The first molecule is a sensible molecule having complete octet of each atom such as C, H and O whereas the second molecule having hydrogen present between the aldehyde and methyl group and thus showing hydrogen is making bond with aldehyde and methyl as well which is not possible because hydrogen only having one electron in its octet due to which it can only form a single bond by sharing its valence electron.
what is a non-polar molecule? Atoms are made of small particles. When atoms bond together to form molecules. They share or give electrons. If the electrons are shared equally by the atoms, then there is no resulting charge making the molecule nonpolar. Polar molecules are the complete opposite.