History of the Atom

The first discovery of the Atom

450 BCE

Democritus wondered what would happen if you kept cutting an apple in half, you would eventually reach a point where you can’t cut anymore. He decided that Invisible particles make up all life on Earth. He named them Atomos which means ‘uncuttable’. His ideas ended up being forgotten for 2000 years.

The return of Democritus’s theory and the first Atomic theory

1810

John Dalton brought back Democritus’s ideas and created the first atomic theory. The atomic theory Dalton developed consists of three ideas:
• All substances are made of atoms.
• All atoms of the same element are alike and have the same mass.
• Atoms join together to form compounds.
Dalton incorrectly thought that atoms are tiny solid particles of matter. He used solid wooden balls to model them. His models were too simple and didn’t show the subatomic particles.

First subatomic particles discovered

1897

J. J. Thompson discovered the first subatomic particle. He named them electrons. He thought that atoms were like plum pudding with plums (or electrons in this case) spread throughout it. He also thought that the empty space inside the atom was positively charged. He knew that electrons were negatively charged.

The discovery of the Nucleus and the Protons

1911

Ernest Rutherford discovered many things about the Atom. He discovered the Nucleus, the Protons and Atoms are not solid. Later, he discovered that the nucleus contains positively charged particles. He named the positive particles protons. He predicted the existence of Nutrons but could prove their existence. He also thought that electrons orbit the Nucleus at random.

The Model of the Atom is updated

1915

Niels Bohr thought that electrons are located at fixed distances away from the Nucleus.

The discovery of the Neutron.

1932

James Chadwick discovered the Neutron

The Electron Cloud

2000

The electron cloud is the area where people thought electrons are likely to be.