Bases are substances that accept protons from acids.
Let's look at an example. Its two atoms are held together by a shared pair of electrons. However, the chlorine atom is so much more electronegative than hydrogen, that the bond between them is polar covalent . Link to a discussion of electronegativity and its role in bond formation. When hydrogen chloride is bubbled through water, the nucleus of the hydrogen atom leaves and takes up residence at one of the unshared pairs of electrons in the water molecule.