Structure 2. Models of bonding and structure - The covalent model

A covalent bond is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the bonded atoms.

The Octet Rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons, similar to a noble gas. Exceptions include hydrogen ($H_{2}$), which requires only 2 electrons, and elements in Period 3 or below that can expand their octet.

Bond polarity is determined by the difference in electronegativity ($\Delta \chi$) between atoms. If $0.0 < \Delta \chi < 0.5$, the bond is non-polar covalent; if $0.5 \le \Delta \chi < 1.8$, the bond is polar covalent.

Bond length and bond strength: As the number of shared electron pairs increases (single to double to triple), bond length decreases and bond enthalpy (strength) increases.

Coordinate (Dative) Covalent Bonding occurs when one atom provides both electrons for the shared pair. This is represented by an arrow ($\rightarrow$) in Lewis structures.

VSEPR Theory (Valence Shell Electron Pair Repulsion) predicts molecular geometry based on minimizing repulsion between electron domains. Total domains determine the electron domain geometry: 2 = Linear ($180^{\circ}$), 3 = Trigonal Planar ($120^{\circ}$), 4 = Tetrahedral ($109.5^{\circ}$).

Resonance occurs when more than one valid Lewis structure can be drawn for a molecule (e.g., $O_{3}$, $CO_{3}^{2-}$, $C_{6}H_{6}$). The actual structure is a resonance hybrid with delocalized electrons.

Giant covalent structures (Network solids) like Diamond, Graphite, and Silicon Dioxide ($SiO_{2}$) have high melting points and hardness because atoms are linked by a continuous network of strong covalent bonds throughout the entire crystal.