Ionic bonding

Ionic bonding is a type of chemical bonding that results from the electrostatic attraction of oppositely charged ions that are typically produced by the transfer of electrons between metallic and nonmetallic atoms.

Compounds composed of ions are called ionic compounds (or salts), and their constituent ions are held together by ionic bonds: electrostatic forces of attraction between oppositely charged cations and anions. The properties of ionic compounds shed some light on the nature of ionic bonds.

Ionic solids exhibit a crystalline structure and tend to be rigid and brittle; they also tend to have high melting and boiling points, which suggests that ionic bonds are very strong. Ionic solids are also poor conductors of electricity for the same reason—the strength of ionic bonds prevents ions from moving freely in the solid state.

Most ionic solids, however, dissolve readily in water. Once dissolved or melted, ionic compounds are excellent conductors of electricity and heat because the ions can move about freely.

Ionic bond is a strong bond between two atoms that attract each other because of different electronegativity. The product of this bond cannot be defined as a molecule because its characteristics and the way it is joined are very different from covalent bond. Generally we indicate this product as an agglomerate and the parts that compose it are called ions, which are classified in anions (negative charge) and cations (positive charge). In an ionic bond there is a transfer of electrons between the two or more combining species. Below is a simple example with the common table salt NaCl (sodium chloride).

Sodium (Na) has only one electron in the outermost orbital, 3s. It will necessarily have to lose this electron to assume the configuration of neon, the noble gas before it, and stabilize. Chlorine (Cl) has seven electrons in the outermost orbital which leads it to acquire another one to reach stability and assume, so, the electronic configuration of argon. When Na and Cl meet, the electronegativity difference between them is so great that sodium instantly gives up an electron to chlorine. Both atoms have a common interest in this transfer: Na gets rid of the one valence electron in the 3s level and becomes stable, Cl acquires the one electron it needed to become stable. The product of this bonding, denoted as Na+Cl or simply NaCl, is a stable agglomerate in which the ions composing it electrostatically attract each other with a force expressed by Coulomb’s law.

Therefore, unlike covalent bonding, for this type of bond there is an effective transfer of one or more electrons from one atom to another, which consequently generates ions. A further characterization of ionic bonding is possible: it is not directional. The electrostatic force, assumed atoms as spheres, is exerted in all directions and with the same intensity. The consequence is that the two ions attract each other in any direction, none of them is preferred over others, nor there are directions energetically more advantageous. Now it is fully understood why such reaction products cannot be defined as molecules: the atomic orbitals of the reacting species do not combine to form molecular orbitals, but remain independent and maintain their integrity.

The formation of ionic compounds

Binary ionic compounds are composed of just two elements: a metal (which forms the cations) and a nonmetal (which forms the anions). For example, NaCl is a binary ionic compound. We can think about the formation of such compounds in terms of the periodic properties of the elements. Many metallic elements have relatively low ionization potentials and lose electrons easily.

These elements lie to the left in a period or near the bottom of a group on the periodic table. Nonmetal atoms have relatively high electron affinities and thus readily gain electrons lost by metal atoms, thereby filling their valence shells. Nonmetallic elements are found in the upper-right corner of the periodic table.

As all substances must be electrically neutral, the total number of positive charges on the cations of an ionic compound must equal the total number of negative charges on its anions. The formula of an ionic compound represents the simplest ratio of the numbers of ions necessary to give identical numbers of positive and negative charges.

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