Sunday, May 15, 2011

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.

-         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.

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

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