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1.3.6 (HL)—Ionization energy (IE)

Syllabus
First assessment 2025
Objective
1.3.6
Level
HL

First Ionization Energy

HL only

First ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms. The convergence limit in an emission spectrum corresponds to ionization.

noble-gas maxima occur at He, Ne, Ar, Kr, and Xe; post-noble-gas minima occur at Li, Na, K, and Rb; highlighted element labels and atomic-number positions match the source; major within-period discontinuities are preserved.
Trend Explanation
Across a period Generally increases as effective nuclear charge increases
Down a group Generally decreases because the outer electron occupies a higher shell
Be → B dip The electron removed from B is in a higher-energy p subshell
N → O dip Pairing in a p orbital makes one electron easier to remove

E=hfandc=λfE = hf and c = λf

Worked example — hydrogen convergence limit

The local course book gives λ=9.12×10−8 m\lambda=9.12\times10^{-8}\,\mathrm{m}. First, f=c/λ=(3.00×108 m s−1)/(9.12×10−8 m)=3.29×1015 s−1f=c/\lambda=(3.00\times10^8\,\mathrm{m\,s^{-1}})/(9.12\times10^{-8}\,\mathrm{m})=3.29\times10^{15}\,\mathrm{s^{-1}}. Then Ephoton=hf=(6.63×10−34 J s)(3.29×1015 s−1)=2.18×10−18 JE_{\text{photon}}=hf=(6.63\times10^{-34}\,\mathrm{J\,s})(3.29\times10^{15}\,\mathrm{s^{-1}})=2.18\times10^{-18}\,\mathrm{J}. Convert one-photon energy to one mole and joules to kilojoules: IE=(2.18×10−18)(6.02×1023)/1000=1.31×103 kJ mol−1IE=(2.18\times10^{-18})(6.02\times10^{23})/1000=1.31\times10^3\,\mathrm{kJ\,mol^{-1}}. This is the molar energy for the first ionization process H(g)→H+(g)+e−\mathrm{H(g)\rightarrow H^+(g)+e^-}.

At the convergence limit, convert wavelength or frequency to energy per photon with E = hf, then multiply by the Avogadro constant and convert J mol⁻¹ to kJ mol⁻¹. Across-period trends are general patterns; subshell energy and electron pairing explain the named dips.

X(g)→X+(g)+e−firstionizationenergyinkJmol−1X(g) → X⁺(g) + e⁻ first ionization energy in kJ mol⁻¹

Explaining and Calculating Ionization Energy

HL only

[ 2 ]

Determine the frequency of electromagnetic radiation, in s−1\mathrm{s}^{-1}, equivalent to the first ionization energy of phosphorus. Use sections 1, 2 and 9 of the data booklet.

Electron Configurations Summary

Retrieve the chain: emission lines reveal discrete levels; capacities, sublevels, orbitals, and spin rules build configurations; first and successive ionization energies then reveal how electrons are held and arranged.

When checking an answer, ask: Did I link a line to a transition? Did I use 2n² and the filling rules? Did I explain an ionization trend or count electrons before a successive-IE jump?