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An electrostatic potential map of the nitrate ion (NO3). Areas coloured red are lower in energy than areas colored yellow

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. An anion (pronounced /ˈænaɪən/; an-eye-on), from the Greek word ἀνω (anο), meaning "up", is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged). Conversely, a cation (pronounced /ˈkætaɪən/; cat-eye-on), from the Greek word κατά (kata), meaning "down", is an ion with more protons than electrons.

An ion consisting of a single atom is a monatomic ion. If it consists of two or more atoms, it is a polyatomic ion. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanions. When writing the chemical formula for an ion, its charge is written as a superscript "+" or "−" following a number indicating the difference between the number of protons and the number of electrons. The number is omitted if it is equal to 1. For example, the sodium cation is written as Na+, the "+" indicating that it has one less electron than it has protons. The sulfate anion is written as SO2−4, the "2−" indicating that it has two more electrons than it has protons. If an ion contains unpaired electrons, it is called a radical ion. Just like neutral radicals, radical ions are very reactive.




Formation of monatomic ions

Monatomic ions are formed by the addition of electrons to the valence shell of the atom, or the losing of electrons from this shell. The inner shells of an atom are filled with electrons that are tightly bound to the positively-charged atomic nucleus, and so do not participate in this kind of chemical interaction. The process of gaining or losing electrons from a neutral atom or molecule is called ionization.

Atoms can be ionized by bombardment with radiation, but the more usual process of ionization encountered in chemistry is the transfer of electrons between atoms or molecules. This transfer is usually driven by the attaining of stable ("closed shell") electronic configurations. Keep in mind that it takes an atom just as much energy to gain an electron as it takes to lose one. Therefore, atoms will gain or lose electrons depending on which action takes the least energy.

For example, a sodium atom, Na, has a single electron in its valence shell, surrounding a stable, closed inner shell of 10 electrons. Since the 10-electron configuration is very stable, a sodium atom tends to lose its extra electron and attain this stable configuration, becoming a sodium cation in the process:

Na → Na+ + e

On the other hand, a chlorine atom, Cl, has 7 electrons in its valence shell, which is one short of the stable, filled shell with 8 electrons. Thus, a chlorine atom tends to gain an extra electron and attain a stable 8-electron configuration, becoming a chloride anion in the process:

Cl + eCl

This driving force is what causes sodium and chlorine to undergo a chemical reaction, where the "extra" electron is transferred from sodium to chlorine, forming sodium cations and chloride anions. Being oppositely-charged, these cations and anions form ionic bonds and combine together to form sodium chloride, NaCl, more commonly known as rock salt.

Na+ + Cl → NaCl

Formation of polyatomic and molecular ions

Polyatomic and molecular ions are often formed by the gaining or losing of elemental ions such as H+ in neutral molecules. For example, when ammonia, NH3, accepts a proton, H+, it forms the ammonium ion, NH+4. Ammonia and ammonium have the same number of electrons in essentially the same electronic configuration, but ammonium has an extra proton that gives it a net positive charge.

Ammonia can also lose an electron to gain a positive charge, forming the ion ·NH+3. However, this ion is unstable, because it has an incomplete valence shell around the nitrogen atom, making it a very reactive radical ion.

Due to the instability of radical ions, polyatomic and molecular ions are usually formed by gaining or losing elemental ions such as H+, rather than gaining or losing electrons. This allows the molecule to preserve its stable electronic configuration while acquiring an electrical charge.

Ionization potential

The energy required to detach an electron in its lowest energy state from an atom or molecule of a gas with less net electric charge is called the ionization potential, or ionization energy. The nth ionization energy of an atom is the energy required to detach its nth electron after the first n − 1 electrons have already been detached.

Each successive ionization energy is markedly greater than the last. Particularly great increases occur after any given block of atomic orbitals is exhausted of electrons. For this reason, ions tend to form in ways that leave them with full orbital blocks. For example, sodium has one valence electron in its outermost shell, so in ionized form it is commonly found with one lost electron, as Na+. On the other side of the periodic table, chlorine has seven valence electrons, so in ionized form it is commonly found with one gained electron, as Cl. Caesium has the lowest measured ionization energy of all the elements and helium has the greatest.[1] The ionization energy of metals is generally much lower than the ionization energy of nonmetals, which is why metals will generally lose electrons to form positively-charged ions while nonmetals will generally gain electrons to form negatively-charged ions.

Ionic bonding

Ionic bonding is a kind of chemical bonding that arises from the mutual attraction of oppositely charged ions. Since ions of like charge repel each other, they do not usually exist on their own. Instead, many of them may form a crystal lattice, in which ions of opposite charge are bound to each other. The resulting compound is called an ionic compound, and is said to be held together by ionic bonding. In ionic compounds there arise characteristic distances between ion neighbors from which the spatial extension and the ionic radius of individual ions may be derived.

The most common type of ionic bonding is seen in compounds of metals and nonmetals (except noble gases, which rarely form chemical compounds). Metals are characterized by having a small number of electrons in excess of a stable, closed-shell electronic configuration. As such, they have the tendency to lose these extra electrons in order to attain to the stable configuration. This property is known as electropositivity. Non-metals, on the other hand, are characterized by having an electron configuration just a few electrons short of a stable configuration. As such, they have the tendency to gain more electrons in order to attain to the stable configuration. This tendency is known as electronegativity. When a highly electropositive metal is combined with a highly electronegative nonmetal, the extra electrons from the metal atoms are transferred to the electron-deficient nonmetal atoms. This reaction produces metal cations and nonmetal anions, which are attracted to each other to form a salt.


A collection of non-aqueous gas-like ions, or even a gas containing a proportion of charged particles, is called a plasma, often called the fourth state of matter because its properties are quite different from solids, liquids, and gases. Astrophysical plasmas containing predominantly a mixture of electrons and protons, may make up as much as 99.9% of visible matter in the universe.[2]


Ions are essential to life. Sodium, potassium, calcium and other ions play an important role in the cells of living organisms, particularly in cell membranes. They have many practical, everyday applications in items such as smoke detectors, and are also finding use in unconventional technologies such as ion engines. Inorganic dissolved ions are a component of total dissolved solids, an indicator of water quality in the world.

Common ions

Common Cations
Common Name Formula Historic Name
Simple Cations
Aluminium Al3+
Calcium Ca2+
Copper(II) Cu2+ cupric
Hydrogen H+
Iron(II) Fe2+ ferrous
Iron(III) Fe3+ ferric
Magnesium Mg2+
Mercury(II) Hg2+ mercuric
Potassium K+
Silver Ag+
Sodium Na+
Polyatomic Cations
Ammonium NH+4
Hydronium H3O+
Mercury(I) Hg2+2 mercurous
Common Anions
Formal Name Formula Alt. Name
Simple Anions
Carbide C4-
Chloride Cl
Fluoride F
Hydride H-
Oxide O2−
Sulfide S2-
Carbonate CO2−3
Hydrogen carbonate HCO3 bicarbonate
Hydroxide OH
Nitrate NO3
Phosphate PO3−4
Sulfate SO2−4
Thiosulfate S2O2−3
Anions from Organic Acids
Acetate CH3COO ethanoate
Formate HCOO methanoate
Oxalate [C2O4]2− ethandioate
Cyanide CN

See also


  1. ^ Chemical elements listed by ionization energy
  2. ^ Plasma, Plasma, Everywere Science@NASA Headline news, Space Science n° 158, September 7, 1999.

Simple English

Cations are atoms that have lost electrons. Since they now have more protons than electrons, cations have a positive charge.

Cations are one of the two types of ion. The other type is called an anion.


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