What are Covalent and Ionic Bonds?

Atoms of different elements except noble gases do not have a complete octet, so they combine with other atoms to form a chemical bond. When two atoms of the same and different elements mutually share two electrons, one by one, to form a bond between them, the bond is called a covalent bond. On the other hand, an electrovalent or ionic bond is formed when a metal atom transfers one or more electrons to a non-metal atom.

Types of Covalent Bonding

Single bond

In this, atoms of the same element or different elements mutually share a single electron to form a covalent bond. Examples are H₂, CH₄, Cl_2, etc.

Double bond

In this, atoms of the same or different elements mutually share two electrons from each of them to form a double bond between them. Examples are O₂, C₂H₄, CO₂,etc.

Triple bond

In this, three electrons are shared by each atom of the same and different elements to form three covalent bonds between two atoms involved in the covalent bond formation. Examples are N₂, C₂H_2, etc.

Characteristics of Covalent Bond

• Covalent substances exist as gases or liquids under normal conditions of temperature and pressure. Some of them are soft solids.
• Diamond, silica (SiO₂), carborundum (SiC), AlN, etc., have a giant three-dimensional network of structures. Hence, have exceptionally high melting points.
• A covalent bond between two similar atoms is non-polar, while it becomes polar when two atoms of different elements having different electronegativities (EN) combine.
• The compounds with covalent bonds are not counted as good conductors of electricity, but when the compound is polar, it can carry out electricity in an aqueous solution.
• These molecules are generally insoluble in water but soluble in a polar solvents like benzene.
• A covalent bond is rigid and directional. These compounds thus show isomerism, i.e., the structural difference in space.
• The ionic character of a covalent bond increases with increases in the EN of the participated elements in the bond formation. 

Covalency of Covalent Compounds

It is defined as the number of electrons contributed by an atom of the element and shared with another atom to achieve a noble gas configuration. The usual covalency of an element except for H is equal to the group 18th, i.e., nearest noble gas element to which the element belongs in Mendeleev's periodic table. For example, O (oxygen), a group 16 element, has a covalency of two or shares two electrons in covalent bond formation with another atom to fulfill electronic configuration like Ne, a group 18 noble gas. The number of covalent bonds present in a molecule depends on its covalency.

Dipole Moment

The product of the magnitude of positive or negative charge (q) and the distance (d) between the centers of positive and negative charges is called the dipole moment. It is denoted as μ, where μ = Electric charge × bond length. The higher the value of dipole moment, the more ionic character the covalent bonding has. Symmetric polyatomic molecules are non-polar, having dipole moment zero. For example, CO₂, CS₂ are linear molecules thus, have a zero dipole moment. On the other hand, compounds with different EN attached with covalent bonding have a positive dipole moment. Examples are HCl, CH₃Cl, H₂O etc.

Coordinate Covalent or Dative Bond

A coordinate covalent bond is a special type of covalent bond where one species contributes its pair of electrons in the form of lone pair in the bond formation, and the other one accepts the lone pair. Both species share the e- pair in the coordinate compound. The atom which contributes electron pair is called the donor or Lewis base, while the other which only shares the electron pair is known as the acceptor or Lewis acid. This special class of covalent bond is represented by an arrow (→) from the donor to the acceptor. For example, in the BF₃ molecule, B is short of two electrons, so, to complete the octet, it shares the lone pair of N in ammonia (NH₃) forming a coordinate covalent substance NH₃→BF₃.

NH₃ + BF₃ → NH₃→BF₃

Characteristics of Co-ordinate Substances

• Their melting and boiling points are higher than pure covalent substances and lower than purely ionic compounds.
• These are sparingly soluble in polar solvents like water but readily soluble in an apolar solvent.
• Like compounds with covalent bonding, these are poor conductors of electricity. Their solutions or fused masses do not allow the passage of electricity.
• Unlike ionic bonds, these bonds are rigid and directional. Hence, these compounds show isomerism.
• Like ionic bonds, coordinate bond possesses high values of dielectric constant.

How are Ionic Bonds Formed?

Atoms of metals are electropositive and can easily donate one or more electrons from their outermost orbital to maintain their electronic configuration. The electronic configuration should be similar to the nearest noble gas and forms a stable positively charged ion or cation. Sodium (Na) donates the 3s electron to a nearby electron-accepting group to convert its electronic structure as Ne (1s²2s²2p⁶) and forms Na⁺ cation. On the other hand, non-metals are capable of accepting electrons based on their electronegativity (EN). They accept electrons from one or more elements to their valence shell to fulfill the electronic configuration similar to their nearby noble gas. By accepting electrons, it becomes a negatively charged ion or anion. For example, chlorine atom (Cl) having the electronic configuration 1s²2s²2p⁶3s²3p⁵ accepts one electron from nearby electron-donating group to fulfil the electronic configuration like Ar (1s²2s²2p⁶3s²3p⁶) and becomes Cl^-. When such positively charged ions come in proximity with oppositely charged anion, the electrostatic attraction between them results in the formation of ionic bonds. This ionic interaction leads to the formation of ionic compounds. Sodium chloride, an ionic molecule formed by the interactions between Na⁺ and Cl^-. The charge of ions depends on the number of valence electrons involved in ionic bonding to form an ionic molecule.

Na → Na⁺ + e^-

Cl + e^- → Cl^-

Na⁺ + Cl^- → NaCl

Characteristics of the Ionic Compounds and their Ionic Bonds

• These compounds are generally crystalline solids at room temperature
• They possess high melting and boiling point. The order of melting and boiling points in halids of Na and oxides of group II elements are as follows: NaF > NaCl > NaBr > NaI; MgO > CaO > BaO.
• They are generally hard and brittle.
• In solid-state, they do not conduct electricity, but when solubilized in an aqueous medium or molten-state, they can conduct electricity.
• They are soluble in a polar solvent and insoluble in a hydrophobic medium.
• The ionic bonds are non-rigid and directional. These compounds do not show space isomerism, e.g., geometrical or optical isomerism.

Conditions for Formation of Ionic Bonds

• A high difference in electronegativity around two units of the two atoms is necessary to form ionic bonding. A bond with ionic character is not possible between similar atoms.
• There must be an overall decrease in energy, i.e., energy must be released during the chemical bond formation. For this reason, an atom should have a low value of ionization potential, such as metals, and the other one should have a high value of electronegativity, such as nonmetals.

Lattice Energy

It is defined as the amount of energy released when free ions combine to form one mole of crystal. The lattice energy is denoted as U. It is one of the most important factors involved in forming the ionic character of an ionic compound. It is directly proportional to the ionic character of a compound. The higher the lattice energy, the greater will be the case of forming an ionic bond.

Fajan's Rule

The magnitude of polarization or increased covalent character depends upon different factors. These factors are the small size of a positively charged ion, the large size of a negatively charged ion, a large charge on either of the two ions, and the electron configuration of the positively charged ion.

Context and Applications

This topic is significant in professional exams in graduate and postgraduate courses, especially for

• Bachelor of Science in Chemistry.
• Master of Science in Chemistry.