Editing Chemical bond (section)

=== Ionic bond ===
{{Main|Ionic bonding}}

[[File:NaCl octahedra.svg|thumb|left|Crystal structure of [[sodium chloride]] (NaCl) with sodium [[cation]]s ({{color|purple|Na<sup>+</sup>}}) in {{color|purple|purple}} and [[chloride]] anions ({{color|green|Cl<sup>−</sup>}}) in {{color|green|green}}. The yellow stipples represent the [[electrostatic force]] between the [[ion]]s of opposite charge.]]
Ionic bonding is a type of electrostatic interaction between atoms that have a large electronegativity difference. There is no precise value that distinguishes ionic from covalent bonding, but an electronegativity difference of over 1.7 is likely to be ionic while a difference of less than 1.7 is likely to be covalent.<ref>{{cite book
 | last = Atkins
 | first = Peter
 | author-link = Peter Atkins
 |author2=Loretta Jones
 | title = Chemistry: Molecules, Matter and Change
 | publisher = W.H. Freeman & Co.
 | year = 1997
 | location = New York
 | pages = 294–295
 | isbn = 978-0-7167-3107-8 }}</ref> Ionic bonding leads to separate positive and negative [[ions]]. Ionic charges are commonly between −3[[elementary charge|e]] to +3[[elementary charge|e]]. Ionic bonding commonly occurs in [[Salt (chemistry)|metal salts]] such as [[sodium chloride]] (table salt). A typical feature of ionic bonds is that the species form into ionic crystals, in which no ion is specifically paired with any single other ion in a specific directional bond. Rather, each species of ion is surrounded by ions of the opposite charge, and the spacing between it and each of the oppositely charged ions near it is the same for all surrounding atoms of the same type. It is thus no longer possible to associate an ion with any specific other single ionized atom near it. This is a situation unlike that in covalent crystals, where covalent bonds between specific atoms are still discernible from the shorter distances between them, as measured via such techniques as [[X-ray diffraction]].

Ionic crystals may contain a mixture of covalent and ionic species, as for example salts of complex acids such as [[sodium cyanide]], NaCN. X-ray diffraction shows that in NaCN, for example, the bonds between sodium [[cation]]s (Na<sup>+</sup>) and the cyanide [[anion]]s (CN<sup>−</sup>) are ''ionic'', with no [[sodium]] ion associated with any particular [[cyanide]]. However, the bonds between the [[carbon]] (C) and [[nitrogen]] (N) atoms in cyanide are of the ''covalent'' type, so that each carbon is strongly bound to ''just one'' nitrogen, to which it is physically much closer than it is to other carbons or nitrogens in a sodium cyanide crystal.

When such crystals are melted into liquids, the ionic bonds are broken first because they are non-directional and allow the charged species to move freely. Similarly, when such salts dissolve into water, the ionic bonds are typically broken by the interaction with water but the covalent bonds continue to hold. For example, in solution, the cyanide ions, still bound together as single CN<sup>−</sup> ions, move independently through the solution, as do sodium ions, as Na<sup>+</sup>. In water, charged ions move apart because each of them are more strongly attracted to a number of water molecules than to each other. The attraction between ions and water molecules in such solutions is due to a type of weak [[intermolecular force|dipole-dipole]] type chemical bond. In melted ionic compounds, the ions continue to be attracted to each other, but not in any ordered or crystalline way.