Ionization Potential

What is Ionization Potential?

Ionization potential, also known as ionization enthalpy or ionization energy, is the amount of energy that must be supplied to a gaseous atom/molecule to remove a valence electron from it. It is often denoted by the symbol ‘Ei’ and expressed in terms of kilojoules per mole (kJ/mol).

Atoms consist of a positively charged nucleus which is surrounded by electrons that are placed in definite orbitals. Since the electrons are negatively charged, they are attracted to the nucleus via electrostatic forces. In order to remove an electron from an atom, an input of energy is required to overcome the electrostatic force acting on the electron. This energy input is called the ionization potential of the atom in question.

Ionization Potential

The amount of energy required to remove an electron from a gaseous atom is generally expressed via the following equation.

X + Ei → X+ + e

Where:

  • X is the neutral gaseous atom.
  • Ei is the ionization potential or ionization enthalpy
  • e is an electron.
  • X+ is the cation formed with the atom ‘X’ loses one electron.

It is important to note that the ionization process in which an atom loses an electron is almost always endothermic (since it involves the absorption of energy for the liberation of an electron). The ionization potential of an atom is inversely proportional to the distance between the nucleus of the atom and its valence shell, i.e. the greater the distance between the nucleus and the outermost electrons, the lower the ionization potential.

Factors that Affect the Ionization Potential of an Atom

Magnitude of Nuclear Charge

All the positively charged protons in an atom are concentrated in the nucleus. Therefore, elements with higher atomic numbers have a greater number of protons in their respective nuclei, resulting in a greater nuclear charge.

The greater the magnitude of the positive charge held by the nucleus, the stronger the force of attraction between the nucleus and the electrons and the greater the ionization potential of the atom.

Atomic Radii

The force of attraction between the nucleus and the valence electrons depends on two factors:

  1. Effective nuclear charge
  2. Distance between the nucleus and the valence electron

As the distance between the nucleus as the valence electrons increases (due to the addition of new electron shells), the force of attraction between the nucleus and the outermost shell decreases. Therefore, it is relatively easier to isolate an electron from an atom with a large atomic radius than an atom with a small radius.

Shielding Offered by Inner-Shell Electrons

The electrons in the valence shell of an atom are subject to repulsive forces from the inner shell electrons (electron-electron repulsions). In a way, the valence shell is shielded from the nuclear attractive force by the inner shells.

Therefore, the net attractive force between the valence electron and the nucleus depends on the magnitude of nuclear charge and the extent of inner-shell shielding. The greater the extent of shielding, the lower the ionization potential of the atom.

The effective nuclear charge acting on an electron, often denoted by Zeff, takes into account the extent of shielding and the magnitude of nuclear charge to denote the net positive charge acting on the electron in question. Needless to say, the lower the value of Zeff, the lower the ionization potential of the atom.

Occupancy of the Atomic Orbital

Atoms with highly stable electron configurations have a lower tendency to lose electrons. For example, half-filled or completely filled atomic orbitals add stability to the atom. The added stability makes it difficult to remove electrons from the atom.

Therefore, the occupancy of the atomic orbitals and the stability of the electron configuration affect the ionization enthalpy of the atom.

nth Ionization Potential

The removal of multiple electrons from an atom is a step-by-step process in which a single electron is removed in each step. The energy required to remove an electron from a cation holding a positive charge of magnitude +1 is called the second ionization potential and can be expressed via the following reaction.

X+ + IE2 → X2+ + e

Where IE2 is the second ionization potential of the atom in question. Similar equations can be written for the third and fourth ionization energies of the atom.

X2+ + IE3 →  X3+ + e

X3+ + IE4 → X4+ + e

The nth ionization potential of an atom is almost always greater in magnitude than the (n-1)th potential. This is because the effective nuclear charge acting on the electrons increases as the magnitude of the positive charge held by the ion increases. The nth ionization energy of an atom can be defined as the amount of energy required to remove an electron from the atom when it holds a charge of (n-1).

Periodic Trends in Ionization Potential

Based on the factors that affect it, periodic trends in the ionization potentials of elements can be observed in the modern periodic table. These trends have been listed below.

  • The atomic radii of the elements decrease when traversing from left to right on the periodic table, due to an increase in effective nuclear charge. This is accompanied by an increase in ionization energy.
  • The exceptions to this trend include the alkaline earth metals and the pnictogen group (group 15). Since both these groups hold half-filled orbitals, their relatively greater ionization potentials are explained by their more stable electron configurations.
  • All noble gases have high ionization energies. This is because their electron configurations are very stable.
  • Ionization potentials decrease while traversing down a group. This can be explained by the increase in atomic radii brought on by the addition of new electron shells and the greater shielding offered in elements with a greater number of shells.

The periodic trends in the ionization potentials of the elements are graphically illustrated below.

Periodic Trends in Ionization Potential

From these trends, it can be understood that helium and neon have the highest first ionization potentials whereas cesium and francium have the lowest.

Ionization Energies of the First 20 Elements (Tabular Data)

The first ionization energies (in kilojoules per mole) of the first 20 elements are tabulated below.

Element and Symbol First Ionization Energy (kJ/mol)
Hydrogen (H) 1312.0
Helium (He) 2372.3
Lithium (Li) 520.2
Beryllium (Be) 899.5
Boron (B) 800.6
Carbon (C) 1086.5
Nitrogen (N) 1402.3
Oxygen (O) 1313.9
Fluorine (F) 1681.0
Neon (Ne) 2080.7
Sodium (Na) 495.8
Magnesium (Mg) 737.7
Aluminum (Al) 577.5
Silicon (Si) 786.5
Phosphorus (P) 1011.8
Sulfur (S) 999.6
Chlorine (Cl) 1251.2
Argon (Ar) 1520.6
Potassium (K) 418.8
Calcium (Ca) 589.8

Thus, the concept of ionization potential, the factors that affect it and its periodic trends are briefly discussed in this article. To learn more about ionization energy and other related concepts, such as electron gain enthalpy, register with BYJU’S and download the mobile application on your smartphone.


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