Covalent Bonds

Introduction to Covalent Bonding

A covalent bond forms when two nonmetal atoms share pairs of valence electrons. Unlike ionic bonding where electrons are transferred completely from one atom to another, covalent bonding involves mutual sharing so that each atom attains a stable electron configuration, usually resembling a noble gas. This type of bonding holds together many substances essential to life and the physical world, including water, carbon dioxide, and the atmospheric oxygen we breathe. Understanding this concept is fundamental for midwifery students when studying pharmacology, as many medicinal drugs rely on covalent carbon-based molecular structures to function within the human body.

Electron Sharing and the Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons until they have eight valence electrons, achieving the stable configuration of a noble gas. Hydrogen is a notable exception, as it is stable with just two electrons in its valence shell. When drawing Lewis structures to visualize this sharing, each shared pair of electrons counts toward the outer shell of both participating atoms.

• Single pair: Two shared electrons forming one bond.
• Lone pairs: Valence electrons not involved in bonding.
• Shared pairs: Electrons counted for both atoms simultaneously.

Single, Double, and Triple Covalent Bonds

Covalent bonds can involve the sharing of more than one pair of electrons between two atoms to satisfy the octet rule.

• Single bond: Formed by sharing one pair of electrons, represented by a single line (e.g., $H-H$).
• Double bond: Formed by sharing two pairs of electrons, represented by two parallel lines (e.g., $O=O$).
• Triple bond: Formed by sharing three pairs of electrons, represented by three lines (e.g., $N \equiv N$).

As the number of shared electron pairs increases between the same two atoms, the bond length becomes shorter and the bond strength increases, making triple bonds much harder to break than single bonds.

Polar versus Nonpolar Covalent Molecules

Not all atoms share electrons equally. Electronegativity is an atom's ability to attract shared electrons toward itself.

• Nonpolar covalent bond: Electrons are shared equally because the bonded atoms have similar or identical electronegativities (e.g., $Cl_2$).
• Polar covalent bond: Electrons are pulled closer to the more electronegative atom, creating partial positive ($\delta+$) and partial negative ($\delta-$) poles (e.g., $H_2O$).

Symmetry also plays a role: a molecule can contain polar bonds, but if its geometry is perfectly symmetrical, the individual bond dipoles cancel out, making the overall molecule nonpolar (e.g., $CO_2$).

Properties of Covalently Bonded Substances

Because covalent molecules are held together internally by strong shared electron pairs, but bound to each other externally by weak intermolecular forces, they exhibit distinct physical properties. Most covalent substances have relatively low melting and boiling points compared to ionic compounds, and many exist as gases or liquids at room temperature. Solid covalent networks, such as diamond or quartz, are notable exceptions that form massive crystalline structures with extreme hardness and high melting points. Generally, simple covalent substances do not conduct electricity in water because they lack free ions to carry an electrical charge.