History of Atoms

Democritus

460 BCE

Democritus had the first concept of the atom. He believed that if you cut something smaller, and smaller, it had to reach a point where it couldn't be cut any further, and you would come to an indivisible particle, which he called an atom. He built on this idea that everything was not made up of water and air, but of atoms. He was also a central figure in the development of the atomic theory of the universe.

Aristotle

384 BCE

He did not believe in atomic theory, unlike Democritus, and thought that all elements on Earth were not made of atoms except for Earth, Fire, Water and Air, themselves. This was believed to be true until other scientists started experimenting with chemical reactions.

Antoine Lavoisier

1743

Antoine Lavoisier investigated the changes that took place when two chemicals reacted and formed a new compound. He weighed the chemicals before and after the reaction, and he found the total mass of the chemicals were the same as the compound that was produced. After doing the same thing a few more times, he came to the conclusion that Matter is neither created nor destroyed during a chemical reaction.-law of conservation of mass.

Joseph Proust

1754

Proust found when he broke up copper carbonate into its elements, and then weighed them, they always contained the same proportions. He found that other substances were made from different amounts of elements, and these amounts were always the same too, no matter how large or how small the numbers of elements were used. From his work, he devised the law of definite proportions, which states that a given chemical compound always contains the same elements in the exact same proportions by mass.

Marie Anne

1758

Marie Ann was Antoine Lavoisier’s wife. She acted as his laboratory companion, using her linguistic skills to write up his work and bring it to an international audience. She was known as the mother of modern chemistry.

John Dalton

1766

John Dalton discovered atoms in 1804, and he came up with the atomic theory. He thought they were the smallest particles of matter, which could not be broken down into smaller particles. He envisioned them as solid, hard spheres. His theory helped scientists at the time, but research showed that the theory was not completely correct.

J.J. Thomson

1856

J.J. Thomson helped revolutionise the knowledge of the atomic structure with his discovery of the electron. He discovered it with the help of a cathode ray tube. A cathode ray tube is a glass tube from which all of the air has been removed. It contains a piece of metal called an electrode at each end. One electrode is negatively charged and known as a cathode. The other electrode is positively charged and known as an anode. He tested to find out what the ray was.

J.J. Thomson- Continued

1856

To test out what the ray was, he placed negative and positive plates along the sides of the cathode ray tube to see how the cathode ray would be affected. The cathode ray appeared to be repelled by the negative plate and attracted by the positive plate. This meant that the ray was negative in charge and that it must consist of particles that have mass. He called the particles “corpuscles,” but they were later renamed electrons.

The Plum Pudding Model

1904

In 1904, J.J. Thomson devised a model of the atom he called the plum pudding model. He knew that electrons were negatively charged, and that atoms were neutral, so he said that something had to balance out the electron, which would have to be a positive substance in the atom. He described the atom as a plum pudding, with negatively charged particles being surrounded by positively charged “pudding.”