Fluorine is the only element that is more electronegative than oxygen. It is the only substance which, when combined with oxygen, pulls the bonding electron pair away from oxygen and toward itself. It has seven electrons in its outer shell and needs only one more to complete the stable octet. Fluorine gas consists of F₂ molecules with a single F-F bond between atoms. Its hydrogen compound is hydrogen fluoride, HF, in which the F atom is surrounded by three lone pairs and one bonding pair (see right). The H-F bond is more polar than the H-O bond, being 41% ionic compared with 33%. But because HF has only one polar bond, whereas H₂0 has two, the overall dipole moment is smaller, 1.82 D compared to 1.85 D for water. With only one proton, HF can make only one hydrogen bond to another molecule, so the liquid is not as tightly "stitched" together as water is. Therefore HF boils at 19C, whereas water boils at 100C (both values at 1 atm pressure). Neither ammonia nor HF can form the elaborate open-cage structures found in ice. Ammonia can't because it has only one lone pair available for receiving a hydrogen bond, and HF can't because it has only one proton with which to make a hydrogen bond. Water has the fortunate combination of two protons for hydrogen bonding and two lone pairs to receive such bonds from neighbors. The result is the three-dimensional framework structure of ice.