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 is made of small, indivisible particles called atoma

Proposed that the behavior of substances were due to the type of atom

Alchemists

Great many 'experiments' to try to convert one element to another

Many discoveries, but very secretive

Robert Boyle

Robert Boyle: 1627-1691 Performed experiments with controls and

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

were incorrect, and that some 'elements' (like mercury) did not contain the 4 Aristotlean elements (earth, air, fire, water)

Antoine Lavoisier

Lavoisier: 1743-1794 Experimented and established the Law of

Conservation of Mass Investigated combustion and cellular respiration

in terms of energy and mass

Charles de Coulomb

Coulomb 1736-1806 Developed Coulomb's Law, the definition of

electrostatic force

Joseph Proust

Proust: 1754-1826 Controversy with another scientist (Berthollet)

led him to discovery of the Law of Definite Proportions (sometimes called Proust's Law)

A chemical compound always contains the same proportion of elements by mass

There are exceptions, which are called non-stoichiometric (or Berthollide) compounds

John Dalton

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

combine in mass ratios of small whole numbers. Elements may combine with the same element in different proportions to make a different compound.

Atomic theory contains 4 hypotheses

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.

James Clerk Maxwell

1831-1879 Showed that

electricity and magnetism were related, and were related to atoms

Predicted that accelerating charges would make waves (electromagnetic radiation)

Cathode Ray Tube Experiments

Glass tube with wire at each end; as much air pumped out as possible

Charge passed across tube makes fluorescent glow

William Crookes Tube coated with fluorescent material can be made

to glow in one focused dot Rays travel in straight lines

Ray carries negative charge

Joseph John Thomson 1856-1940 Used a study of the

cathode ray tube to determine the presence of electrons 1897

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

Won the Nobel Prize in Physics in 1906

J. J. Thomson’s 4 Experiments Used magnetic field to show cathode rays had

negative charge Used electric charge to show cathode rays were

particles with negative charge Used varying electric currents to determine charge to

mass ratio Either particles very small or Carried huge charge

Evidence suggested particles very small and came from atom

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

Wilhelm Röntgen 1825-1943 Rays produced by

cathode ray tube interaction with other matter could pass through objects, affect photographic plates and cause some materials to fluoresce (X-rays)

Nobel Prize in Physics in 1901

Antoine Henri Becquerel 1852-1908 Investigated possible link

between X-rays and natural phosphorescence

Discovered that the rays emitted by uranium were different from X-rays

Nobel Prize in Physics 1903 (shared with Pierre and Marie Curie)

Marie Curie 1867-1934 Worked with huband

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 1831-1937 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 with dense central nucleus and vast empty areas

Nobel Prize in Chemistry 1938

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

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

Mass and Atomic Number Each element's atoms have a specific number of

protons in the nucleus (Z=atomic number) For a neutral atom, the number of protons is

equal to the number of protons The mass number (A) is the number of protons

plus the number of neutrons (the number of particles in the nucleus).

Some atoms of the same element have different A values. This means they have different numbers of neutrons. They are called isotopes.

Isotope notation Isotope notation is

used to represent an isotope.

Z, A and symbol of element are represented.

Isotope notation can be used to determine the number of protons, neutrons and electrons in an isotope.