A nonpolar covalent bond is unlikely when two different atoms join because the atoms are likely to differ in?
It’s easy! Two different atoms will have different electronegativities. Now, we know that electronegativity is the characteristic of an atom to pull the bonded electrons towards itself when present in a bonded state(with other atom or atoms). Polar covalent bonds are those where the bonded electrons cling towards the more electronegative atoms (instead of staying between the bonded atoms). Non-polar covalent bonds are only formed between molecules made up of similar types of atoms (examples are oxygen molecule,hydrogen molecule,nitrogen molecule,etc.)
Now, if both the atoms are same,their electronegativities will also be the same,as a result of which the bonded electrons will stay at the middle of the bonded atoms (and not cling towards any atom).