Minimize formal charges.

Place negative formal charges on more electronegative atoms.

Avoid adjacent atoms with same formal charge.

4.2. Resonance: The Delocalized "Average"

When a single Lewis structure is insufficient. The actual structure is a hybrid of two or more contributing (canonical) structures, differing only in electron placement.

Resonance Energy: The actual molecule is more stable (lower energy) than any single canonical structure. The energy difference is resonance energy.
Rules for Contributing Structures:
1. Same atomic connectivity.
2. Same number of valence electrons.
3. Obey octet rule (as much as possible).
4. Better structures have: more covalent bonds, less charge separation, negative charge on more EN atom, positive charge on less EN atom.
Examples: $CO_3^{2-}$, $SO_2$, $NO_3^-$, Benzene. Explains equal bond lengths (e.g., C-O bonds in carbonate).

5. Theories of Bonding: Explaining the "How"

5.1. Valence Shell Electron Pair Repulsion (VSEPR) Theory: Predicting 3D Shape

Electron domains (lone pairs and bond pairs) repel each other and arrange to maximize separation, dictating electron and molecular geometry. This is your primary tool for predicting shape.

Draw Lewis structure.
Calculate Steric Number (SN) = (Number of $\sigma$ bonds) + (Number of lone pairs) on the central atom.
Determine Electron Geometry (arrangement of electron domains).
Determine Molecular Geometry (arrangement of atoms only).

SN	Electron Geometry	Lone Pairs	Molecular Geometry	Ideal Angle	Example
2	Linear	0	Linear	$180^\circ$	$BeCl_2$, $CO_2$
3	Trigonal Planar	0	Trigonal Planar	$120^\circ$	$BF_3$, $SO_3$
3	Trigonal Planar	1	Bent / V-shaped	$<120^\circ$	$SO_2$, $O_3$
4	Tetrahedral	0	Tetrahedral	$109.5^\circ$	$CH_4$, $NH_4^+$
4	Tetrahedral	1	Trigonal Pyramidal	$107^\circ$	$NH_3$, $PCl_3$
4	Tetrahedral	2	Bent / V-shaped	$104.5^\circ$	$H_2O$, $SCl_2$
5	Trigonal Bipyramidal	0	Trigonal Bipyramidal	$120^\circ(\text{eq}), 90^\circ(\text{ax})$	$PCl_5$, $AsF_5$
5	Trigonal Bipyramidal	1	Seesaw	$<120^\circ, <90^\circ$	$SF_4$
5	Trigonal Bipyramidal	2	T-shaped	$<90^\circ$	$ClF_3$
5	Trigonal Bipyramidal	3	Linear	$180^\circ$	$XeF_2$, $I_3^-$
6	Octahedral	0	Octahedral	$90^\circ$	$SF_6$, $PF_6^-$
6	Octahedral	1	Square Pyramidal	$<90^\circ$	$BrF_5$, $IF_5$
6	Octahedral	2	Square Planar	$90^\circ$	$XeF_4$, $ICl_4^-$
7	Pentagonal Bipyramidal	0	Pentagonal Bipyramidal	$72^\circ(\text{eq}), 90^\circ(\text{ax})$	$IF_7$

Repulsion Order: Lone Pair-Lone Pair (LP-LP) > Lone Pair-Bond Pai