The 2,400-year search for the atom - Theresa Doud

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How do we know what matter is made of? The quest for the atom has been a long one, beginning 2,400 years ago with the work of a Greek philosopher and later continued by a Quaker and a few Nobel Prize-winning scientists. Theresa Doud details the history of atomic theory.

Lesson by Theresa Doud, animation by TED-Ed.

What do an ancient Greek philosopher

and a 19th century Quaker

have in common
with Nobel Prize-winning scientists?

Although they are separated
over 2,400 years of history,

each of them contributed
to answering the eternal question:

what is stuff made of?

It was around 440 BCE
that Democritus first proposed

that everything in the world
was made up of tiny particles

surrounded by empty space.

And he even speculated
that they vary in size and shape

depending on the substance they compose.

He called these particles "atomos,"
Greek for indivisible.

His ideas were opposed by
the more popular philosophers of his day.

Aristotle, for instance, disagreed completely,

stating instead that matter
was made of four elements:

earth, wind, water and fire,

and most later scientists followed suit.

Atoms would remain
all but forgotten until 1808,

when a Quaker teacher named John Dalton
sought to challenge Aristotelian theory.

Whereas Democritus's atomism
had been purely theoretical,

Dalton showed that common substances
always broke down into the same elements

in the same proportions.

He concluded that the various compounds

were combinations of atoms
of different elements,

each of a particular size and mass

that could neither be created
nor destroyed.

Though he received
many honors for his work,

as a Quaker, Dalton lived modestly
until the end of his days.

Atomic theory was now accepted
by the scientific community,

but the next major advancement

would not come
until nearly a century later

with the physicist J.J. Thompson's
1897 discovery of the electron.

In what we might call
the chocolate chip cookie model of the atom,

he showed atoms as
uniformly packed spheres of positive matter

filled with negatively charged electrons.

Thompson won a Nobel Prize in 1906
for his electron discovery,

but his model of the atom
didn't stick around long.

This was because he happened
to have some pretty smart students,

including a certain Ernest Rutherford,

who would become known
as the father of the nuclear age.

While studying the effects
of X-rays on gases,

Rutherford decided
to investigate atoms more closely

by shooting small, positively charged
alpha particles at a sheet of gold foil.

Under Thompson's model,

the atom's thinly dispersed
positive charge

would not be enough
to deflect the particles in any one place.

The effect would have been
like a bunch of tennis balls

punching through a thin paper screen.

But while most of the particles
did pass through,

some bounced right back,

suggesting that the foil was more
like a thick net with a very large mesh.

Rutherford concluded that atoms
consisted largely of empty space

with just a few electrons,

while most of the mass
was concentrated in the center,

which he termed the nucleus.

The alpha particles
passed through the gaps

but bounced back from the dense,
positively charged nucleus.

But the atomic theory
wasn't complete just yet.

In 1913, another of Thompson's students
by the name of Niels Bohr

expanded on Rutherford's nuclear model.

Drawing on earlier work
by Max Planck and Albert Einstein

he stipulated that electrons
orbit the nucleus

at fixed energies and distances,

able to jump from one level to another,
but not to exist in the space between.

Bohr's planetary model took center stage,

but soon, it too encountered
some complications.

Experiments had shown that rather than
simply being discrete particles,

electrons simultaneously
behaved like waves,

not being confined
to a particular point in space.

And in formulating
his famous uncertainty principle,

Werner Heisenberg showed
it was impossible to determine

both the exact
position and speed of electrons

as they moved around an atom.

The idea that electrons
cannot be pinpointed

but exist within
a range of possible locations

gave rise to the current
quantum model of the atom,

a fascinating theory
with a whole new set of complexities

whose implications
have yet to be fully grasped.

Even though our understanding
of atoms keeps changing,

the basic fact of atoms remains,

so let's celebrate
the triumph of atomic theory

with some fireworks.

As electrons circling an atom
shift between energy levels,

they absorb or release energy in the form
of specific wavelengths of light,

resulting in
all the marvelous colors we see.

And we can imagine Democritus
watching from somewhere,

satisfied that over two millennia later,

he turned out
to have been right all along.