|How did they find atoms?
|Question Date: 1999-02-17|
In around 460 B.C., did a Greek philosopher,
Democritus, developed the idea of atoms. He asked
this question: If you break a piece of matter in
half, and then break it in half again, how many
breaks will you have to make before you can break
it no further? Democritus thought that it ended at
some point, a smallest possible bit of matter. He
called these basic matter particles, atoms. He
also believed that atoms differed from each other
only in size and shape, and different substances
with their distinct qualities were made up of
different shapes, arrangements, and positions of
atoms. Atoms were in continuous motion in the
infinite void and constantly collided with each
other. During these collisions they could rebound
or stick together because of hooks and barbs on
their surfaces. It was thought that atoms were
neither created nor destroyed. At this stage,
atoms were only a hypothesis, without any
experimental evidence to support their
In the 1800's an English
chemist, John Dalton performed experiments with
various chemicals that showed that matter, indeed,
seems to consist of elementary lumpy particles
(atoms). Although he did not know about their
structure, he knew that the evidence pointed to
something fundamental. Dalton figured out the
"stoichiometry" of a lot of different compounds,
that is the amounts of different elements that
they contain, and he showed that compounds are
formed from elements in very definite proportions.
For example compounds can be formed from the
elements carbon and oxygen, but only in whole
number ratios, giving us carbon monoxide (CO)
carbon dioxide (CO2) among others. This only made
sense if separate carbon and oxygen atoms existed.
At this stage in history the existence of atoms
was upgraded to a theory.
Today we have a
huge amount of evidence for the existence of
atoms. We even have experimental tools available
which allow us to "photograph" atoms directly! For
example, take a look at the "quantum corral" of
atoms on IBM's
the existence of atoms is now a definite
This progression from hypothesis to
theory to accepted fact is very common in science
as we learn more about a subject through time. It
is remarkable in this case that the ancient Greeks
got a lot of it right, over 2000 years before
there was any substantial experimental
This is a very long and complicated story.I
suggest that you look up a couple of people that
were involved. Some of them are James Chadwick
(1891-1974), Sir William Crookes, Michael Faraday
(1791-1867), Svante Arrhenius (1859-1927), J.J.
Thompson (1856-1940), G.J. Stoney (1826-1911), and
Earnest Rutherford (1871-1937). As you can see,
there were a lot of different people involved and
none of them individually found the atom but they
found parts of them like the electron, proton, and
neutron. All of their work put together helped us
find the atom. Even today more things are being
discovered about atoms.
This is a great question! For a long time, people
have thought that all matter is made up of tiny,
indestructible elements (atoms).
But atoms are
hard to see. They're much too small to be seen in
a microscope, for example. So it was only about a
hundred years ago that people had experimental
evidence that atoms exist.
Imagine you were
watching a soccer game played with a
glow-in-the-dark ball, and suddenly the stadium
lights went out. Now you can't see anything
except for this glowing ball. You wouldn't be
able to see the soccer players, but if the ball
kept bouncing around back and forth, you'd know
the players were still on the playing
This is an "indirect" observation of the
players, because you don't actually see them
there: you only see the effects of their actions.
A similar effect led to one of the first,
indirect, "observations" of atoms.
If you look
at a little bit of water under a microscope,
you'll see a lot of things - bacteria and so
forth. Some of the things you'll see are little
specks of dirt or dust or something like that.
These specks seem to move around a lot, even
though they don't have any way to propel
themselves. They move around randomly, jiggling
back and forth and wandering around the
The explanation of this motion came from
Einstein, who pointed out that if water was made
up of atoms, then the atoms would be constantly
moving around. Every so often, an atom (or a
molecule, which is a clump of atoms) would happen
to bump into a dust speck, and when it bumped into
the dust speck you would see the dust speck move a
When Einstein carefully analyzed the
motions that a dust speck would have, he found
that it matched the experimental results quite
This is one bit of evidence that atoms
exist. It's indirect evidence, because you still
cannot see the atoms, but you do observe the
effects of their motion.
Click Here to return to the search form.
Copyright © 2017 The Regents of the University of California,
All Rights Reserved.