Ionic bond

Ionic bond

Overview
An ionic bond is a type of chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

 formed through an electrostatic attraction between two oppositely charged ion
Ion
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. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal
Nonmetal
Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal...

. Pure ionic bonding cannot exist: all ionic compounds have some degree of covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

ing. Thus, an ionic bond is considered a bond where the ionic character is greater than the covalent character.
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Encyclopedia
An ionic bond is a type of chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

 formed through an electrostatic attraction between two oppositely charged ion
Ion
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. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal
Nonmetal
Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal...

. Pure ionic bonding cannot exist: all ionic compounds have some degree of covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

ing. Thus, an ionic bond is considered a bond where the ionic character is greater than the covalent character. The larger the difference in electronegativity
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...

 between the two atoms involved in the bond, the more ionic (polar) the bond is. Bonds with partially ionic and partially covalent character are called polar covalent bonds. Ionic bonding is a form of noncovalent bonding
Noncovalent bonding
A noncovalent bond is a type of chemical bond, typically between macromolecules, that does not involve the sharing of pairs of electrons, but rather involves more dispersed variations of electromagnetic interactions. The noncovalent bond is the dominant type of bond between supermolecules in...

.

Ionic compounds conduct electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 when molten or in solution, but not as a solid. They generally have a high melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

 and tend to be soluble in water.

Formation


The formation of an ionic bond proceeds when the cation, whose ionization energy
Ionization energy
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...

 is low, releases some of its electrons to achieve a stable electron configuration. The anion, whose electron affinity
Electron affinity
The Electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion....

 is positive, then accepts the electrons, again to attain a stable electron configuration. Typically, the stable electron configuration is one of the noble gases for elements in the s-block
S-block
The s-block of the periodic table of elements consists of the first two groups: the alkali metals and alkaline earth metals, plus hydrogen and helium.Except in hydrogen and helium, these electrons are very easily lost to form positive ions...

 and the p-block
P-block
The p-block of the periodic table of the elements consists of the last six groups minus helium . In the elemental form of the p-block elements, the highest energy electron occupies a p-orbital.-See also:...

, and particular stable electron configurations
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...

 for d-block
D-block
The d-block is the portion of the periodic table that contains the element groups 3-12. These groups correspond to the filling of the atomic d-orbital subshell, with electron configurations ranging from s2d1 to s2d10...

 and f-block
F-block
The f-block of the periodic table of the elements consists of those elements whose atoms or ions have valence electrons in f-orbitals. Actual electronic configurations may be slightly different from what is predicted by the Aufbau principle...

 elements. The electrostatic attraction between these two entities forms the ionic bond.

For example, common table salt is sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

. When sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 (Na) and chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 (Cl) are combined, the sodium atoms each lose an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

, forming cations (Na+), and the chlorine atoms each gain an electron to form anions (Cl). These ions are then attracted to each other in a 1:1 ratio to form sodium chloride (NaCl).
Na + Cl → Na+ + Cl → NaCl


The removal of electrons from the cation is endothermic, raising the system's overall energy. There may also be energy changes associated with breaking of existing bonds or the addition of more than one electron to form anions. However, the action of the anion accepting the cation's valence electrons and the subsequent attraction of the ions to each other releases energy and thus lowers the overall energy of the system.

Ionic bonding will occur only if the overall energy change for the reaction is favourable – when the reaction is exothermic. The larger the resulting energy change, the stronger the bond. The low electronegativity
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...

 of metals and high electronegativity of non-metals means that the reaction is most favourable between a metal and a non-metal.

Structure


Ionic compound
Ionic compound
In chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. Ions in ionic compounds are held together...

s in the solid state form lattice structures. The two principal factors in determining the form of the lattice are the relative charges of the ions and their relative sizes. Some structures are adopted by a number of compounds; for example, the structure of the rock salt sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

 is also adopted by many alkali halides, and binary oxides such as MgO
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...

.

Bond strength


For a solid crystalline ionic compound the enthalpy change in forming the solid from gaseous ions is termed the lattice energy
Lattice energy
The lattice energy of an ionic solid is a measure of the strength of bonds in that ionic compound. It is usually defined as the enthalpy of formation of the ionic compound from gaseous ions and as such is invariably exothermic. Lattice energy may also be defined as the energy required to completely...

