Understanding the electron was a key discovery in understanding how elements interact or chemically bond with each other.  It was puzzling to have neutral particles to come together as proposed by earlier atomic theories.  The discovery of electrons by J.J. Thomson provided clues about how bonding occurred.

In 1852, E. Frankland (1825-1899), an English chemist first proposed the concept of a valence.  Different elements had specific number of electrons in their outer or valence shell.  German chemist August Kekule (1829-1896) was first to propose the term “covalent bond”  when describing the large molecule that formed due to multiple chemical bonds of carbon.  Scottish chemist Archibald Scott Couper (1831-1892) independently discovered self-linking carbons.  They proposed that carbon had a valence of four and shared electrons.  This was the first description of covalent bonding. 

Couper also proposed a way to write the coupling on paper by using a straight line linking symbol for atoms which is still in practice today.  For example, a molecule of water could be represented by the structural formula: H-O-H. The arrangement of atoms in a molecule is represented by the symbols of the elements present joined by dashed lines that show how the atoms of those elements are bonded to each other.  This is referred to as the Couper dash system.