The best way to understand geometry is by drawing the Lewis dot structure of the molecules.
The Lewis dot structure of #CO_2# is:
The Lewis dot structure of #SO_2# is:
The geometry of #CO_2# is linear (electron geometry is Linear too) because the two bonds are the furthest apart and the bond and the bond angle is #180^@#. This is what makes of #CO_2# a non polar molecule because the dipole moments of both bonds cancel out.
The geometry of #SO_2# is bent (electron geometry is trigonal planar) because the lone pair on the central Sulfur atom pushes the two bonds down for a bond angle of #120^@# due to the electrons repulsion. This is what makes of #SO_2# a polar molecule because the dipole moments of both bonds do not cancel out, and the molecule will have a net dipole moment.
