Why atoms bond and how molecules get their shapes
Atoms bond to reach a stable electron arrangement, often an octet. In ionic bonding, a metal transfers electrons to a non-metal, as in Na⁺Cl⁻, and lattice energy holds the crystal together. In covalent bonding, atoms share electron pairs, shown by Lewis structures. Formal charge helps choose the best Lewis structure, and resonance describes molecules such as ozone and benzene that cannot be shown by one structure. Bond polarity arises from differences in electronegativity, giving molecules like HCl a dipole moment.
VSEPR theory predicts shape from the repulsion between electron pairs: CH₄ is tetrahedral, NH₃ is trigonal pyramidal and H₂O is bent. Valence bond theory explains shapes through hybridisation, such as sp³ in methane, sp² in ethene and sp in ethyne. Molecular orbital theory explains bond order and magnetism; it shows O₂ is paramagnetic with two unpaired electrons. Hydrogen bonding, a strong intermolecular force, explains the high boiling point of water.
Why it matters
Bonding explains why salt dissolves in water, why diamond is hard and graphite slippery, and why ice floats. Drug designers rely on molecular shape to make medicines fit target proteins. Hydrogen bonding holds the two strands of DNA together. This topic connects atomic structure with the properties of every compound studied later.
Key terms
- Ionic bond
- A bond formed by the transfer of electrons, creating oppositely charged ions that attract each other.
- Covalent bond
- A bond formed when two atoms share one or more pairs of electrons.
- Hybridisation
- The mixing of atomic orbitals to form new equivalent orbitals that explain molecular shapes.
- Hydrogen bond
- A strong attraction between a hydrogen atom bonded to N, O or F and a lone pair on another such atom.