Fundamentals of the Quantum Theory of Chemical Bonding

Abstract

As we know from chemistry, many atoms can combine to form particular molecules, e.g. chlorine and sodium atoms form NaCl molecules. But atoms of the same type can also form bonds, as, for example in the case of hydrogen H₂. Before the development of quantum theory, the explanation of chemical bonding was a puzzle to chemists and physicists alike. Bonding between ions, as in the negatively charged chlorine ion and the positively charged sodium ion, could, to be sure, be understood in the hght of the Coulomb attraction between oppositely charged bodies; it remained, however, inexplicable that two similar atoms, which are electrically neutral (as, for example two hydrogen atoms) could form a bound state (homopolar bonding). Here it only became possible with the aid of quantum mechanics to attain a fundamental understanding. Even in the case of ionic bonding (also called heteropolar bonding), basic new insights have been obtained through quantum theory. For example, it must be understood why the ions form in the first place, and why the electron which is transferred from sodium to chlorine thus finds an energetically more favourable state.