© Left: Wikimedia Commons, Public Domain. Right: Lawrence Berkeley National Laboratory.
In the period around 1870, Dmitri Mendeleev (left) created the first periodic table by arranging the chemical elements in a manner not only useful for understanding the relationships between the elements, but also for predicting the existence of as yet undiscovered elements. Gilbert Newton Lewis (right) developed an empirical shell model in the period from 1902 to 1916. Using data based on the periodic table and the observed bonding properties of the atomic elements to form molecules, Lewis proposed that it was the electrons in atoms that gave rise to their physical and chemical properties. In doing so, he developed the idea of filled shells (or kernels, as he called them) of two, and then eight, electrons as leading to special stability, and also the idea of the shared pair covalent bond, which are discussed later in this unit. (Unit: 6)