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450 BCE
Democritus
Approx. 450 BC
He wondered what would happen if you kept cutting an apple into smaller pieces - you would reach a point where you couldn’t cut anymore.
Invisible particles make up all life on Earth
Atomos means ‘uncuttable
His ideas were forgotten for 2000 years -
John Dalton
The atomic theory Dalton developed consists of three ideas:
All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles. They also cannot be created or destroyed.
All atoms of the same element are alike and have the same mass. Atoms of different elements are different and have different masses.
Atoms join together to form compounds. A given compound always consists of the same kinds of atoms in the same ratio. -
J. J. Thomson.
1897
First subatomic particles discovered - electrons
Atom is like a plum pudding, which has plums (electrons) scattered through it. -
Ernest Rutherford
Discovered the nucleus
Atoms not solid
Conducted a gold foil experiment
Later, he discovered that the nucleus contains positively charged particles. He named the positive particles protons.
He predicted the existence of neutrons but failed to find them.
He thought electrons orbit the nucleus at random. -
James Chadwick
Discovered the neutron -
Niels Bohr
orbits of electrons are located at fixed distances from the nucleus - energy levels -
Today: Electron Cloud Model
The electron cloud is an area around the nucleus where electrons are likely to be.
Orbitals in the cloud show where electrons are most likely to be.
