Difference Between Ionic and Covalent Bonds

The key difference between ionic and covalent bonds is that ionic bonds occur between atoms having very different electronegativities whereas covalent bonds occur between atoms with similar or very low electronegativity differences.

As proposed by the American chemist G.N.Lewis proposed that 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.

CONTENTS

1. Overview and Key Difference
2. What are Ionic Bonds
3. Whare is Covalent Bonds
4. Side by Side Comparison – Ionic vs Covalent Bonds in Tabular Form
5. Summary

What are Ionic Bonds?

Atoms can gain or lose electrons and form negative or positive charged particles; which we call ions. There are electrostatic interactions between the ions. Ionic bond is the attractive force between these oppositely charged ions. The electronegativities of the atoms in an ionic bond largely influence the strength of the electrostatic interactions between ions.

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 and chlorine is a nonmetal; 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^–. At the same time, sodium forms 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; Na-Cl bond.

What are Covalent Bonds?

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, atoms of the same element join to form molecules like Cl₂, H₂, or P₄, each atom binds with another via a covalent bond.

Methane molecule (CH₄) also has covalent bonds between carbon and hydrogen atoms; there are four covalent bonds between one central carbon atom and four hydrogen atoms (four C-H bonds). Methane is an example of a molecule having covalent bonds between atoms with very low electronegativity difference.

What is the Difference Between Ionic and Covalent Bonds?

Ionic vs Covalent Bonds
A chemical link between two atoms caused by the electrostatic force between oppositely-charged ions in an ionic compound.	A chemical link between two atoms or ions where the electron pairs are shared between them.
Number of Atoms
Occur between metals and nonmetals.	Most commonly occurs between two nonmetals.
Number of Electrons
A complete transfer of electrons occurs.	Occurs when two (or more) elements share electrons.
Compounds
Usually seen as crystals, in which few positively charged ions surround a negatively charged ion.	The atoms bonded by covalent bonds exist as molecules, which at room temperature, mainly exist as gases or liquids.
Polarity
Ionic bonds have a high polarity.	Covalent bonds have a low polarity.
Physical Properties
Ionic compounds have very high melting points and boiling points, compared to covalent molecules.	Covalent molecules have low melting points and boiling points compared to ionic compounds.
Water Solubility
In polar solvents (such as water), ionic compounds dissolve releasing ions; such solutions are capable of conducting electricity.	In polar solvents, covalent molecules do not dissolve considerably; hence these solutions are incapable of conducting electricity.

Summary – Ionic vs Covalent Bonds

Ionic and covalent bonds are the major two types of chemical bonds that exist in compounds. The difference between ionic and covalent bond is that ionic bonds occur between atoms having very different electronegativities whereas covalent bonds occur between atoms with similar or very low electronegativity differences.

Reference:

1. Helmenstine, Anne Marie, Ph.D. “Ionic Bond Definition.” ThoughtCo, Feb. 10, 2017. Available here
2. Helmenstine, Anne Marie, Ph.D. “Covalent Bond Definition.” ThoughtCo, Feb. 7, 2018. Available here 

Image Courtesy:

1.’IonicBondingRH11’By Rhannosh – Own work, (CC BY-SA 3.0) via Commons Wikimedia  
2.’Covalent’By DynaBlast – Created with Inkscape, (CC BY-SA 2.5) via Commons Wikimedia