The Atom in History
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Most
people think that the atom is a very boring thing to study. However, that can’t possibly be true – after
all, lots of people have studied it for thousands of years. It must be exciting! Let’s take a look at what we’ll discuss here:
Let’s
get started!
The Greeks and
their imaginary atom:
Even
though the Greeks had very little in the way of high technology, they still
felt that they could use the power of their brains to figure out what matter
was made up of at the smallest levels. As a result, lots of them talked about it a
lot.
Democritus was one of
these guys. He came up with a model of
the atom that said:
Because
Aristotle disagreed with him and everybody thought that Aristotle was a big
hotshot, practically nobody paid attention to Democritus. The moral of the story: Don’t mess with Aristotle. Or something like that.
Some ideas that
nobody had ever thought of before:
For
a really long time, nobody really thought that Aristotle was wrong. Eventually, however, with the advance of
science, people started to rethink their devotion to the dead Greek guy. Here are some of the discoveries that changed
this:
In
the 1800’s, some English guy named John
Dalton came up with his own idea of what atoms were like. His theory included the following ideas:
Overall,
people seemed pretty happy with
Thomsen and
his cathode ray tube:
Since
every chemistry textbook in the world shows a picture of the cathode ray tube
experiment, I’m not going to reproduce it – I suggest you turn to it, though,
since it might help with my explanation.
Anyway,
one day Thomsen was goofing around the lab with these cathode ray tubes he
found somewhere. What he found was that
when he connected these big long hollow tubes to batteries, a beam of light
would go from one end to another. Since
he had a lot of time on his hands, he decided to figure out what the deal was with
the light. After all, if there was
nothing in the tube to start with, where’d the light come from? He figured, it must come from the electrodes
– since the electrodes were made of atoms, the atoms must somehow be coming
apart.
Among
other things, Thomsen got a magnet and held it near the beam. When he did this, he found that the beam
would bend toward the positive side of the magnet and away from the negative
side. From this, he figured that the
beam must contain very small particles from the atom and that they must have
negative charge.
This
led directly to his “plum pudding” model of the atom, named after a dessert
that nobody can eat without throwing up.
Think of a chocolate chip cookie, instead. His idea was that the dough in the chocolate chip
cookie made up most of the atom and that it had positive charge. The chips represented the little tiny bits of
negative charge that made up the light he was messing around with – unlike the
dough, they could leave the atom if you gave them a shove (with a battery, for
example).
For
this discovery, Thomsen is forever known.
This, despite the fact that his model was almost instantly disproved.
When
he did this (again, you can find MUCH better pictures of this in your textbook
than I can make), he found that most of the particles went right through the
foil, while some of them either passed through or bounced off at irregular
angles. Why is this?
His
idea was to come up with a model of the atom in which most of the atom is empty
space with electrons floating around in it.
The protons, however, are all concentrated in the middle of the atom
(called the nucleus) – according to this model, the positively charged alpha particles
would go straight through the atom most of the time and only be deflected on
the rare occasions when they passed very close to the tiny nucleus. For this discovery, we will always know
Random
interlude: Chadwick and the discovery of
the neutron
In
1932, James Chadwick discovered the neutron (which has no charge at all) by
doing some really complicated experiments that I don’t understand even a little
bit. Don’t worry, though – your teacher
probably doesn’t understand it either (unless they’re a nuclear scientist or
something), so if you just remember that Chadwick discovered it, you’re
probably fine.
Neils Bohr and
the planetary model:
As
tends to be the case with models of the atom, nobody really bought into
His
idea was that electrons traveled only in certain circular paths around the
nucleus, much as the planets circle the sun.
When energy is added to the electrons, the electrons jump from their
normal orbit (called the “ground state” orbital) to a higher energy orbital
farther from the nucleus (called the “excited state” orbital). Since the world likes to exist at low energy
more than at high energy, the electrons eventually return to their ground state
orbitals. When this happens, the energy
that they absorbed is given off as light.
Since the color of light is very closely related to its energy, you only
see very particular colors of light being given off by the very particular energy
differences between the ground state and the excited state.
His
idea further went on to say that the energies of the orbitals were different
for every atom. As a result, the colors
of light given off by every element is unique, which allows us to identify them
via the magic of spectroscopy
(specifically, this phenomenon is called atomic emission spectroscopy).
Like
all models of the atom, this was overturned in about 15 minutes by a bunch of
guys who invented something called quantum mechanics. However, don’t feel sorry for the planetary
model of the atom – it still has a healthy and thriving life in elementary
school textbooks (which for some reason refuse to acknowledge the existence of
quantum mechanics).
© 2007 Ian Guch – All Rights Reserved