Aristotle (384 BC – 322 BC)
- Believed you could divide matter an infinite number of times
- Believed world was made of earth, fire, water and air.
- These elements were acted upon by gravity and levity
Democritus (460 BC)
- Believed matter was made up of tiny particles called ATOMS
- Matter could not be divided indefinitely
- Could not prove his hypothesis
- founded the law of definite proportions
- discovered oxygen and hydrogen
- reasoned that the ratio between elements would always be the same in a compound no matter how much of that compound was available.
Five Point Atomic Theory
- All matter is made up of atoms
- Atoms of a given element differ from the atoms of another element
- All atoms of a given element are identical
- Chemical reactions do not change the elements but change the way the elements are grouped together
- Atoms cannot be created, divided or destroyed through chemical process.
- discovered radioactivity
- studied the effect of xrays on photographic film
- some chemicals decomposed and gave off penetrating rays
- Discovered the electron
- Used a cathode ray tube to experiment with the magnetic deflection
- Created the plum pudding model
- This model states that negatively charged electrons exist randomly throughout an atom. No mention of protons or a nucleus.
- Created the bohr model of the atom
- Electrons do not spiral around the nucleus – they orbit at certain allotted distances from nucleus
- Atoms radiate energy- jump from higher to lower orbits
- His model had protons, electrons and neutrons
- Changed the arrangement of particles in an atom
- Gold foil experiment- shot alpha particles through gold foil – some of them bounced back
- He concluded that atoms have a positive dense centre and electrons orbit outside it
- This was the first planetary model – electrons spin around the nucleus.
- Discovered the negative charge on the electron
- Used the oil drop experiment and observed the speed of the particles
James Chadwick (1932)
- discovered the neutron
- Observed that previously thought gamma radiation was radiation from neutral particles