4.1.1.3 The development of the model of the atom

Atomic theory

\- Democritus (500 BC)

Everything made from tiny little particles that can’t be broken down further

Separated from each other by empty space

John Dalton

John Dalton- 1800s

Describes atoms as solids spheres

Different types of Spheres make up different elements

Plum pudding model

JJ Thompson

Couldn’t be solid spheres( must have negatively charged particles)

Proposed the atom was a ball of positive charged with electrons/ negative charge in embedded in it

Gold foil expirement

Ernest Rutherford -1909

took positively charged alpha particles and fired them at a thin sheet of gold foil

If atom was like plum pudding alpha particles would pass straight through

Conclusions of the alpha scattering experiment

* Most particles went straight through so most of atom is empty space
* Some particles are slightly deflected the nucleus must be positive repel a positive alpha particles
* Are you part of the deflected by more than 90° the nucleus contains most of the mass

Nuclear model

Compact nucleus (containing all positive charge)

Negative charge exists in a cloud around central nucleus

Niels Bohr

Niels Bohr- 1913

Electrons orbit nucleus- stops atom collapsing

Held in shells at specific distance

Ernest Rutherford 2

Positive charge in the nucleus is small discreet particles (protons)

positive​ ​charge​ ​of​ ​any​ ​nucleus​ ​could​ ​be​ ​subdivided​ ​into​ ​a​ ​whole number​ ​of​ ​smaller​ ​particles,​ ​each​ ​particle​ ​having​ ​the​ ​same​ ​amount​ ​of​ ​positive​ ​charge (protons)

James Chadwick

Discovered neutrons within nucleus