Introduction

The electron shell model for the atom is relatively modern; the model wasn’t possible until the advent of quantum mechanics in the 1930’s. The purpose of this post is to provide some historical background beyond what you’ll find in your textbook.

The Greeks

Perhaps the first mention of atoms may be attributeable to Leucippus, and his student, Democritus (born 460 BCE). Democritus held that matter could not be infinitely divided, and that atoms were the ultimate basis of matter (including human souls). This theory of atoms was in part, a response to Parmenidies and Zeno, who held that the material that formed the universe was an infinite, all encompassing mass that contained no empty space.

Dalton’s Atomic Theory

In 1803 John Dalton proposed a new atomic theory. His theory stated that

  1. All matter was made up of small, indivisible particles called atoms.
  2. Atoms of a given element posses unique properties and weight.
  3. Three types of atoms exist: simple (elements); compound (simple molecules) and complex (complex molecules).

The Nuclear Atom

In 1911 Ernest Rutherford found that when a stream of alpha particles (\alpha particles are helium nuclei) was shot at a gold film, some of the \alpha particles bounced back! He famously stated that this result was as shocking as “if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you.”

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This result required a rethinking of the “plum pudding” atomic model, in which negatively charged electrons were distributed in a positively charged “gel”:

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If the plum pudding model were correct, then the \alpha particles should have gone straight through the foil. The fact that some of the positiviely charged particles were deflected meant that there must be some small, positively charged particles in the foil. A nuclear model of the atom was then proposed.

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Quantum Mechanics

The modern electron shell model of the atom was first developed by Niels Bohr in the early 1920’s. Bohr showed that the energies of electrons in atoms were quantized. In other words, electrons could only have specific energies, or orbits.

As quantum theory developed, the notion of orbits was abandoned in favor of atomic orbitals – maps of the most probable location of an electron in an atom.

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