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

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.