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Historical models of the atom


Is matter continuous and infinitely divisible?
Or is matter divisible only until a basic, tiny particle is reached?
This was a question discussed by the ancient Greek philosophers.


Democritus (Greek: Δημόκριτος, meaning “chosen of the people”; c. 460 – c. 370 BCE)

Ancient Greek pre-Socratic philosopher remembered today for his formulation of an atomic theory of the universe.

He proposed that all matter was composed of small indivisible particles called atoms.

Aristotle (330 BCE) asserted that the elements of fire, air, earth, and water were not made of atoms, but were continuous.

One ancient idea was that there were four basic elements:


Why would ancient thinkers believe that there were four elements?

Think of what they observed when wood burns: Fire being released, smoke (“air”), and after the burning was done you would have ashes (“earth”).

Water being the opposite of fire would extinguish it.

One could even make a rudimentary chemical equation to justify this idea.

Wood (earth/air/fire) —> Earth (ashes) +air (smoke) + fire (flame)

Over time we realized that thinking about this subject wasn’t enough. We needed to learn more through experimentations.

Arabic ideas and models of the atom


Medieval ideas and models of the atom



The Scientific Revolution

By the 1700s scientists defined an element as: a substance that cannot be further broken down by ordinary chemical means.

They also assumed that elements combined to form compounds that have different physical and chemical properties than those of the elements that make them.

By 1800 they had discovered the basic laws of modern day chemistry. For instance, they discovered the law of conservation of mass – mass is neither created nor destroyed during ordinary chemical reactions or physical changes.

Law of conversation of mass CHEMISTRY

Soon after they discovered the law of multiple proportions:

Compounds are made of a fixed proportion of elements.

For example, NaCl (sodium chloride, table salt) always consists of 39.34% by mass of the element sodium, Na, and 60.66% by mass of the element chlorine, Cl.

Law of multiple proportions

John Dalton (1766-1844 CE)

In 1808, an English schoolteacher named John Dalton proposed an explanation that encompassed all observations:

Elements were composed of atoms. Only whole numbers of atoms can combine to form compounds.

His theory is summarized here:

1. All matter is composed of extremely small particles – atoms.

2. Atoms of an element are identical in size, mass, and other properties;
atoms of different elements differ in size, mass, and other properties.

3. Atoms cannot be subdivided, created, or destroyed.

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

5. In chemical reactions, atoms are combined, separated, or rearranged.

  • Adapted from Modern Chemistry, Raymond Davis et al, Holt, Rinehart an Winston


When Dalton proposed his model electrons and the nucleus were unknown.

John Dalton's A New System of Chemical Philosophy (1808)


J. J. Thomson, 1897

Discover of the electron

Thomson discovery electron

Thomson discovery of the electron


Here’s one way to model the atom.


Ernest Rutherford experiment in 1899


At this point, here is how they imagined that they atom worked.

Neils Bohr, first quantum model of the atom

Borh discovered:

In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus.

To remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits.

Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump.

Flaw in classical model electron orbits

Here’s a model of what happens when an electron “falls” from one orbital down to a lower energy orbital. As the e- loses energy, this energy is converted into a photon.

Here we see what happens when a photon (light particle) interacts with an atom.



Erwin Schrödinger, electron cloud model, 1920s

Erwin Rudolf Josef Alexander Schrödinger animated GIF

Erwin Schrodinger


Werner Heisenberg
matrix based version of quantum mechanics

Section tba


Early development of our understanding of the Atom
Historical development of atomic models

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