History of the Atom

Democritus of Abdera

Democritus: 460-370 BCE Greek mathematician and philosopher Co-author (with Leucippus) of the idea that all

matter made of small parts atoma

Proposed that the behaviour of substances was due to the type of atom

Aristotle

Greek Philosopher: 384-322 BCE Not an ‘atomist’ Proposed all matter made up of 5 elements:

Fire Air Water Earth Aether

Robert Boyle

Robert Boyle: 1627-1691 Performed experiments with controls and

published detailed results 1661: Published claiming that Aristotle's ideas

were incorrect Some 'elements' (like mercury) did not contain the

Greek elements (earth, air, fire, water)

John Dalton

Dalton: 1766-1844 Law of Multiple Proportions (1803): elements

combine in mass ratios of small whole numbers. Small whole numbers reflect the atoms involved,

therefore there are atoms Atomic theory contains 4 hypotheses

Dalton’s Atomic Theory

1. Elements are made of small particles called atoms. (Dalton viewed atoms as small spheres of differing characteristics.)

2.All atoms of an element are identical. All atoms of one element are different from atoms of other elements.

3.Compounds are made of atoms of more than one element. The ratio of the elements is a simple fraction.

4.A chemical reaction involves separation, combination or rearrangement of atoms.

Joseph John Thomson 1856-1940 Used a cathode ray

tube to determine the presence of electrons 1897

Suggested the plum pudding model of the atom and the existence of isotopes

Won the Nobel Prize in Physics in 1906

Robert A. Millikan 1858-1953 Used the 'falling drop

method' to determine the charge of the electron (-1.6022 x 10-19 C) and mass of electron as 9.10 x 10-28 g

Investigated photoelectric effect and spectroscopy of elements

Won the Nobel Prize in Physics in 1923

Marie Curie

1867-1934 Worked with husband

Pierre to study radioactivity

Isolated elements polonium and radium

Shared Nobel Prize in Physics 1903, won Nobel Prize in Chemistry 1911 for work with radioactive elements.

Ernest Rutherford 1871-1937 Investigated release and

measurement of different forms of radiation from elements (α, β, γ)

Gold foil experiment with Geiger and Marsden led to new model of an atom Model:ruther14.swf

Nobel Prize in Chemistry 1908

Model of the atom in the early 1900s

Nucleus: has positive charge, very small, but very dense Proton: same charge as electron, but opposite.

Mass=1.67262 x 10-24 g Vast empty space around nucleus contains

electrons

Where are the electrons? Niels Bohr: 1885-1962

Collaborated with Rutherford

Used emission spectra to propose Bohr model of atom 1913

Electrons found in specific ‘orbits’ around nucleus

Most of atom is empty space

Nobel Prize Physics 1922

James Chadwick 1891-1974 Discovered the

neutron and worked on fission of atoms

Discovery of neutron (mass = 1.67493 x 10-24 g) explained the mass problem of many atoms

Nobel Prize in Physics 1935

Current Model Based on the previous work, as well as

Einstein, Heisenberg, Born, Schrödinger and others

Nucleus contains p+ and no

Made of smaller particles called quarks uud, ddu (baryons)

Electron cloud e- are in areas of probability, predicted by wave

equations leptons