Unit 4
Chemical bonds
Why do atoms join at all? In table salt, sodium and chlorine are held by opposite charges; in hydrogen, two atoms share a pair of electrons. This unit turns “bonding” from a slogan into an energy account you can compute and check — with potential-energy curves, Born–Haber cycles, Lewis structures and electronegativity.
- 1 Ionic bonding: opposite charges together Moving an electron from sodium to chlorine costs energy — so why does salt form at all? The answer is the Coulomb attraction between ions, and above all the collective attraction of a whole crystal. 30 min
- 2 Covalent bonding: sharing a pair Why does the energy of two approaching hydrogen atoms first fall and then rise? Read bond length, bond energy and zero-point energy off a curve computed from spectroscopic data, then see how bond order changes length and strength. 25 min
- 3 Lewis structures and formal charge Count the valence electrons and draw Lewis structures step by step; use formal charge to pick the most reasonable one, and meet resonance, electron deficiency and the expanded octet. 35 min
- 4 Bond polarity and electronegativity A shared pair is not always shared equally. The electronegativity difference decides which way the electron cloud leans and how far — from H–H to Cs–F, covalent and ionic are ends of one continuum. 25 min