Reality → Matter → Molecules → Bonds
There are two main types of atomic bonds: covalent bonds and ionic bonds [1] . In covalent bonding, atoms share a pair of electrons, whereby each of the two bonded atoms shares one of its outer electrons with the other atom. Covalent bonding plays a major role in organic chemistry and in biochemistry, mainly due to the structure and properties of the carbon atom [2] . A special, much weaker type of covalent bonding is the hydrogen bond, which acts between water molecules and parts of macromolecules [3] . The ionic bond is a strong bond based on the electrostatic force acting between a cation (a positively charged atom) and an anion (a negatively charged atom). This is the classical chemical bond between a metal atom (cation, electron donator) and a non-metal atom (anion, electron receiver) which together form a salt molecule. A special type of bonding is present in metals [4] . The number of bonds an atom can make is determined by the number of valence electrons, which normally (for main group elements) are the electrons in the outermost ‘shell’.
The type of bond is largely determined by the complex electromagnetic interaction of the outer electron orbitals of matching atoms or molecules, described by Pauling with the concept of electronegativity (see also hybridization and ionization energy). X-ray crystallography and theoretical models (e.g., VSEPR theory) try to unravel the molecular structure (see Pauling's Nobel Prize award for the difficulties involved).
Intramolecular hydrogen bonds, as well as the still weaker van der Waals force, play an important role in the structure of DNA and the folding of proteins.
Metals do not consist of electrically neutral molecules. Instead, positively charged nuclei are held together in a tight bond by a common cloud of free electrons providing the metallic bond and electric conductivity.