20. History of the Atom

what name did Democritus suggest for the small indivisible particles?

atomos

John Dalton atomic theory point 1

all matter is composed of tiny particles

John Dalton atomic theory point 2

atoms are indivisible

John Dalton atomic theory point 3

atoms could not be created nor destroyed

John Dalton atomic theory point 4

all atoms of the same element are identical to each other

why was Dalton's atomic theory significant?

it was the first complete attempt to describe all matter as atoms

what did William Crookes do?

carried out experiements to investigate the properties of cathode rays

what did Crookes discover about cathode rays?

found they travelled in straight lines, have enough energy to move a small paddle wheel, and cause glass to fluoresce

what did JJ Thompson conclude about cathode rays?

they were made of negatively charged particles - electrons

cathode rays

beams of negatively charged sub-atomic particles called electrons

how did JJ Thompson represent an atom?

plum pudding model

who named the electron?

George Stoney

plum pudding model

a sphere of positive charge with negative charges embedded at random

what did Robert Millikan do?

measured the size of the charge on the electron through his oil drop experiment

what experiment is associated with Ernest Rutherford?

gold foil/alpha particle experiment

gold foil/alpha particle experiment

bombarded a very thin sheet of golf foil with alpha particles to test if the plum pudding model was correct

what were Rutherford's expectations?

expected most of the alpha particles to pass straight through the gold foil undeflected, or be slightly deflected when passing near an electron

Rutherford observation 1

most alpha particles passed straight through the gold foil undeflected

Rutherford conclusion 1

most of the atom is made of empty space

Rutherford observation 2

some alpha particles were deflected at large angles

Rutherford conclusion 2

there must be a positive centre, which he called the nucleus

Rutherford observation 3

very few alpha particles deflected straight back along their original paths

Rutherford conclusion 3

the positively charged nucleus must be very small and very dense

what new model did Rutherford propose?

small, dense, positive nucleus surrounded by an electron cloud

what model did Bohr propose?

planetary model which said electrons are found in energy levels

who discovered and named the proton?

Rutherford

what did James Chadwick do?

discovered the neutron

why was the neutron discovered last?

neutral things are hard to detect