Ion
From Wikipedia, the free encyclopedia
An ion is an atom or molecule where 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 ἀνά (ana), 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 SO42−, 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.
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[edit] Formation
[edit] 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. 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, sodium "likes" to lose its extra electron so that it can attain to this stable configuration, becoming the 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, chlorine "likes" to gain an extra electron in order to attain to the stable 8-electron configuration, becoming the chloride anion in the process:
- Cl + e− → Cl−
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 combine together to form sodium chloride, NaCl, more commonly known as salt.
[edit] 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.
[edit] 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.
[edit] 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, they are bound to ions of the opposite charge. The resulting compound is called an ionic compound, and is said to be held together by ionic bonding.
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.
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