I am unsure with regards to the specific reason why alkanes are non-polar as I have heard different explanations. My teacher said that it is because, although there is a difference in electronegativity between carbon and hydrogen atoms, the carbon is shielded by the hydrogen atoms from reacting with the external environment and therefore, since only the hydrogen can react, it is non-polar (something along the lines of it is the difference between delta + and delta - ends that actually make a molecule polar). From searching on the internet, however, I have found the explanation that it is because the difference in electronegativity is so small (0.3) that it is non-polar, which makes sense, however it contradicts what my textbook states: That only bonds between elements with equal electronegativity are non-polar. Can someone clear this up for me? Thank you!
There are two reasons why molecules are polar or not: the symmetry of the molecule and the polarity of the bonds in it.
Bonds can be thought to have electric dipoles if the elements in the bonds are different. Bonds between more polar elements will have stronger dipoles. But, in a molecule with many bonds, you need to know the overall structure to know whether the molecule is polar or not. Imagine taking all the individual bond dipoles and adding them up (as vectors, in other words taking account of the direction they point as well as the size of the dipole).
So polar bonds are not enough to make a molecule polar. Carbon tetrachloride has 4 fairly polar bonds but is non-polar because the dipoles in each bond cancel each other out because of symmetry. Many simple hydrocarbons are non-polar for this reason: they have enough symmetry that any polarity in their C-H bonds cancel out. Many simple hydrocarbons are too symmetric to have any polarity, but some are not: isobutane, HC(CH3)3 is polar (dipole moment 0.13 D) because it isn't symmetric and the slight dipoles in its C-H bonds don't cancel out. But compare to chloromethane which is asymmetric and has a very polar bond: it is fairly polar (dipole moment 1.9 D) as it is dominated by just one polar C-Cl bond.
But even the non-symmetric hydrocarbons are not very polar compared to things like chlorinated hydrocarbons so are often grouped with completely non-polar compounds.