What Is Ionization Energy? Definition and Trend

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Ionization energy is the energy required to remove an electron from an atom.
Ionization energy is the energy required to remove an electron from an atom. It is a periodic table trend that increases moving across the table and decreases moving down it.

By definition, ionization energy is the minimum energy needed to remove the most loosely bound electron from a gaseous atom or ion. The term is also spelled ionisation energy (British English). Ionization energy is denoted by the symbols IE, IP, ΔH° and has units of kilojoule per mole ((kJ/mol) or electron volts (eV).

Ionization energy is the energy required to remove an electron from an atom or ion.

Ionization energy increases moving across a period and decreases moving down a group. There are exceptions to this periodic table trend.

Francium (an alkali metal) has the lowest ionization energy, while helium (a noble gas) has the highest ionization energy.

The first ionization energy is the lowest. Removing each subsequent electron requires more energy.

Importance of Ionization Energy

Ionization energy reflects how difficult it is to remove an electron from an atom, so it is a useful predictor of reactivity and the strength of chemical bonds that atom forms. The higher the ionization energy, the harder it is to remove an electron. So, atoms with low ionization energies (such as alkali metals) tend to be highly reactive and readily form chemical bonds. Atoms with high ionization energies (such as the noble gases) display low reactivity and are less likely to form chemical bonds and compounds.

Ionization Energy Trend on the Periodic Table

In chemistry, periodicity refers to repeating trends in the elements of the periodic table, such as ionization energy, atomic radius, electron affinity, and electronegativity.
In chemistry, periodicity refers to repeating trends in the elements of the periodic table, such as ionization energy, atomic radius, electron affinity, and electronegativity.

The element with the highest ionization energy is helium, which is located in the upper right side of the periodic table and is one of the noble gases. Francium, an alkali metal located on the bottom left of the table, has one of the lowest ionization energies. Ionization energy displays a trend on the periodic table.

  • Ionization energy generally increases moving from left to right across an element period (row). The reason is that the atomic radius tends to decrease moving across a period. This happens because more protons are added, increasing the attraction between the nucleus and electrons and drawing the electron shells in closer.
  • Ionization energy generally decreases moving from top to bottom down an element group (column). The reason is that principal quantum number of the outermost (valence) electron increases moving down. Atoms have more protons moving down a group, which does pull in the electron shells. But, each row adds a new shell, so the outermost electrons are still further from the nucleus.

Exceptions to the Trend

There are some exceptions to the ionization energy trend. For example, the first ionization energy of boron is lower than the first ionization energy of beryllium. The ionization energy of oxygen is lower than that of nitrogen. Exceptions occur because of Hund’s rule and the electron configurations of the atoms. Basically, a full sublevel is more stable than one that is half-filled, so neutral atoms naturally move to this configuration. Also, it matter whether or not a sublevel has two electrons with opposing spin values.

For beryllium, the first ionization potential electron comes from the 2s orbital, although ionization of boron involves a 2p electron. For both nitrogen and oxygen, the electron comes from the 2p orbital, but the spin is the same for all 2p nitrogen electrons, while there is a set of paired electrons in one of the 2p oxygen orbitals.

Ionization Energies of Atoms
Ionization energies increase within a period moving from the alkali metals to the noble gases. The maximum values are reached as the s, p, d, and f orbitals fill. (image: Ponor, CC BY-SA 4.0)

First, Second, and Third Ionization Energies

The first ionization energy is the energy required to remove the outer valence electron, so it is the lowest value. Generally, the second ionization energy is highest than the first, while the third is higher than the second. Removing subsequent electrons is harder than removing the first one because these electrons are more tightly bound to the nucleus and may be closer to it.

For example, consider the first (I1) and second (I2) ionization energies of magnesiu:

Mg (g) → Mg (g) + e− I1 = 738 kJ/mol
Mg+ (g) → Mg2+ (g) + e− I2 = 1451 kJ/mol

Electron Affinity Trend

Electron affinity is a measure of how readily a neutral atom can gain an electron to form a negative ion. Electron affinity and ionization energy follow the same trend on the periodic table. Electron affinity increases moving across a period and decreases moving down a group.

References

  • Cotton, F. Albert; Wilkinson, Geoffrey (1988). Advanced Inorganic Chemistry (5th ed.). John Wiley. ISBN 0-471-84997-9.
  • Lang, Peter F.; Smith, Barry C. (2003). “Ionization Energies of Atoms and Atomic Ions”. J. Chem. Educ. 80 (8). doi:10.1021/ed080p938
  • Miessler, Gary L.; Tarr, Donald A. (1999). Inorganic Chemistry (2nd ed.). Prentice Hall. ISBN 0-13-841