Atomic History

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

Lavoisier (1777)

-         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.

Dalton (1800s)

            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. 

Henri Bequerel(1851)

-         discovered radioactivity

-         studied the effect of xrays on photographic film

-         some chemicals decomposed and gave off penetrating rays

Thomson (1897)

-         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.

Bohr

-         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

Rutherford

-         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.

Millikan (1921)

-         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

- Candace