Hydrochloric acid is a strong acid, whereas hydrofluoric acid is a weak acid. But what determines their strength? The strength of binary acids, with only two elements, is determined by bond energy and bond polarity. An acid with a higher bond energy has a stronger bond and, therefore, will be a weaker acid. Comparing acids in a group, the bond in a weak acid, like hydrofluoric acid, is harder to break, so the acid is less likely to donate protons. In contrast, an acid with lower bond energy has a weaker bond and, therefore, will be a stronger acid. For example, the bond in a strong acid, like hydrochloric acid, breaks more easily and donates protons more readily than hydrofluoric acid. A bond, like that in hydrochloric acid, is polar when one atom is more electronegative than the other atom. A compound can act as an acid when the atom attached to hydrogen has a higher electronegativity than hydrogen. The atom will have a partial negative charge, allowing the hydrogen to have a partial positive charge so it can be released as a proton. An acid with higher bond polarity has a weaker bond and, therefore, will be a stronger acid. Comparing compounds across a period, hydrochloric acid is stronger than hydrogen sulfide as chlorine is more electronegative than sulfur and, therefore, releases protons more easily. If hydrogen has an equal or greater electronegativity than the other atom, that molecule can not donate protons and, therefore, can not act as an acid. Oxyacids are acids where an OH is attached to a third atom that is more electronegative than hydrogen. The strength of an oxyacid depends upon the electronegativity and the number of oxygens attached to the third atom. The higher the electronegativity of the atom, the more it polarizes, weakening the bond between the oxygen and hydrogen. If the central atom is attached to additional oxygen atoms, it further increases the polarity of the bond between the oxygen and the hydrogen. For example, perchloric acid with three additional oxygen atoms is stronger than chloric acid with two additional oxygen atoms. Chloric acid, in turn, is stronger than chlorous acid, which has only one additional oxygen and hypochlorous acid, with no additional oxygen atoms. Carboxylic acids are weak acids that contain a carboxyl group. The second oxygen atom makes the oxygen-hydrogen bond more polar, and thereby allows the molecule to donate a proton. Acetic acid and formic acid are examples of carboxylic acids.