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JOHN DALTON
(1766-1844) An
English chemist and
founder of modern
atomic theory. Born in
Cumberland, he moved
to Manchester where
he taught mathematics
and natural
philosophy. He studied
the behaviour of gases
and the atmosphere,
but his most famous
insights were with
the atomic theory of
chemistry, with
which his name
is associated.DMITRI MENDELEEV
(1834-1907) A Russian
chemist most famous
for his Periodic Table
of Elements, which
he discovered while
writing a textbook on
chemistry in 1869. He
was twice nominated
for the Nobel Prize,
in 1906 and 1907, but
this was rejected
after claims that his
discovery was too old.NIELS BOHR
(1885-1962) A Danish
physicist who made
major contributions
to the foundations of
quantum mechanics
and to the theory of
atomic structure. His
planetary model was
the forerunner of the
modern picture
of the atom. He won
the Nobel Prize for
physics in 1922.ERNEST RUTHERFORD
(1871-1937) The New
Zealand-born British
physicist is famous for
discovering the atomic
nucleus, identifying
forms of radioactivity,
and fathering the
field of nuclear
physics. Although
he is best known
for his discovery of
the nuclear atom,
his 1908 Nobel Prize
was for chemistry
and his discovery of
transmutation of
the elements.JOSEPH JOHN
THOMSON
(1856-1940) Better
known as JJ Thomson,
he was born in
Manchester and
joined Trinity College,
Cambridge, in 1876.
He spent the rest of
his life there and his
work on the properties
of gases and atomic
structure led to his
discovery of the
electron, in 1897, and a
Nobel Prize in 1906.ingredients, the atoms of the heavier
elements having ‘more’ of t he myster y
material than the lighter ones. In other
words: atoms are made of something
even smaller.Mystery components
With hindsight, by the middle of
the 19th century two discoveries held
the clue that atoms have an inner
structure. First was the phenomenon
of atomic spect ra. Here, when light
emitted by hot elements was split into
component colours, characteristic sets
of lines showed up, in effect a n atomic
ba rcode u nique to each element. While
chemists used the phenomenon to
identify known elements and discover
new ones, such as helium in the Sun,
physicists found it too complicated to
explain and initially ignored it.
Second, the Russian Dmitri
Mendeleev discovered that, when he
listed the atomic elements in order of
their atomic weights, from the lightest,
hydrogen, up to uranium, elements
having similar chemical properties
periodically reoccurred. His
celebrated Periodic Table of Elements
contained gaps, which led him to
predict that further elements must
exist to fill them. The discoveries of
gallium, germanium and scandium,
found in France, Germany and
Scandinavia followed – you can easily
tell which was found where!
Dalton had believed that atoms were
indivisible spheres. But by the end of
the 19th century, clues were
accumulating suggesting that atoms
have a n inter nal st r uctu re. In addition
to spectra and the periodic table,
radioactivity showed that one element
could transform spontaneously into
another by emitting particles, a
process known as transmutation. This
raised two questions: what were the
constituent parts of atoms and how
were they arranged?
Answers came in 1897, when Joseph
John Thomson found that electric
current is carried by negatively
charged particles: electrons.
Measuring the ratio of an electron’s
charge to its mass, he found this was
very large and common to all elements
that he used. From this he deduced 5The pioneers who have peeled back the layers
of the ‘atomic onion’CAST OF CHARACTERS