Question

How do you know i a molecule is polar or not?

Answer 1

It all has to do with the dipole moment of the molecule. If the individual vector dipoles of the atoms that make up the molecule cancel each other out then the molecule is not polar. If there is a net dipole moment then the molecule is polar. The dipole vectors of CO2 cancel each other out to create a net-zero dipole moment. The H2O, however, has a net (summed) dipole moment with the oxygen 'part' begin delta negative and the hydrogen 'part' delta positive. Also, you could know that linear molecules have no net dipole moment and are thus non-polar [ie CO2]. And bent molecules have a dipole moment and are thus polar [ie H2O]. Your basic approach should be to use the atom's electronegativity to determine the dipole moments and sum their vectors to see what the resulting net vector (and thus net dipole moment) is.