Nuclear Power Plant Isar, Germany

Spent nuclear fuel pool

Turbine of a nuclear plant



In physics and chemistry, an ion is an atom or molecule that does not have a neutral electrical charge. A cation with a positive electric charge is called a cation, and an ion with a negative electric charge is anion.

The process of winning or losing electrons (with respect to the neutral atom or molecule) is called ionization. Cations and anions are usually represented by the symbol of the corresponding atom and the symbol "+" or "-", respectively. If the number of electrons gained or lost is greater than one, this is also indicated.

The cations and anions are attracted to the cathode and the anode, respectively.

Michael Faraday was the first to propose the existence of ions, in 1830, but it was Arrhenius who developed the corresponding theory in 1884. This earned him the Nobel Prize in Chemistry in 1903.


The phenomenon that follows an atom loses or gains one or more electrons is called ionization. In physics, fully ionized atoms, like those of alpha particles, are commonly called charged particles. Ionization is usually done by applying high energy to the atoms, in the form of electric potential or radiation. An ionized gas is called plasma.

The negatively charged ions are known as anions (which are attracted to the anodes) and the positively charged ions are called cations (and are attracted to the cathodes). Ions are divided into monoatomics and polyatomomics.

Ionization energy

For individual atoms in a vacuum, there is a physical constant associated with the ionization process. The energy required to remove electrons from an atom is called ionization energy or ionization potential. These terms are also used to describe the ionization of molecules and solids, but the values ​​are not constant, since the ionization is influenced by local chemical bonds, geometry and temperature.

The ionization energy decreases along a group of periodic tables, and increases from left to right throughout the period. These tendencies are exactly opposite to those of the atomic ray, this because, since the purpose of an atom is to form an octet (thanks to the valence electrons), then it moves more towards the groups on the right of the periodic table (towards the "noble gases" ") we find atoms with a high value of ionization energy.

It is called the first ionization energy, the energy required to eliminate an electron, the second ionization energy that is required to eliminate two electrons, and so on. Subsequent ionization energies are always significantly larger than the previous ones. This is why ions tend to be formed in certain ways. For example, sodium is found as Na +, but usually not as Na 2+, due to the high second ionization energy required, which is much higher than the first ionization energy. Similarly, magnesium is found as Mg 2+ and not Mg 3+, and aluminum exists as a 3+ cation

Generally, the ionization potentials decrease from top to bottom, and grow from left to right in the periodic table. This trend is inverse of that found for the atomic radius. This is because, in small atoms, the electrons are more strongly attracted by the nucleus and there is more energy to pull them out.

The first ionization potential is what is needed to start the first electron of a neutral atom; the second potential is that which is needed to start two electrons, and so on. The ionization potentials are gradually increased. Generally, there is a considerable jump of energy at some point in the series. This causes each atom to tend to form a certain type of cation.

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Last review: January 24, 2019