Ionization Energy

Ionization energy is the minimum energy required to remove an electron from an isolated neutral atom or molecule in its ground state. The first ionisation process forms a positive ion. Values may be given per particle in electron-volt eV or per mole in kJ/mol. Atomic ionisation energies are obtained from spectroscopic and collision measurements, then evaluated against the energy difference between the neutral ground state and the ionisation limit.


The Bohr model of an atom gives a useful energy-level formula for hydrogen and hydrogen-like one-electron ions. Multi-electron atoms require quantum-mechanical orbitals, electron shielding and electron-electron interactions, so the simple Bohr expression does not predict all of their ionisation energies.

In the Bohr expression, Z is nuclear charge and n is the positive-integer principal quantum number. For ground-state hydrogen, the electron binding energy is about 13.6 eV, so that is also the first ionisation energy needed to reach the continuum.

The zero-energy reference is taken for a free electron infinitely far from the ion. Removing an electron from the n = 1 state therefore requires energy equal in magnitude to that state’s negative binding energy:Discrete Bohr levels explain the hydrogen spectrum, but successive ionisation of a multi-electron atom also changes shielding and the remaining electronic configuration. The second ionisation energy removes an electron from the singly charged ion, not from the original neutral atom.

For example, the first ionization energy of sodium (Na) is represented by:
The second ionisation removes an electron from Na+ and is represented by:

For one element, IE₂ > IE₁ because the second electron is removed from a positive ion. The successive sequence is IE₁ < IE₂ < IE₃……….< IE_k, with large jumps when ionisation begins to remove electrons from a filled inner shell.

Many metals have relatively low first ionisation energies, but that atomic property does not by itself set bulk electrical conductivity. Representative values near room temperature are about 6.2 × 10⁷ S/m for silver and 5.9 × 10⁷ S/m for copper. In solid conductors, energy-band structure supplies mobile states, while impurities, lattice defects, grain boundaries and phonon scattering limit electron mobility. Conductivity also changes with temperature and material purity.

Factors Affecting Ionisation Energy

First ionisation energy generally rises from left to right across a period and falls down a group. Subshell energies and electron pairing create exceptions, so atomic radius alone does not explain every value. The main factors are:

• Size of the Atom: An outer electron that occupies a shell farther from the nucleus is usually less strongly attracted and easier to remove. Atomic radius is therefore linked to the broad downward trend within a group.
• Shielding Effect: Inner-shell electrons reduce the effective nuclear attraction felt by an outer electron. Greater shielding tends to lower ionization energy, but s, p, d and f electrons shield with different effectiveness, so the result depends on orbital configuration.
• Nuclear Charge: Across a period, proton number increases while the electron removed usually remains in the same principal shell. The resulting rise in effective nuclear charge tends to increase first ionisation energy.
• Electronic Configuration: Filled and half-filled subshells, electron pairing and the energy of the occupied orbital cause departures from a smooth periodic trend. A large jump in successive energies often shows that removal has reached an inner shell.