208 ERNEST RUTHERFORD
T
he discovery at the turn
of the 20th century that
the basic constituent of
matter—the atom—could be
broken into smaller fragments
was a watershed moment for
physics. This astonishing
breakthrough revolutionized ideas
about how matter is constructed
and the forces that hold it and
the universe together. It revealed
an entirely new world at the
subatomic level—one that required
a new physics to describe its
interactions—and a slew of
tiny particles that filled this
infinitesimally small domain.
Atomic theories have a long
history. The Greek philosopher
Democritus developed the ideas of
earlier thinkers that everything is
composed of atoms. The Greek
word átomos, which is credited to
Democritus, means indivisible and
referred to the basic units of matter.
Democritus thought that the
materials must reflect the atoms
they are made of—so atoms of iron
are solid and strong, while those of
water are smooth and slippery.
At the turn of the 19th century,
English natural philosopher John
Dalton proposed a new atomicIN CONTEXT
BRANCH
PhysicsBEFORE
c.400 BCE Greek philosopher
Democritus envisages atoms
as solid, indestructible
building blocks of matter.1805 John Dalton’s atomic
theory of matter marries
chemical processes to physical
reality and allows him to
calculate atomic weights.1896 Nuclear radiation is
discovered by Henri Becquerel,
and is used to reveal the
internal structure of the atom.AFTER
1938 Otto Hahn, Fritz
Strassman, and Lise Meitner
split the atomic nucleus.2014 Firing increasingly
energetic particles at the
nucleus continues to reveal
a slew of new subatomic
particles and antiparticles.theory based on his “law of multiple
proportions,” which explained
how elements (simple, uncombined
substances) always combine in
simple, whole-number ratios. Dalton
saw that this meant that a chemical
reaction between two substances
is no more than the fusing of
individual small components,
repeated countless times. This
was the first modern atomic theory.A stable science
A self-congratulatory mood was
detectable in physics at the end of
the 19th century. Certain eminent
physicists made grandstanding,
declarations to the effect that the
subject was all but finished—that
the principal discoveries had all
been made and the program going
forward was one of improving the
accuracy of known quantities “to
the sixth decimal place.” However,
many research physicists of the
time knew better. It was already
clear that they were facing an
entirely new and strange set of
phenomena that defied explanation.
In 1896, Henri Becquerel,
following a lead from Wilhelm
Röntgen’s discovery of mysterious
“X-rays” the previous year, hadNow I know
what the atom
looks like.Alpha particles fired
into atoms sometimes travel
straight through, sometimes are
deflected, and sometimes
bounce back.This means that
an atom must have
a small, dense
central nucleus.Electrons are
found to have specific
orbits around
the nucleus.So, the atom is made of
a small, massive nucleus
with electrons orbiting
it in shells.