group as the bonding electrons become further away from the nucleus (atomic radius increases) and therefore the attraction of the bonding electrons is decreased.

Periodic trend

Electronegativity increases on going across a period as the effective nuclear charge increases so the bonding electron pair is more strongly attracted by the nucleus.