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Ionic Bonding

An ionic bond is a strong mutual attraction of oppositely charged ions. Such bonds do not usually form by the direct transfer of an electron from one atom to another; rather atoms that have already become ions stay close together because of their opposite charges.

Ions may consist of single atom or multiple atoms, in which a group of atoms is called a "polyatomic ion". Examples of polyatomic anions include: carbonate ion, which is composed of carbon and oxygen; and sulfate ion, which is composed of sulfur and oxygen. An example of a polyatomic cation is ammonium ion, which consists of nitrogen and hydrogen. Cation are usually metal atoms and anions are either nonmetals or polyatomic ions. The attraction of the two charges holds the atoms or molecules together. Electrostatic forces hold ionic bonds together.

Ionic bond is defined as
"An ionic bond is the force of attraction between oppositely charged ions in a compound."

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Features Favoring Ionic Bond

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The main features favoring ionic bond are
  1. The metal has a low ionization energy
  2. The non-metal has a high electron affinity (electron gain energy)
  3. The metal forms large ions of low charge
  4. The non-metal forms small ions of low charge
The extent to which the bonds formed are ionic depends on the effect which the cation, generally the smaller ion, has on the anion. Small cations of high charge have a high charge density; and this may be able to distort the electron cloud around the anion so that there is some degree of electron sharing between the cation and anion.

Examples of Ionic Bonding

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Ionic compounds are made up of ions. For example, sodium chloride consists of sodium ions and chloride ions. A strong electrostatic force of attraction between the oppositely charged ins holds them together. This force of attraction is called an ionic bond.

Two examples illustrate the formation of ionic bonds. They are

Sodium chloride

Hot sodium metal reacts with chlorine gas, Cl2 to produce a white solid. The white solid is the compound sodium chloride. In these reaction, the sodium atom loses an electron to become a sodium ion, Na+. The electron is taken by a chlorine atom to become a chloride ion, Cl-. There is a transfer of an electron from the sodium atom to chlorine atom.

Ionic Bonding in NaCl

The positive sodium ion Na+ and the negative chloride ion Cl- are attracted by an electrostatic force. This force of attraction is very strong and it is the ionic bond.

Magnesium Fluoride

Magnesium fluoride MgF2 is an ionic compound. The magnesium atom gives up two electrons to form a magnesium ion Mg2+. The two electrons are transferred to two fluorine atoms to form two fluoride ions F-. The magnesium fluoride has the formula MgF2. Each unit of magnesium fluoride consists of one magnesium ion and two fluoride ions.

Ionic Bonding in MgF2

Structure of Ionic Compounds

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Ionic compounds consist of positive ions of metals and negative ions of non-metals. An example is sodium chloride. A crystal of sodium chloride consists of large numbers of Na+ and Cl- ions arranged in an orderly manner.

Ionic Compound Structure

This diagram shows a model of the structure of sodium chloride. Notice that in this structure, we cannot identify any NaCl pairs. Instead there is a repetition of positive and negative ions throughout the whole structure. Because the structure consist of large number of ions, it is called a giant ionic structure or giant lattice structure. All compounds formed from metals and non-metals have these giant structures.
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