Molecular Structure and Simple Covalent Bonding Models
Covalent bonding can be thought of as the situation when groups of atoms
stay associated with each other independent of the phase
Valence Bond Theory: bonding occurs when pairs of electrons accumulate between nuclei
Why?
Paired electrons screen the nuclear charge, so nuclear-electron attraction
overcomes electron-electron repulsion giving a net attraction of nuclei.
Lewis Dot structures
Electrons are distributed about atoms pictorially.
Only use valence electrons.
Lines represent an electron pair, either as a bond or a lone pair.
The formal charge about each atom should be minimized (neutral is best).
The octet "rule" works well for C, N, O, F but otherwise is merely
advisory.
Resonance is a superposition of degenerate (equivalent) VB structures
- this helps minimize charge distribution.
The net structure is a weighted average of all possible dot structures; the weighting is based on the relative energy of each resonance structure: low-energy structures contribute a lot and high-energy structures contribute little.
Neutral structure perhaps contributes less than ionic structures (Why?)