Lipids are made up of long hydrocarbon chains, composed of all carbon and hydrogen atoms. this characteristic makes lipids insoluble in water ("oil and water don't mix") because:

This is because their tails are hydrophobic and their heads are hydrophilic. Hydrophobic meaning dislikes being near water and hydrophilic meaning likes to be near water. Therefore, they will orientate themselves in such a manner that the heads are facing externally and all the tails are facing together protected by the hydrophilic heads. google a photo of lipid chains in water if you are still confused. I'm not sure if that is what you are asking, but I hope it helps.

The correct answer is "atomic weight".