Credit for the development of the modern theory of chemical bonding belongs largely to the American chemist Linus Pauling (1901-1994).  He built an electron diffraction instrument in 1930 which allowed him to look at elements by their electronegativity.  Pauling explained that two atoms react to allow the lowest possible energy state. He developed the concept of electronegativity, which is the tendency of an atom to attract the electrons in a covalent bond. The difference in electronegativity can suggest whether the bond is more ionic (a small difference,) or more covalent   (a large difference).