Atom's structure

Ionization Energy: Definition, Importance and Examples

Ionization Energy: Definition, Importance and Examples

Ionization energy is a physical property that refers to the amount of energy required to remove an electron from an atom or molecule in its gaseous state and transform it into a positive ion. This process is known as ionization, and the electron that is removed is called an ionized electron.

When enough energy is applied to an atom or molecule, one of its outermost electrons can be "released" and escape the attraction of the positively charged nucleus.

Ionization energy is usually expressed in units of energy per molecule or atom, such as electron volts (eV) or kilojoules per mole (kJ/mol).

What does the ionization energy depend on?

The ionization energy depends on several factors related to the electronic structure of the atom or molecule:

  1. Nucleus size: The higher the positive charge of the nucleus (atomic number), the greater the attraction it exerts on electrons and, therefore, the more energy required to ionize the atom.

  2. Electronic configuration: The distribution and organization of the electrons in the different energy levels and sublevels of the atom influences the ionization energy. The electrons closest to the nucleus are more strongly attracted and require more energy to be removed.

  3. Number of Electrons: In general, ionization energy increases as more electrons are removed from an atom. This is because, once the outermost electrons have been removed, the remaining electrons experience a stronger net attraction from the nucleus.

  4. Distance of the electron to be ionized: The ionization energy decreases as the electron to be ionized is further away from the nucleus. Electrons in the outermost levels experience less attraction from the nucleus and therefore require less energy to be removed.

  5. Electron-electron interactions: Repulsive interactions between electrons also play a role. Electrons tend to repel each other, making it difficult to remove an electron from an atom.

  6. Screening Effect: The inner electrons exert a screening effect on the outer electrons, which decreases the net attraction they experience. As a result, the outermost electrons are ionized with lower energy.

Because it is important?

Ionization energy is a fundamental property in chemistry and physics, and its importance lies in several key aspects:

  1. Ion formation: Ionization energy determines whether an atom can lose one or more electrons to form a positive ion (cations). Ions play an essential role in various chemical reactions and in the formation of compounds. The ability of atoms to form ions is directly related to their ionization energies.

  2. Chemical Reactivity: Elements with low ionization energies tend to be more reactive, as they can more easily lose electrons and form positive ions. On the other hand, elements with high ionization energies are less reactive, since they firmly hold their electrons.

  3. Electrical conductivity: In conductive materials, such as metals, electrons can move freely between atoms forming a "cloud" of delocalized electrons. The ionization energy determines the ease with which these electrons can be released, which is directly related to the electrical conductivity of the material.

  4. Atomic and Molecular Structure: Ionization energy is essential to understanding the electronic structure of atoms and molecules. Knowing the ionization energies of the elements allows us to predict the configuration of electrons and the chemical behavior of atoms in various situations.

  5. Spectroscopy: Spectroscopy is a technique used to study the interaction between light and matter. Electronic transitions and the energies required to excite or ionize atoms or molecules are used to identify and analyze substances in various applications, such as in chemical and astronomical analysis.

  6. Technology and electronics: Ionization energy is relevant in the design and operation of electronic devices, such as transistors and diodes, since it is related to the electronic characteristics and conductivity of the materials used in their construction.

Examples of ionization energies in some chemical elements

Below is a table with some prominent chemical elements along with their approximate ionization energy and a brief description of their relevance:


Ionization Energy (eV)




Hydrogen is the simplest and most abundant element in the universe. Its low ionization energy makes it susceptible to the formation of H+ ions and it is essential in water chemistry and redox reactions. It is also a crucial component in nuclear fusion and in the production of energy in the Sun.



Carbon is a fundamental element for life and organic chemistry. Its relatively high ionization energy allows the formation of strong covalent bonds, giving it great versatility in the formation of complex molecules and macromolecules such as amino acids and nucleic acids.



Oxygen is essential for respiration and combustion. Its ionization energy allows the formation of negatively charged oxygen ions (O-), which play an important role in forming ionic bonds with other elements to form oxides. It is also essential in atmospheric chemistry and in shielding against ultraviolet radiation in the stratosphere.



Sodium is a highly reactive metal due to its low ionization energy. It forms important ionic compounds, such as sodium chloride (common salt), and is crucial for the transmission of nerve impulses and the balance of fluids in the human body. It is also used in the industry for the production of chemical products and in the manufacture of glass and detergents.



Iron is an essential transition metal in everyday life and in biology. Its moderate ionization energy allows the formation of Fe2+ and Fe3+ ions, which are essential components in oxygen-carrying proteins such as hemoglobin and myoglobin. It is also used in the steel industry for the production of steel.



Gold is known for its beauty and economic value. Its moderate ionization energy allows it to form stable positive ions (Au+), making it useful in jewelry and high-end electronics manufacturing. It is also used in medicine and in the space industry due to its high reflectivity and low chemical reactivity.

Publication Date: July 25, 2023
Last Revision: July 25, 2023