The geometrical shapes of some simple molecules can be predicted based on the following general patterns:

Molecular Shapes with Two Electron Sets (Linear Arrangement-AX₂ type)

The repulsion of two electron groups with each other results in the assignment on opposite sides of the central atom in a straight line. The linear arrangement of electron sets results in a linear molecular shape and bond angle of 180°. All AX2 type molecules or ions are linear in geometry. The examples of the molecules of this type are CO2, BeCl2, CS2 , HCN, etc.

Molecular shapes with three electron sets (Trigonal Planar Arrangement, AX₃ type)

Three electron sets around the central atom repel each other to lie at the corners of an equilateral triangle. This is the trigonal planar arrangement and the ideal bond angle is 120°: Example: BF3, NO3-, HCHO (formaldehyde) etc. Another molecular shape is possible within this electron set arrangement, with two bonding and one lone pair (AX2E type). The examples of this type include PbCl2, SnBr2, SO2, O3, etc. They have bent (V-shaped or angular) geometry. This is the first example of the effect of a lone-pair on adjacent bonding-pairs. Since a lone-pair is held on the central atom, it exerts a stronger repulsion than bonding pair.

Boron trifluoride contains three covalent bonds, or bonding pairs. In the most stable arrangement, the three BF bonds point to the corners of an equilateral triangle with B in the center of the triangle:

Molecular Shapes with Four Electron Sets (Tetrahedral Arrangement, AX₄ Type)

Note that AX3E and AX2E2 types are with four electron sets like AX4 type but differ in shape and bond angle because of the presence of lone pair(s) on the central atom.

In three dimensions, the four electron sets can move farther apart and point to the corners of a tetrahedron, giving a bond angle of 109.5^o. Some of the examples of this type are CH₄, NH₄⁺ , SO₄²⁻. When one of the four electron sets in the tetrahedral arrangement is a lone pair, the molecular shape is that of a trigonal pyramid (AX_` E type). Recall that one of the shapes in the trigonal planar arrangement - that with two bonding sets and one lone pair-is also called angular or bent or V-shaped (AX₂E type), but it is ideal bond angle is 120°, not 109.5°. Water is the most important V-shaped molecule in the tetrahedral arrangement of electrons. We would expect the repulsions between its two lone-pairs have a greater effect on bond angle than the repulsions from the single lone pair in NH₃, and observation confirms this: two lone-pairs on the central O atom compress the H–O–H bond angle to 104.5°.