Chemical bonding explains how and why atoms combine to form compounds. The type of bond that forms depends on the electronegativity of the atoms involved — electronegativity is an atom's ability to attract shared electrons in a bond. The difference in electronegativity between two atoms determines bond type: a large difference (above ~1.7) produces an ionic bond, a moderate difference produces a polar covalent bond, and a small or zero difference produces a nonpolar covalent bond.
Ionic bonding occurs when one atom transfers electrons to another. Metals with low electronegativity (like sodium, EN = 0.9) lose electrons to become cations, while nonmetals with high electronegativity (like chlorine, EN = 3.0) gain electrons to become anions. The electrostatic attraction between oppositely charged ions forms the ionic bond. Ionic compounds form crystal lattice structures where each ion is surrounded by ions of opposite charge, giving them high melting points and the ability to conduct electricity when dissolved in water.
Covalent bonding involves sharing electron pairs between nonmetal atoms. A single covalent bond shares one pair of electrons (2 electrons total), a double bond shares two pairs (4 electrons), and a triple bond shares three pairs (6 electrons). Carbon uniquely forms four covalent bonds, making it the backbone of organic chemistry. In polar covalent bonds (like H-O in water), electrons are shared unequally because one atom is more electronegative, creating partial positive and negative charges.
Metallic bonding involves a "sea" of delocalized electrons that are free to move among a lattice of metal cations. This explains why metals conduct electricity (free-moving electrons carry charge), are malleable and ductile (layers of atoms can slide without breaking the bond), and have lustrous surfaces (delocalized electrons absorb and re-emit light). Different metals have different numbers of delocalized electrons, which is why copper conducts electricity better than iron.
Lewis structures are diagrams that show how electrons are arranged in molecules. Each dot represents a valence electron, and each line represents a shared pair (a covalent bond). The octet rule states that most atoms are most stable with 8 electrons in their outer shell. Atoms achieve this by gaining, losing, or sharing electrons. Lewis structures help predict molecular geometry, polarity, and reactivity.