.
The experimental value for the lattice energy
Lattice energy
The lattice energy of an ionic solid is a measure of the strength of bonds in that ionic compound. It is usually defined as the enthalpy of formation of the ionic compound from gaseous ions and as such is invariably exothermic. Lattice energy may also be defined as the energy required to completely...

 can be determined using the Born-Haber cycle
Born-Haber cycle
The Born–Haber cycle is an approach to analyzing reaction energies. It was named after and developed by the two German scientists Max Born and Fritz Haber....

. It can also be calculated using the Born-Landé equation as the sum of the electrostatic potential energy, calculated by summing interactions between cations and anions, and a short range repulsive potential energy term. The electrostatic potential can be expressed in terms of the inter-ionic separation and a constant (Madelung constant
Madelung constant
The Madelung constant is used in determining the electrostatic potential of a single ion in a crystal by approximating the ions by point charges. It is named after Erwin Madelung, a German physicist....

) that takes account of the geometry of the crystal. The Born-Landé equation gives a reasonable fit to the lattice energy of e.g. sodium chloride where the calculated value is −756 kJ/mol which compares to −787 kJ/mol using the Born-Haber cycle
Born-Haber cycle
The Born–Haber cycle is an approach to analyzing reaction energies. It was named after and developed by the two German scientists Max Born and Fritz Haber....

.

Polarization effects


Ion
Ion
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. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s in crystal lattices of purely ionic compounds are spherical
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

; however, if the positive ion is small and/or highly charged, it will distort the electron cloud of the negative ion, an effect summarised in Fajans' rules
Fajans' rules
In inorganic chemistry, Fajans' Rules, formulated by Kazimierz Fajans in 1923, are used to predict whether a chemical bond will be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion...

. This polarization of the negative ion leads to a build-up of extra charge density between the two nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

, i.e., to partial covalency. Larger negative ions are more easily polarized, but the effect is usually only important when positive ions with charges of 3+ (e.g., Al3+) are involved. However, 2+ ions (Be2+) or even 1+ (Li+) show some polarizing power because their sizes are so small (e.g., LiI is ionic but has some covalent bonding present). Note that this is not the ionic polarization effect which refers to displacement of ions in the lattice due to the application of an electric field.

Comparison with covalent bonds


In an ionic bond, the atoms are bound by attraction of opposite ions, whereas, in a covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

, atoms are bound by sharing electrons to attain stable electron configurations. In covalent bonding, the molecular geometry
Molecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...

 around each atom is determined by VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing
Close-packing
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement . Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occupied by spheres – that can be achieved by a regular lattice...

 rules.

Purely ionic bonds cannot exist, as the proximity of the entities involved in the bond allows some degree of sharing electron density
Electron density
Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...

 between them. Therefore, all ionic bonds have some covalent character. For example, Na–Cl and Mg–O bonds have a few percent covalency, while Si–O bonds are usually ~50% ionic and ~50% covalent. Predominantly covalent bonds with partial ionic character are called polar covalent
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...

.

Electrical conductivity



Ionic compounds, if molten or dissolved, can conduct electricity because the ions in these conditions are free to move and carry electrons between the anode and the cathode. In the solid form, however, they cannot conduct because the electrons are held together too tightly for them to move. However, some ionic compounds can conduct electricity when solid. This is due to migration of the ions themselves under the influence of an electric field. These compounds are known as fast ion conductor
Fast ion conductor
In solid-state ionics, fast ion conductors, also known as solid electrolytes and superionic conductors, are materials that act as solid state ion conductors and are used primarily in solid oxide fuel cells. As solid electrolytes they conduct due to the movement of ions through voids, or empty...

s.

See also

  • Coulomb's law
    Coulomb's law
    Coulomb's law or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism...

  • Ionic potential
    Ionic potential
    Ionic potential is the ratio of electric charge to the radius of an ion.As such, the proportion measures the charge density at the surface of the ion; usually the denser the charge, the stronger will be the bond that the ion forms....

  • Linear combination of atomic orbitals
  • Hybridization
  • Chemical polarity
    Chemical polarity
    In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...


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