Developing ideas of the atom

History of the atomic theory - Democritus

• First proposed that all matter is made of tiny, indivisible, and indestructible particles called "atomos" around 400 BC.

History of the atomic theory - John Dalton (1803)

• First wrote the atoms were the basic unit of chemistry

• He thought that atoms were spherical, and different spheres constituted different elements.

• Dalton thought atoms were the fundamental units of matter and were indivisible.

What was the name of John Dalton’s Model?

Solid sphere model

History of the atomic theory - J.J Thomson (1896)

• Discovered that atoms were divisible

• Discovered and measured the mass of electron

• Electron was measured to be several orders of magnitude lighter than an atom, proving that smaller particles than atoms existed.

• This lead to the plum pudding model, where the atom was viewed to be a large positively charged sphere, with embedded smaller, negatively charged, electrons.

What was the name of J.J Thomson’s model?

Plum-pudding model

History of the atomic theory - Ernest Rutherford (1911)

• Fired alpha particles at a thin sheet of gold

• If plum pudding model was true, the particles would have been deflected by the gold.

• Instead, most particles passed straight through the film.

• This led to the nuclear model, where the atom was viewed as being mostly empty space, with a positive nucleus and orbiting electrons.

History of the atomic theory - Niels Bohr (1913)

• The nuclear model had a problem; classically, an electron should spiral into the nucleus, and atoms should collapse.

• Niels Bohr solved this problem by developing the first quantum theory of the atom, with electrons assigned to fixed orbits of defined energy.

• When electrons move between orbits, they must emit or absorb electromagnetic radiation of a particular frequency.

Who came up with the Nuclear model?

Ernest Rutherford

Who discovered electrons?

J.J Thomson