Dictionary of Chemistry [6th Ed.]

atomic mass of an element divided
by its density.

atomic weightSee relative
atomic mass.

atom-probe Üeld-ion micros-
copyA technique for identifying in-

dividual atoms on surfaces. In the

atom-probe *Üeld-ionization micro-

scope (FIM) there is a hole in the

Ûuorescent screen, with which the

FIM image of an adsorbed atom is

brought into coincidence. The gas

causing the imaging is removed. The

49 atom-probe Üeld-ion microscopy

ATOMIC THEORY a

c.430BC Greek natural philosopher Empedocles (d. c. 430 BC) proposes that all

matter consists of four elements: earth, air, fire, and water.

c. 400 BC Greek natural philosopher Democritus of Abdera (c.460–370BC)

proposes that all matter consists of atoms.

c.306BC Greek philosopher Epicurus (c.342–270BC) champions Democritus’

atomic theory.

1649 French philosopher Pierre Gassendi (1592–1655) proposes an atomic

theory (having read Epicurus).

1803 John Dalton proposes Dalton’s atomic theory.

1897 J. J. Thomson discovers the electron.

1904 J. J. Thomson proposes his ‘plum pudding’ model of the atom, with

electrons embedded in a nucleus of positive charges.

Japanese physicist Hantaro Nagaoka (1865–1950) proposes a ‘Saturn’

model of the atom with a central nucleus having a ring of many

electrons.

1911 New Zealand-born physicist Ernest Rutherford (1871–1937) discovers the

atomic nucleus.

1913 Niels Bohr proposes model of the atom with a central nucleus

surrounded by orbiting electrons.

British physicist Henry Moseley (1887–1915) equates the positive charge

on the nucleus with its atomic number.

Frederick Soddy discovers isotopes.

1916 German physicist Arnold Sommerfield (1868–1951) modifies Bohr’s

model of the atom specifying elliptical orbits for the electrons.

1919 Ernest Rutherford discovers the proton.

1920 Ernest Rutherford postulates the existence of the neutron.

1926 Erwin Schrödinger proposes a wave-mechanical model of the atom (with

electrons represented as wave trains).

1932 British physicist James Chadwick (1891–1974) discovers the neutron.

Werner Heisenberg proposes a model of the atomic nucleus in which

protons and neutrons exchange electrons to achieve stability.

1939 Niels Bohr proposes a ‘liquid drop’ model of the atomic nucleus.

1948 German-born US physicist Marie Goeppert-Meyer (1906–72) and

German physicist Hans Jensen (1907–73) independently propose the

‘shell’ structure of the nucleus.

1950 US physicist Leo Rainwater (1917–86) combines the ‘liquid-drop’ and

‘shell’ models of the nucleus into a single theory.