Atomic Theory Development

Development of the Atomic Theory


Approx. 460 BCE

Democritus was an ancient Greek philosopher in the 5th century BC. He believed that if you were to break something and then break those pieces, over and over again, you would eventually get to a piece so small, it would be microscopic. He called these pieces “atomos”, the Latin word for indivisible. Democritus theorised that:
- All matter consists of invisible particles called atoms.
- Atoms are indestructible.
- Atoms are solid but invisible.
- Atoms are homogenous (of the same kind/alike).
- Atoms differ in size, size, shape, position and arrangement.

John Dalton

Approx. 1803

Throughout the late 1700s, multiple scientists had a crack at filling in the blanks of the past model set by Democritus, but it wasn’t until John Dalton made any major improvements/adjustments in 1803 to the model. Dalton’s atomic theory stated five basic assumptions:
- All matter consists of tiny particles called atoms.
- Atoms are indestructible and unchangeable.
- Elements are characterised by the weight of their atoms.
- In chemical reactions, atoms combine in small, whole number ratios.
- When elements react, their atoms may combine in more than one whole number ratio.

J.J. Thomson

Approx. 1900

Around 1900, J.J. Thomson, found that there were other particles smaller than an atom called electrons, that were negatively charged through completing multiple experiments. His advancements along with Goldstein in 1886, concluded that within an atom, there are both positively charged protons and negatively charged electrons.
In years to come, Thomson put forward the first model of the arrangement of particles. His model stated that an atom is a positively charged sphere and electrons are set within the sphere. This was known as the plum pudding model and is still known as that today. It was called this because he compared the sphere to the actual pudding and the randomly distributed electrons to the random pieces of fruit within it.

Ernest Rutherford

Approx. 1902

In 1902, Ernest Rutherford discovered through thorough experimentation with alpha particles that an atom must have one concentrated area of positive charge within the larger atom. He called this area the nucleus. He concluded that most of the atom was simply, empty space.

Niels Bohr

Approx. 1913

In 1913, Niels Bohr went to study with Ernest Rutherford. Using mathematics and Rutherford’s past discoveries, Bohr found the most likely position for electrons in an atom. Bohr’s resulting model commonly referred to as the planetary model or the Bohr model showed points on round rings representing the electrons in an orbit around the central nucleus. He found that each orbit can have a specific number of electrons in it.

James Chadwick


Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles/radiation. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists.