Ionisation Energies

Ionisation energies provide some of the most powerful evidence for the arrangement of electrons in atoms. By measuring how much energy is needed to remove electrons, we can map out the shell and subshell structure of atoms — and explain the trends we see across the periodic table.

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Definitions

The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions:

X(g) → X⁺(g) + e⁻

Key conditions:

• The element must be in the gaseous state (to remove intermolecular force effects)
• We measure per mole of atoms
• We remove the most loosely held electron

The second ionisation energy is the energy required to remove a second electron:

X⁺(g) → X²⁺(g) + e⁻

Each successive ionisation energy is higher than the last because you are removing an electron from an increasingly positive ion.

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Factors Affecting Ionisation Energy

Four factors determine how strongly an electron is held:

1. Nuclear Charge

More protons in the nucleus means a greater attractive force on the electrons. Higher nuclear charge → higher ionisation energy.