Atomic Theory: Definition, History & Timeline

Instructor: Marion Carroll

Marion is a 30-year veteran of industry and academia primarily in the fields of biochemistry and genomics.

Imagine developing a concept without any physical evidence or laws to guide your theory! Early atomic theory relied solely on observation, hypothesis and experimentation. Let's visit some of the scientists and discoveries leading to what we know today.

What is the Atomic Theory?

A simplistic depiction of matter according to Aristotle: fire, air, water and earth

Atomic theory is what scientists have come to recognize as the explanation of how matter and energy cooperate to produce the elements and and their properties. Early philosophers Aristotle, Plato and Democritus disagreed on the concept of matter. Some like Aristotle held that invisible particles could not explain the formation of matter. Democritus (370 B.C.) however held to his belief that matter was more than just fire, air, earth and water.

The Eastern sage Acharya Kashyap Kanad, 600 B.C.

Democritus proposed what the philosopher and sage Acharya 'Kashyap' Kanad (600 B.C.) had described years before in his scientific work entitled 'Vaisheshik Darshan'. Indivisible particles called anu or atoms connect to make up the properties of matter. This was not determined empirically, but by inference from physical interactions that can be observed, much like time and space.

Isaac Newton and Robert Boyle, both 17th century physicists, laid the foundation of modern atomic theory. Newton described the attractive and repulsive forces of objects having some impact on pressure. Boyle actually defined this pressure within gases as the product of atomic forces. Using the scientific method, Boyle determined the quantitative properties of air, and other gases, under varying conditions: i.e. temperature, pressure and volume. Air was a significant experimental medium to work with as it was so abundant.

The scientific method involves a series of steps, including observation and hypothesis, which are tested before developing a theory or law. Boyle's discoveries led to laws that apply to most gases. He also deduced that elements were distinctly different from compounds; compounds consisted of more than one element. Separating and isolating components of compounds usually produced materials that were pure. This process, particularly as part of metallurgy, led to the understanding that atoms of a single purified element are identical.

Total pressure is the sum of the partial pressures of the individual gases.

Historical Moments

A number of events led to scientists defining the boundaries of atomic theory. Most notable include the discoveries made by John Dalton. John Dalton's publication of A New System of Chemical Philosophy (1808) framed the continued work on atomic theory in four tenets quoted below:

  1. Each element is made up of tiny particles called atoms.
  2. The atoms of a given element are identical; the atoms of different elements are different in some fundamental way(s).
  3. Chemical compounds are formed when atoms of different elements combine with each other. A given compound always has the same relative number and types of atoms.
  4. Chemical reactions involve reorganization of the atoms - changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

Dalton continues his study of elements by defining atomic weights, the total physical mass of protons and neutrons, and atomic numbers, the total number of protons which reflect the chemical as well as physical properties of neutral atoms.

Also significant was Dalton's understanding of the Proust Law of Definite Proportions a few years early. Proust's law explains how atoms are arranged in specific proportions within molecules, and how those molecules can then interact and participate in specific chemical reactions to form new compounds. For example, water consists of oxygen and hydrogen. It was Dalton's analysis of the ratio of oxygen to hydrogen, 8:1 that enabled the identification of hydrogen having an atomic number of 1 and oxygen's atomic number of 8.

Total mass is the sum of individual masses.


By the 20th century, the atomic theory had been thoroughly tested. Investigations began focusing on actually isolating the particles to visualize what had only been inferred by the behavior of elements and molecules. In 1926, Werner Heisenberg and Erwin Schrödinger interpreted the electrons in terms of mathematical wave functions, as part of the atomic model formed by Niels Bohr. Heisenberg's Uncertainty Principle and Schrödinger's wave experiments served to define and unify the wave vs. particle models that caused much debate surrounding the nature of atomic theory and atomic structure.

During this era of quantum mechanics, the principles of Dalton's Atomic Theory were being refined, enabling the theoretical models to be visualized by electron density mapping. Electron density can be determined physically by X-ray diffraction, and mathematically by radial probability. X-ray diffraction uses a beam of light to shine on an object; the 'shadow' cast by the object can then be assembled into the original object's shape. Radial probability is the measure of the atomic radius given various quantum numbers according to Niels Bohr's atomic radii structure. The probability of a given radii is used to estimate a shape or pattern for the electrons around the atomic nucleus.

Electron density of the blue field illustrates Heisenbergs Uncertainty Principle

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