Ionization Energy - Alevel for OCR and Edexcel

Ionization Energy
For OCR and Edexcel exam boards When an atom becomes ionised it loses an electron and turns into a positive ion. Ionization energy is the energy required to remove this electron.

Definition of the first ionisation energy:
“the first ionization energy of an element is the energy required to remove one electron from one mole of atoms in the gaseous state.”

The symbol for ionization energy is ΔH_i
The first ionization energy ΔH_i1

An example of the first ionization energy for Calcium
Ca(g) →Ca⁺(g) + e^-                                             ΔH_i1 = +590kJ mol^-1

Definition of the second ionisation energy:
“the second ionization energy is the energy required to remove one electron from one mole of gaseous ions in the gaseous state.”

An example of the second ionization energy for Calcium
Ca⁺(g) →Ca²⁺(g) + e^-                             ΔH_i2 = +1150kJ mol^-1

A general rule
The ionisation energies increase as each electron is removed. This is because the ion becomes more positively charged thus a greater force on the remaining electrons.

The three main factors influencing ionisation energies are:
1. The size of the positive nuclear charge.
This affects all the electrons. The increase of nuclear charge with atomic number will tend to increase ionisation energies.
2. The distance of electron from nucleus.
The attraction follows the inverse square law, as the distance increases the force greatly decreases. Thus electrons in the shells far from the nucleus have dramatically lower ionization energy.
3. The shielding effect by the filled inner shells.
All electrons have the same negative charge. Like charges repel thus the electrons repel each other. Electrons in filled inner shells repel electrons in outer shells and reduce the effect of the positive nuclear charge, this is called the shielding effect.