A simple explanation of a chemical bond was given by G. N. Lewis in 1914: A bond is formed between two atoms when a pair of electrons is shared between them. This is the electron-pair, or covalent bond, which is the subject of Chapter 4.

Two hydrogen atoms, each with a single electron, can share their electrons and form a covalent bond, as shown at center left. If you were to perform a quantum-mechanical calculation to see how the electrons in an H-H bond were distributed, you would find that most of the time they are between the two H nuclei.

One positive nucleus is attracted to the two electrons, which simultaneously attract the other nucleus. At the same time, the two electrons shield or screen the nuclei from one another and decrease the repulsion between their positive charges.

The negatively charged electrons are the "glue" that holds the positive nuclei together.