Trends in ionization energy

Today my students created their own explanations for the trends and discontinuities in first ionisation energies across a period and down a group.

Using only the assumptions provided and the IB Chemistry data booklet each pair of students was assigned a trend to explain to the class. Students were encouraged to use Bohr diagrams and electrons configurations to support their explanations.

The Assumptions

  • If enough energy is absorbed by an electron it can leave the electron altogether n = 1 to n = infinity
  • The first ionization energy is the energy required to completely remove the first electron from an atom in its gaseous state. E.g      K(g)   → K+(g) + e     I.E = 419 kJmol-1
  • Valence electrons are the electrons found in the outer subshell of an atom. They are also called outer shell electrons.
  • During the first ionization the electrons are removed from the outermost or valence shell.
  • As the amount of energy needed to remove a valence electron increases the ionization energy increases.
  • An energetically stable atom will have subshells that are full or half full.
  • The attraction between the protons in the nucleus of an atom and the valence electrons increases when the number of protons increases.
  • The attraction between the positive protons and negative valence shell electrons decreases as the distance between them increases.
  • The inner energy levels shield the valence shell electrons from the protons in the nucleus decreasing the force of attraction between them.


  1. Explain why the first ionization energies for the group 18 elements decrease down the group.
  2. Explain why the first ionization energies for the group 1 elements decrease down the group.
  3. Explain why the first ionization energies for the group 18 elements (He, Ne and Ar) are higher that the group 1 elements groups.
  4. Explain why the first ionization energies of the elements of the third period (Na, Mg, Al, Si, P, Cl, Ar) have a general increasing trend.
  5. Explain why the ionization energy of Al is lower than Mg.
  6. Explain why S has a lower than expected first ionization energy than P.

Trends in ionization Energy Student Work

Chemistry syllabus & cross curricular links 12.1
Prior knowledge 2.2. Evidence for the existence of electrons on energy levels provided by the emission spectrum of hydrogen was used as the basis of this new learning
Learner Profile Thinkers, Communicators
Approach to Teaching Inquiry based.
Approach to Learning Thinking skills
Nature of Science (NOS) Experimental evidence supports theories about the existence of electrons in energy levels (1.8)
Assessment Formative



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