C1.5 - History Of An Atom

• first theory of an atoms • dated early 1800s • believed an atom was a tiny sphere that couldn’t be divided

• second theory = dated end of 1800s • aka ‘plum pudding theory’ • electron was discovered -> negatively charged particles were found • scientists the sphere had a positive charge and electrons were spread throughout it

• third theory = dated 1908-1913 • proposed the nuclear model • proposed that electrons orbited the nucleus • discovered the positively charged nucleus • Rutherford designed an experiment which was carried out by Geiger and Marsden • Experiment involved an atom of gold foil and alpha particles

• they fired alpha particles at a piece of atom thick gold foil • many particles passed straight through but some were deflected • a mass (nucleus) was discovered in the centre of the gold foil atom • this prevented some alpha particles from passing

• made up of 2 protons and 2 neutrons = positive charge - aka the helium atom nucleus

• 4th theory= dated 1914 - discovered that electrons orbit the nucleus at a specific distances in certain fixed energy levels (or shells)

• dated 1932 - discovered that neutrons were apart of the atomic structure