Ionization Energy, or Ionisation Energy, and Electronic Configuration Chemistry Tutorial

Key Concepts

• Ionisation energy is also referred to as ionization energy.
• Electron configuration is also referred to as electronic configuration.
• Ionization energy, or ionisation energy, is the energy required to remove an electron from a gaseous atom or ion.
• First ionization energy is the energy required to remove one electron from the gaseous atom

  First Ionization for Hydrogen: H_(g) → H⁺_(g) + e^-

  First Ionization for Carbon: C_(g) → C⁺_(g) + e^-

• Ionization energy is measured in kilojoules per mole (kJ mol^-1) or electronvolts per atom (eV).

  1 eV = 1.602 × 10^-19 J

• Ionization energy is given a number of symbols including I and I.E.

  The first ionization energy of an atom would therefore be given the symbol I₁ or I.E₁

• The greater the value of the ionization energy, the harder it is to remove an electron from the gaseous species.

  The lower the value of the ionization energy, the easier it is to remove an electron from the gaseous species.

• Ionization energy depends on :

1. The charge on the nucleus:

   ionization energy increases with increasing numbers of protons in the nucleus, that is, with increasing positive nuclear charge.
   (The number of protons in the nucleus of an atom = the atomic number of the element (Z))

2. The distance of the electron from the nucleus:

   ionization energy decreases as the distance between the electron and the nucleus increases.

3. Repulsion between electrons :

   ionization energy decreases if 2 negatively charged electrons are in close proximity to each other.

• Ionization energy of atoms provides evidence for the electronic configuration of the atoms.

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Predicting Electronic Configuration Using First Ionization Energy

Using hydrogen as a base with a first ionization energy of 1312 kJ mol^-1, we can predict the first ionization energy for helium to be 2 × 1312 = 1624 kJ mol^-1 since helium has twice as many protons in the nucleus as hydrogen.
By comparing the predicted value for helium's first ionization energy with its measured value, we can model what the electronic configuration of an atom of helium is.

Lithium has 3 protons, so its nuclear charge is ³/₂ × nuclear charge of helium.
We predict that lithium's first ionization energy would be ³/₂ × helium's first ionization energy.
By comparing the predicted first ionization energy of lithium with its measured value we can then model the electronic configuration for lithium.

This process can be continued for each element.

Ionization Energy: Evidence for Energy Levels and Orbitals

Each of the huge decreases in first ionization indicates an electron at much greater distance from the nucleus than expected, for example, the huge decrease in first ionization for lithium and for sodium indicates the electron being removed is much, much further from the nucleus than expected.
These jumps in distances equate to the beginning of new energy levels in the electronic configuration of the atom.

Each volume of space capable of holding 2 electrons is an orbital
(see Quantum Mechanics model of the atom tutorial).

Each energy level contains only one s orbital.

Each energy level can contain a maximum of three p orbitals.
(see Quantum Numbers tutorial).

The p orbitals are designated as p_x, p_y and p_z.
In a given energy level, the energy of an s orbital is less than the energy of a p orbital.