Let's begin by defining what a covalent bond is. A covalent bond is a bond in which we have the sharing of electrons. If the sharing is equal between two atoms, we have a non polar covalent. If the sharing is not equal, if it's slanted to one side, then we have a polar covalent bond. So let's suppose we're considering the following molecule. In this biological molecule that exists on DNA molecules and RNA molecules, this guanine molecule, we have four types of atoms that are arranged to form the molecule.