Ionic and Covalent Bonds

As proposed by the American chemist G.N.Lewis, atoms are stable when they contain eight electrons in their valence shell. Most of the atoms have less than eight electrons in their valence shells (except the noble gases in the group 18 of the periodic table); therefore, they are not stable. These atoms tend to react with each other to become stable. Thus, each atom can achieve a noble gas electronic configuration. Ionic and covalent bonds are the two major types of chemical bonds, which connect atoms in a chemical compound.

Ionic Bond

Atoms can gain or lose electrons and form negative or positive charged particles. These particles are called ions. There are electrostatic interactions between the ions. Ionic bond is the attractive force between these oppositely charged ions. The strength of the electrostatic interactions is largely influenced by the electronegativities of the atoms in an ionic bond. Electronegativity is a measurement of the atoms’ affinity for electrons. An atom, with high electronegativity can attract electrons from an atom with low electronegativity to form an ionic bond. For example, sodium chloride has an ionic bond between sodium ion and chloride ion. Sodium is a metal; therefore, it has a very low electronegativity (0.9) compared to Chlorine (3.0). Because of this electronegativity difference, Chlorine can attract an electron from Sodium and form Cl- and Na+ ions. Because of this, both atoms gain the stable noble gas electronic configuration. Cl- and Na+ are held together by attractive electrostatic forces, thus forming an ionic bond.

Covalent Bond

When two atoms, having similar or very low electronegativity difference, react together, they form a covalent bond by sharing electrons. In this way, both atoms can obtain the noble gas electronic configuration by sharing electrons. Molecule is the product that results from the formation of covalent bonds between atoms. For example, when the same atoms are joined to form molecules like Cl2, H2, or P4, each atom is bonded to another by a covalent bond. Methane molecule (CH4) also has covalent bonds between carbon and hydrogen atoms. Methane is an example for a molecule having covalent bonds between atoms with very low electronegativity difference.

Ionic bonds vs Covalent bonds - Ionic bonds occur between atoms having very different electronegativities, whereas, covalent bonds occur between atoms with similar or very low electronegativity differences. - Ionic bonds occur between metals and non metals. Covalent bonding most commonly occurs between two nonmetals. - In ionic bonding, a complete transfer of electrons occurs, whereas covalent bonding occurs when two (or more) elements share electrons. - Ionic substances are usually seen as crystals. In crystals, a negatively charged ion is surrounded by few positively charged ions and vice versa. - Unlike the ionic compounds, the atoms bounded by covalent bonds exist as molecules. At room temperature, they are mainly seen as gases or liquids. - Since ionic compounds are in crystalline form, they have very high melting points and boiling points compared to covalent molecules. - Ionic bonds have a high polarity and covalent bonds have a low polarity. - In polar solvents (water), ionic compounds are solvated releasing ions as opposed to molecules with covalent bonding. Such solutions are capable of conducting electricity.