Shapes of molecules

Draw the Lewis structure, then count the bond pairs and lone pairs on the central atom. Their total gives the electron geometry (2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral), and the lone pairs decide which corners hold atoms, which gives the shape.

In CO2 the carbon has two double bonds and no lone pair, so the two bonds point opposite each other: linear. In H2O the oxygen has two bonds and two lone pairs, four pairs in a tetrahedron, so the two O-H bonds make a bent shape at 104.5 degrees.

A molecule is non-polar when its bond dipoles cancel: linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral and square planar shapes with identical outer atoms, such as CO2, BF3, CH4, PCl5, SF6 and XeF4. Bent, pyramidal, see-saw, T-shaped and square pyramidal shapes are polar.