A hyrogen bond is a special type of dipole-dipole force that exists between an electronegative atom and a hydrogen atom bonded to Nitrogen, Oxygen or Fluorine. This type of force always involves a hydrogen atom and the energy of this attraction is close to that of weak covalent bonds (155 kJ/mol), thus the name - Hydrogen Bonding. A covalent bond is only 20 times stronger than an intermolecular hydrogen bond. These attractions can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly). The hydrogen bond is a very strong fixed dipole-dipole van der Waals-Keesom force, but weaker than covalent, ionic and metallic bonds. The hydrogen bond is somewhere between a covalent bond and an electrostatic intermolecular attraction.