Ionisation energies

• first ionisation energy is the energy required to remove an electron from each atom in one mole of atoms in the gaseous state
• second ionisation energy is the energy required to remove an electron from each singly charged positive ion in one mole of positive ions in the gaseous state
• ionisation energy = energy of electron when removed - energy of electron when in the orbital
• a higher subshell / orbital has a higher energy so needs less energy to be removed (ionisation energy)
• an increased atomic radius decreases the first ionisation energy
• an increased nuclear charge / pull (proton number) increases the first ionisation energy
• increased electron shielding (more electrons in same subshell / orbital or between outer electron and nucleus) decreases the first ionisation energy
• electron-electron repulsion means that each electron shields the other from the effect of the nuclear charge which increases the energy of the electrons, decreasing the ionisation energy
• across a period - nuclear pull is more significant than electron-electron repulsion so the first ionisation energies increase across the period (general trend so not all elements apply to this)
• down a group - extra shells (atomic radius) and increased electron-electron repulsion is more significant than increased nuclear charge, decreasing the first ionisation energy down the group (1, 2, 4 (Pb is an anomaly), 5, 6, 7, 8)(group 3 has no general trend)