Chemistry · Chemical bonding. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
VSEPR theory predicts the shape of a molecule from the electron pairs around its central atom. The pairs repel each other and move as far apart as possible. Lone pairs repel more strongly than bond pairs, so they squeeze the bond angles. A double or triple bond counts as one region of electrons.
Valence bond theory says a covalent bond forms when half-filled atomic orbitals overlap and the two electrons pair up. Head-on overlap gives a sigma (σ) bond; sideways overlap of parallel p orbitals gives a pi (π) bond.
In hybridisation, atomic orbitals of one atom mix to form the same number of new, equivalent orbitals. This explains why the four bonds of CH4 are identical.
Students name the shape from all the electron pairs instead of from the atoms. NH3 has four pairs arranged tetrahedrally, but its shape is trigonal pyramidal because one pair is a lone pair.
Answer: sp3, sp2, sp2; 8 σ bonds and 1 π bond.
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