Covalent Bonding and Molecular Shapes

Covalent bonding primarily occurs between non-metal atoms where there is a small or negligible difference in electronegativity. Instead of transferring electrons (as in ionic bonding), atoms achieve a stable electron configuration, typically an octet, by sharing valence electrons. This sharing leads to the formation of a covalent bond, which is a strong electrostatic attraction between the shared electrons and the nuclei of the bonded atoms.

For example, in a hydrogen molecule (H₂), each hydrogen atom has one valence electron. By sharing their electrons, both atoms achieve a stable duet configuration, similar to helium. In a chlorine molecule (Cl₂), each chlorine atom has seven valence electrons. They share one pair of electrons to achieve a stable octet. The strength of a covalent bond depends on factors like bond length and the number of shared electron pairs (single, double, or triple bonds). Covalent compounds typically have lower melting and boiling points compared to ionic compounds due to weaker intermolecular forces, although the covalent bonds themselves are very strong.

Covalent bonds can be classified based on the number of electron pairs shared between two atoms:

• Single Covalent Bond: Formed when two atoms share one pair of electrons. Represented by a single line (e.g., H-H, Cl-Cl). These are generally the longest and weakest covalent bonds between two specific atoms.
• Double Covalent Bond: Formed when two atoms share two pairs of electrons. Represented by two parallel lines (e.g., O=O in O₂, C=C in ethene, C=O in carbon dioxide). Double bonds are shorter and stronger than single bonds between the same atoms.
• Triple Covalent Bond: Formed when two atoms share three pairs of electrons. Represented by three parallel lines (e.g., N≡N in N₂, C≡C in ethyne). Triple bonds are the shortest and strongest covalent bonds between two specific atoms.

The formation of multiple bonds allows atoms to satisfy the octet rule when single bonds are insufficient. For instance, in carbon dioxide (CO₂), carbon forms double bonds with each oxygen atom to achieve an octet, and each oxygen also achieves an octet.

A dative covalent bond, also known as a coordinate bond, is a special type of covalent bond where both shared electrons come from only one of the bonding atoms. The other atom contributes an empty orbital to accept the electron pair. Once formed, a dative bond is indistinguishable from a regular covalent bond in terms of its properties and strength.

Common examples include:

• Ammonium ion (NH₄⁺): Ammonia (NH₃) has a lone pair on the nitrogen atom. When it reacts with a hydrogen ion (H⁺), the lone pair on nitrogen is donated to the empty 1s orbital of the H⁺ ion, forming a dative bond. All N-H bonds in NH₄⁺ are identical.
• Hydronium ion (H₃O⁺): Water (H₂O) has two lone pairs on the oxygen atom. One lone pair can be donated to an H⁺ ion to form H₃O⁺.
• Carbon monoxide (CO): In CO, one dative bond is formed from oxygen to carbon, along with two regular covalent bonds, to satisfy the octet rule for both atoms. It is often represented with an arrow pointing from the donor atom to the acceptor atom (e.g., N → H⁺ or O → C).