SCIENCE&^SOCIETYX^2,^ NASA1803
John Dalton proposes that all matter is
made of indestructible atoms; that atoms
of different elements are distinguished by
their weights and that chemical reactions
occur when atoms are rearranged.1897
Joseph John ‘JJ’ Thomson (above) discovers
the electron – a constituent of all atomic
elements. Negatively charged, it suggests
there must also exist positively charged
constituents to neutralise the atom.1911
Ernest Rutherford discovers the positively
charged atomic nucleus following
experiments by Hans Geiger and Ernest
Marsden. He realises the nucleus is massive
and compact, and that an atom is mostly
empty space.1913
Niels Bohr (left)
creates a conceptual
picture of the atom
like a miniature
Solar System, where
‘planetary’ electrons
orbit a central
nuclear ‘Sun’.1925-28
Erwin Schrödinger produces a quantum
theory of electron behaviour in the
hydrogen atom in 1925. Three years
later, Paul Dirac completes the theory,
making it consistent with the theory of
Special Relativity.1932
Atomic nucleus established to consist of
protons and neutrons. The proton and
neutron are today known to be made of
more fundamental seeds: quarks. The
electron still appears to be indivisible.5 that electrons are a component part
that features in all elements.
American Robert Millikan
measu red t he elect ric cha rge of t he
electron, which, combined with
Thomson’s result for the ratio of charge
to mass, showed this ratio is large
because the mass of an electron is
tiny, only about 1/2,000th that of a
hydrogen atom, the lightest atom
known. This led to two inferences:
one, as electrons are so light, there
must be other more massive particles
in t here too. And, two, as atoms have
no overall electric charge, the massive
particles must therefore be positively
charged in order to neutralise the
electrons’ negativity.
When Ernest Rutherford and his
assista nts Ha ns Geiger a nd Er nest
Marsden bombarded atoms of gold
with alpha particles – massive,
positively charged particles emitted in
radioactivity – they found that most of
them passed through, but occasionally
one would recoil violently (see ‘The
Key Experiment’, page 34). In 1911,
Rutherford deduced that the gold atom
must be mostly empty space, but with
a dense massive central region,
capable of deflecting the alpha
particles. He called this the nucleus.
The nucleus of a hydrogen atom is
t he simplest of all, consisting of a
single positively charged ‘proton’.
The nuclei of heavier elements contain
several protons – helium has two,
uranium has 92 – whose combined
positive charge ensnares negatively
charged electrons to form the atom.
It is t he la rger number of protons in
atoms of elements, such as uranium,
that helps give them a larger atomic
weight than hydrogen.Weighty issue
But protons alone don’t explain the
exact values of the atomic weights: in
addition to protons, all elements other
than hydrogen contain neutrons,
which have no electric charge.
Neutrons add to the mass of the atom
but leave its chemical properties
unchanged. A given elemental atom
can occur with different numbers
of neutrons. Such alternatives are
known as isotopes. Even hydrogenhas isotopes: ‘heavy water’ is the result
of a hyd rogen atom having a proton
and a neutron.
When Rutherford’s discovery of the
positively charged atomic nucleus
and Thomson’s discovery of the
lightweight, negatively charged
electron were married with the rule
t hat opposite elect rical cha rges
attract, a seductively simple picture
emerged of the atom as a miniature
Solar System. In this naive analogy,
the nucleus plays the role of the Sun
and electrons are like the remote
planets in orbit around it.
However, had electrons in atoms
encircled the central nucleus like
planets orbiting the Sun, obeying
New ton’s laws of motion, t hey would
have spiralled into the nucleus within
a mere fraction of a second. An atom,
once formed, would self-destruct in a
flash of light almost immediately;
matter would not exist. Something
was missing. The final ingredient was
the discovery of quantum theory: very
small things, such as atoms, follow
different laws from
those of Newton,
which explain the
behaviour of objects
that are large enough
to see. Instead of an
electron being able to
go anywhere in an
atom, it is limited,
like someone on a
ladder who can only
stand on one rung at a
time. Electrons in
atoms follow a
fundamental
regularity, each rung
corresponding to a
state where the
electron has a unique
amount of energy.
Danish physicist
Niels Bohr suggested
the idea in 1912. When
an electron drops from
a rung with highTIMELINE
Mendeleev’s 1869 periodic
table had gaps that led him to
believe that some elements
remained undiscoveredTHE FUNDAMENTALS OF PHYSICS