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Ionic bond



 

An ionic bond (or electrovalent bond) is a type of chemical bond that can often form between metal and non-metal ions (or polyatomic ions such as ammonium) through electrostatic attraction.

The metal donates one or more electrons, forming a positively charged ion or cation with a stable electron configuration. These electrons then enter the non metal, causing it to form a negatively charged ion or anion which also has a stable electron configuration. The electrostatic attraction between the oppositely charged ions causes them to come together and form a bond.

For example, common table salt is sodium chloride. When sodium (Na) and chlorine (Cl2) are combined, the sodium atoms each lose an electron, forming a cation (Na+), and the chlorine atoms each gain an electron to form an anion (Cl-). These ions are then attracted to each other in a 1:1 ratio to form sodium chloride (NaCl).

Na + ½Cl2 → Na+ + Cl- → NaCl

  The removal of electrons from the atoms is endothermic and causes the ions to have a higher 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 attraction of the ions to each other lowers their energy. Ionic bonding will occur only if the overall energy change for the reaction is favourable – when the bonded atoms have a lower energy than the free ones. The larger the resulting energy change the stronger the bond. The low electronegativity of metals and high electronegativity of non-metals means that the energy change of the reaction is most favorable when metals lose electrons and non-metals gain electrons.

Pure ionic bonding is not known to exist. All ionic bonds have a degree of covalent bonding or metallic bonding. The larger the difference in electronegativity between two atoms, the more ionic the bond. Ionic compounds conduct electricity when molten or in solution. They generally have a high melting point and tend to be soluble in water.

Additional recommended knowledge

Contents

Polarization effects

Ions in crystal lattices of purely ionic compounds are spherical; however, if the positive ion is small and/or highly charged, it will distort the electron cloud of the negative ion. This polarization of the negative ion leads to a build-up of extra charge density between the two nuclei, 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 (e.g., pure AlCl3 is a covalent molecule). 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).

Ionic structure

Ionic compounds in the solid state form a continuous ionic lattice structure in an ionic crystal. The simplest form of ionic crystal is a simple cubic. This is as if all the atoms were placed at the corners of a cube. This unit cell has a weight that is the same as 1 of the atoms involved. When all the ions are approximately the same size, they can form a different structure called a face-centered cubic (where the weight is 4 * atomic weight), but, when the ions are different sizes, the structure is often body-centered cubic (2 times the weight). In ionic lattices the coordination number refers to the number of connected ions.

Ionic versus covalent bonds

In an ionic bond, the atoms are bound by attraction of opposite ions, whereas, in a covalent bond, atoms are bound by sharing electrons. In covalent bonding, the molecular geometry around each atom is determined by VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing rules.

Electrical conductivity

Main article: Electrolyte

Ionic substances in solution conduct electricity because the ions are free to move and carry the electrical charge from the anode to the cathode. Ionic substances conduct electricity when molten because atoms (and thus the electrons) are mobilised. Electrons can flow directly through the ionic substance in a molten state.

Substances in ionic form

Common Cations
Stock System Name Formula Historic Name
Simple Cations
AluminiumAl3+
BariumBa2+
BerylliumBe2+
CaesiumCs+
CalciumCa2+
Chromium(II)Cr2+Chromous
Chromium(III)Cr3+Chromic
Chromium(VI)Cr6+Chromyl
Cobalt(II)Co2+Cobaltous
Cobalt(III)Co3+Cobaltic
Copper(I)Cu+Cuprous
Copper(II)Cu2+Cupric
Copper(III)Cu3+
Gallium Ga3+
Gold(I)Au+
Gold(III)Au3+
HeliumHe2+(Alpha particle)
HydrogenH+(Proton)
Iron(II)Fe2+Ferrous
Iron(III)Fe3+Ferric
Lead(II)Pb2+Plumbous
Lead(IV)Pb4+Plumbic
LithiumLi+
MagnesiumMg2+
Manganese(II)Mn2+Manganous
Manganese(III)Mn3+Manganic
Manganese(IV)Mn4+Manganyl
Manganese(VII)Mn7+
Mercury(II)Hg2+Mercuric
Nickel(II)Ni2+Nickelous
Nickel(III)Ni3+Nickelic
PotassiumK+
SilverAg+
SodiumNa+
StrontiumSr2+
Tin(II)Sn2+Stannous
Tin(IV)Sn4+Stannic
ZincZn2+
Polyatomic Cations
AmmoniumNH4+
HydroniumH3O+
NitroniumNO2+
Mercury(I)Hg22+Mercurous
Common Anions
Formal Name Formula Alt. Name
Simple Anions
ArsenideAs3−
AzideN3
BromideBr
ChlorideCl
FluorideF
HydrideH
IodideI
NitrideN3−
OxideO2−
PhosphideP3−
SulfideS2−
PeroxideO22−
Oxoanions
ArsenateAsO43−
ArseniteAsO33−
BorateBO33−
BromateBrO3
HypobromiteBrO
CarbonateCO32−
Hydrogen carbonateHCO3Bicarbonate
ChlorateClO3
PerchlorateClO4
ChloriteClO2
HypochloriteClO
ChromateCrO42−
DichromateCr2O72−
IodateIO3
NitrateNO3
NitriteNO2
PhosphatePO43−
Hydrogen phosphateHPO42−
Dihydrogen phosphateH2PO4
PermanganateMnO4
PhosphitePO33−
SulfateSO42−
ThiosulfateS2O32−
Hydrogen sulfateHSO4Bisulfate
SulfiteSO32−
Hydrogen sulfiteHSO3Bisulfite
Anions from Organic Acids
AcetateC2H3O2
FormateHCO2
OxalateC2O42−
Hydrogen oxalateHC2O4Bioxalate
Other Anions
Hydrogen sulfideHSBisulfide
TellurideTe2−
AmideNH2
CyanateOCN
ThiocyanateSCN
CyanideCN

See also

 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ionic_bond". A list of authors is available in Wikipedia.
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