Ionic and Covalent Bond

Bonding in covalent and ionic compounds is classified according to the character of electrons, which are delocalized. This classification is based on electronic effects, for which bonding is a result of interaction among electrons of different orbitals and hence is a chemical bonding. As a result of this interaction, electrons become shared by two or more nuclei, leading to a reduction in the electron energy and therefore stabilization of the compound. A compound whose nucleus accepts an electron is termed an electron acceptor and a nucleus that donates a single electron is an electron donor.

The delocalized electron is called the valence electron, which can be donated by a nucleus. Electrons in orbitals can be assigned energies concerning the nucleus of the atom. For example, an electron in the 1s orbital, which is localized around a nucleus, has high energy, and when the atom gains energy, the electron is donated to the nucleus to make the atom more stable. For an atom to be stable, it must gain the maximum amount of energy, and hence it is called a 'maximum donor'. Electrons with high energy are called 'high valence' or 'heavier' electrons.

The bond between atoms can be classified as either covalent or ionic following the number of electrons transferred. The number of transferred electrons to bond with a molecule to make it more stable is termed the valence. In a chemical bonding reaction, a bond with one electron transferred is termed a single bond, two electrons transferred is a double bond, three electrons transferred is a triple bond, and the maximum bond strength is denoted by a covalent bond. In general, the higher the valence of an atom, the stronger the bond between them is.

The chemical bonding of atoms can be classified as one of the following: ionic, covalent, metallic and van der Waals. Among these, ionic is a bond formed when two or more elements share electrons in their outer shells. However, all ionic bonds are not created equally. There are covalent, metallic and van der Waals bonds in ionic bonding.

Covalent Bonding

In a covalent bond, the nucleus of an atom accepts electrons of the other atom. The electron configuration is as follows:

• There is one outer shell electron for both the donor and the acceptor atom.

• One orbital is the highest in energy, which accounts for the greater bonding in covalent bonding.

• The bond energy is the highest among all of the types of chemical bonding, owing to the greatest attractive force.

In a covalent bond, the bond energy is approximately 2-3 kcal mol⁻¹ more than in an ionic bond. To illustrate the above-stated principle, in an electron exchange reaction in which one electron is transferred from donor atom A to the acceptor atom B, the nuclei of the atoms become ionic and the electron configuration of the atoms change from 2 _n_ − 2 to _n_ + 2. A covalent bond is considered when the electron configuration becomes 2 _n_ + 1 (in a diatomic molecule, the two atoms are of the same type).

Metallic Bonding

In a metallic bond, the electrons of the nucleus of both the atoms are shared between them, unlike the covalent and ionic bonds. The electrons form new chemical species which do not exist individually. The electrons cannot be assigned individually to the nucleus, and the two molecules share each electron in common.

Metallic bonds are extremely strong and they form through s-p mixing in d-orbitals.

In a metallic bond, the bond energy is lower than covalent bonding. However, owing to strong forces and a low percentage of electrons, the bond energy is weak and the bonds are easily broken.

Van der Waals Bonding

Van der Waals (vdW) bonding is similar to ionic bonding. One of the atoms in the molecule is ionic and the other is electrically neutral. In van der Waals bonding, the electrically neutral atom attracts the ionic atom. The interaction between the neutral atom and the ionic atom is of an opposite sign than that of the covalent bonds. This leads to attractive force and attraction between them. The strength of van der Waals bonding is not so strong.

Ionic Bonding

An ionic bond is very strong, owing to the presence of ions. The positive ions of one of the atoms are shared with the negative ions of the other atom. One of the atoms of the ionic compound has more electrons than the other. This one atom is positively charged. The electrically neutral atom has fewer electrons than the other atom. This atom is negatively charged. The charge on an atom increases or decreases if it loses or gains electrons, respectively.

Ionic bonding also involves an exchange interaction between the positive charge on one atom and the negative charge on the other atom. The covalent bonds of both atoms exist when the positive charge on one atom is neutralized by the negative charge on the other. Ionic bonding is different from ionic bonding in the following ways:

• In an ionic bond, all the valence electrons are shared between two different atoms. In a covalent bond, only one valence electron of an atom remains.

• In ionic bonding, the interaction between the positive charge on one atom and the negative charge on the other atom is not the same as the interaction between two like charges. In covalent bonding, both the interaction and force between two like charges are similar.

• The charge separation of ions from an atom causes attraction. In covalent bonds, both the atom and the bonding electrons have the same charge.

• Ions have higher mobility than non-polarized charges.