Chemical Bonding
The forces that hold atoms together in molecules and crystals: ionic, covalent, and metallic bonds determined by electronegativity differences.
Chemical bonding is the set of forces that hold atoms together in molecules, salts, and metals. Atoms bond because the resulting arrangement has lower energy than the separated atoms — electrons are shared or transferred to complete stable electron configurations. The type of bond that forms depends mainly on electronegativity, the tendency of an atom to attract bonding electrons, which increases across the periodic table toward fluorine.
Covalent bonds form when atoms share electron pairs. Two hydrogen atoms share their electrons equally, forming a nonpolar covalent bond; when the atoms differ in electronegativity, the shared pair is pulled toward the more electronegative atom and the bond is polar — water's O–H bonds are polar, which is why water is a good solvent and why its acid–base behavior is so important. Covalent bonds are strong and directional, and the electrons involved are called the valence electrons; Lewis structures draw them as shared pairs.
Ionic bonds form when the electronegativity difference is large: an electropositive metal such as sodium transfers an electron to an electronegative nonmetal such as chlorine, producing Na⁺ and Cl⁻ ions held together by electrostatic attraction in a crystal lattice. Ionic compounds are brittle solids with high melting points that conduct electricity when molten or dissolved. Metallic bonds, in contrast, arise when metal atoms share their outer electrons in a delocalized electron sea, which explains the electrical conductivity, malleability, and luster of metals.
Most real bonds are intermediate: the covalent–ionic boundary is a continuum measured by percent ionic character. Beyond the simple models, quantum mechanics describes bonds as shared electron probability densities — the molecular orbital picture explains properties that Lewis structures cannot, such as bond order and magnetism. Bonding determines nearly all material behavior — hardness, conductivity, solubility, melting point — and the reactivity patterns of functional groups in organic chemistry are the behavior of specific bonds. Bond-breaking and bond-forming with electron transfer is precisely what oxidation–reduction reactions describe.
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chemical bonds chemistry electronegativity molecules