To describe the characteristics of ionic bonding. To describe quantitatively the energetic factors involved in the formation of an ionic bond.

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Ions are atoms or molecules which are electrically charged. Cations are positively charged and anions are negatively charged. Ions form when atoms gain or lose valence electrons. Since electrons are negatively charged, an atom that loses one or more electrons will become positively charged; an atom that gains one or more electrons becomes negatively charged. Ionic bonding is the attraction between positively- and negatively-charged ions. These oppositely charged ions attract each other to form ionic networks, or lattices. Electrostatics explains why this happens: opposite charges attract and like charges repel. When many ions attract each other, they form large, ordered, crystal lattices in which each ion is surrounded by ions of the opposite charge. Generally, when metals react with non-metals, electrons are transferred from the metals to the non-metals. The metals form positively-charged ions and the non-metals form negatively-charged ions.

The properties of ionic compounds follow from the orderly crystal lattice arrangement of tightly bonded charged particles that make them up. Ionic compounds tend to have high melting and boiling points, because the attraction between ions in the lattice is very strong. Moving ions out of the lattice disrupts the structure, so ionic compounds tend to be brittle rather than malleable. Ionic compounds do not conduct electricity in the solid state because ions are not free to move around the lattice; however, when ionic compounds are dissolved, they may dissociate into individual ions which move freely through the solution and therefore conduct electricity well.


Generating Ionic Bonds

Ionic bonds form when metals and non-metals les-grizzlys-catalans.orgically react. By definition, a metal is relatively stable if it loses electrons to form a complete valence shell and becomes positively charged. Likewise, a non-metal becomes stable by gaining electrons to complete its valence shell and become negatively charged. When metals and non-metals react, the metals lose electrons by transferring them to the non-metals, which gain them. Consequently, ions are formed, which instantly attract each other—ionic bonding. In the overall ionic compound, positive and negative charges must be balanced, because electrons cannot be created or destroyed, only transferred. Thus, the total number of electrons lost by the cationic species must equal the total number of electrons gained by the anionic species.

Ionic compounds are held together by electrostatic forces, which are described in classical physics by Coulomb"s Law. According to this law, the energy of the electrostatic attraction ((E)) between two charged particles is proportional to the magnitude of the charges ((Q)1) and (Q_2)) and inversely proportional to the internuclear distance between the particles ((r)):

The energy of attraction ((E)) is a type of potential energy, since it is based on the position of the charged particles relative to each other. If the two particles have opposite charges (as in ionic compounds), the value of ((E)) will be negative, meaning that energy is released by bringing the particles together—that is, the particles naturally attract each other. According to Coulomb"s Law, the larger the magnitude of the charges on each particle, the stronger the attraction will be. So, for example, Mg2+ and O2- will have a stronger attraction than Na+ and Cl-, because of the larger charges. Also, the closer together the charges are, the stronger the attraction. Therefore, smaller ions also form stronger ionic bonds.

In an ionic lattice, many more than two charged particles interact simultaneously, releasing an amount of energy known as the lattice energy. The lattice energy is not exactly the same as that predicted by Coulomb"s Law, but the same general principles of electrostatic attraction apply. In an ionic compound, the value of the lattice energy corresponds to the strength of the ionic bonding.


Example (PageIndex1): Sodium Chloride

For example, in the reaction of Na (sodium) and Cl (chlorine), each Cl atom takes one electron from a Na atom. Therefore each Na becomes a Na+ cation and each Cl atom becomes a Cl- anion. Due to their opposite charges, they attract each other to form an ionic lattice. The formula (ratio of positive to negative ions) in the lattice is (ceNaCl).

These ions are arranged in solid (ceNaCl) in a regular three-dimensional arrangement (or lattice):

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NaCl lattice. (left) 3-D structure and (right) simple 2D slice through lattice. Images used with permission from Wikipedia and Mike Blaber.

The chlorine has a high affinity for electrons, and the sodium has a low ionization energy. Thus the chlorine gains an electron from the sodium atom. This can be represented using Lewis dot symbols showing the valence electrons in each atom (here we will consider one chlorine atom, rather than Cl2):

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not form a noble gas core:

Fe: 4s23d6 Fe2+: 3d6 Fe3+: 3d5

Some metal ions can form a pseudo noble gas core (and be colorless), for example:

Ag: 5s14d10 Ag+ 4d10 Compound: AgCl Cd: 5s24d10 Cd2+ 4d10 Compound: CdS

Note: The silver and cadmium atoms lost the 5s electrons in achieving the ionic state. Remember that atoms always lost electrons from the subshell with the highest n quantum number first (i.e. 5s before 4d).

When a positive ion is formed from an atom, electrons are always lost first from the subshell with the largest principle quantum number.

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Polyatomic Ions

Not all ionic compounds are formed from only two elements. Many polyatomic ions exist, in which two or more atoms are bound together by covalent bonds. They form a stable grouping which carries a charge (positive or negative). The group of atoms as a whole acts as a charged species in forming an ionic compound with an oppositely charged ion. Polyatomic ions may be either positive or negative, for example:

NH4+ (ammonium) = cation SO42- (sulfate) = anion

The principles of ionic bonding with polyatomic ions are the same as those with monatomic ions. Oppositely charged ions come together to form a crystalline lattice, releasing a lattice energy. Based on the shapes and charges of the polyatomic ions, these compounds may form crystalline lattices with interesting and complex structures.