Ionisation energy levels
- In I/GCSE Chemistry, it measured in kilojoules per mole (kJ mol-1)
- 1st Ionisation energy: Energy required to remove one mole of electrons from one mole of gaseous atoms, forming one mole of uni-positive ions in the gaseous state.
X (g) → X⁺ (g) + e⁻
- 2nd Ionisation energy: Energy required to remove one mole of electrons from one mole of unipositive ions in the gaseous state.
X⁺ (g) → X²⁺ (g) + e⁻
- 3rd Ionisation energy: Energy required to remove one mole of electrons from one mole of X²⁺ ions in the gas phase.
X²⁺ (g) → X³⁺ (g) + e⁻
- Electron shielding: The repulsion between electrons in different inner shells. Shielding reduces the net attractive force from the positive nucleus on the outer-shell electrons.
Factors affecting ionisation energy
- ATOMIC RADIUS: The greater the atomic radius, the greater the nuclear attraction experienced by inner electrons
- NUCLEAR CHARGE: The greater the nuclear charge, the greater the attractive force on outer electrons
- ELECTRON SHIELDING: Inner shells of electrons repel outer shell electrons. This is called electron shielding
- The more inner shells there are, the larger the shielding effect, the smaller the nuclear attraction experienced by the outer electrons
Trend in 1st ionisation energy in GROUPS
- As you go down a group, the electron being removed occupies a higher energy and is FURTHER AWAY from the nucleus.
- The force of attraction between the negatively charged electron being removed and the positively charged nucleus DECREASES.
Trend in 1st ionisation energy in PERIODS
- In I/GCSE Chemistry, elements INCREASES as you go across a period because the electron being removed is more strongly bound to the nucleus (greater attraction) and has an increasing nuclear charge
- As the atomic radius decreases, the negatively charged electron being removed is CLOSER to the positively charged nucleus- DIFFICULT to remove electron
Trend in Successive ionisation energy
- 2nd ionisation energy is GREATER than 1st ionisation energy because as each electron is moved, there is LESS repulsion between electrons, and each shell will be drawn CLOSER to nucleus - LESS shielding from inner electons
- Also because the distance of each electrons from the nucleus DECREASES- the nuclear attraction INCREASES, and MORE ionisation energy is needed to remove the successive electron
- 3rd ionisation energy is GREATER than the 2nd ionisation energy because you are trying to remove a negatively charged electron from an ion with a 2+ charge
You got it now!