Station 1: Early Atomic Theory and John Dalton
The concept of atoms was first suggested by the Greek philosopher
Democritus, who suggested that if you kept splitting an object in half, you would
eventually get to a tiny, fundamental piece that can’t be broken down any further.
He called these pieces “atoms,” but it was mainly a philosophical idea, and wasn’t
supported by any science.
The British chemist John Dalton was the first modern scientist to develop a
theory about atoms. According to Dalton, atoms could not be subdivided into
smaller particles. Atoms were the smallest pieces of matter. Many of Dalton’s
ideas about atoms were correct, for example that atoms of one type of element
were different from atoms of all different types of elements. Dalton was also
correct in his description of how atoms behave during chemical reactions. He
hypothesized that atoms are combined, separated, or rearranged during chemical
reactions, and that compounds were made of different elements that combined in
simple, whole number ratios.
We represent Dalton’s theory about the atom as a “pool ball” because he
thought that the atom was made of the same “stuff” throughout (like a pool ball).
Dalton said nothing about the smaller pieces that make up atoms.
Think about what a “pool ball” looks like and think about how Dalton’s
model might look to you. When you finish thinking about it, try to draw what you
think it would look like.
Station 2: The Electron and J. J. Thomson
The British physicist J.J. Thomson discovered that atoms contain small,
negatively charged particles called electrons. He discovered electrons by accident,
when he was studying electricity. Thomson had a glass tube (called a cathode ray
tube) that connected two metal plates, one of which had a positive charge and one
had a negative charge. When Thomson pumped all the air out of the connecting
tube, he noticed that a beam of particles came from the negatively charged metal to
the positively charged metal. He decided that these negative particles must be tiny
pieces of the atoms on the negatively charged metal (we call them electrons!)
However, Thomson knew that overall atoms have a neutral charge (atoms
have no charge), which meant that some other part of the atom must be positive, to
balance the negative electrons. He proposed the plum pudding model of the atom,
where negatively charged electrons sat in a positively charged ball of matter. We
call this the plum pudding model after a British desert of a pudding (the positively
charged ball) that has raisins in it (the electrons).
Think about what “plum pudding” might look like and think about how
Thomson’s model might look to you. When you finish thinking about it, try to
draw what you think it would look like.
Station 3: The Nucleus and Ernest Rutherford
In 1909, Ernest Rutherford proved that the plum-pudding model of the atom
was incorrect. Rutherford fired a bunch of small, positively charged particles
(alpha particles) at a sheet of gold foil. If the plum-pudding model had been
correct, the alpha particles would have bounced back at Rutherford because they
would hit the positive balls of atoms. A few of the alpha particles did bounce back
like this. However, most of them passed straight through the foil. Rutherford
concluded that most of the atom was empty space. He also suggested that the
positive charge of the atom was concentrated in a central nucleus, and that the
alpha particles that had been deflected were the ones that had hit the nucleus.
Rutherford suggested that electrons were on orbits spinning around the positive
nucleus.
The main problem with this model is that according to classical physics the
orbiting electrons would lose energy and collapse into the nucleus. This would
mean that all matter in the universe was completely unstable, which is clearly not
the case.
We call Rutherford’s model “the Peach” because there is a hard and dense
center of the atom. The rest of the atom is not as dense or heavy as the nucleus
(like a peach pit), but the rest of the atom is a lot of space (the peach itself).
Think about what “the peach” might look like and think about how
Rutherford’s model might look to you. When you finish thinking about it, try to
draw what you think it would look like.
Station 4: Electron Orbits and Niels Bohr
The Danish physicist Niels Bohr came up with a model that describes
hydrogen atoms (one electron and one proton) very successfully. According to
Bohr, the electron could only orbit the electron on fixed energy levels. It was
impossible for the electron to collapse into the proton, because the electron could
not have energy less than its lowest orbit. Adding energy to the electron would
excite it, and cause it to jump up to an orbital that was further away from the
proton.
The problem with this model is that it couldn’t explain the behavior of
electrons in atoms or ions where there was more than one electron.
Bohr’s model can be thought of like the “Solar system”. Think about the
solar system: planets rotate around the sun in specific paths. The planets never
leave these path’s and they are continuously moving.
Think about what “the solar system” might look like and think about how
Bohr’s model might look to you. When you finish thinking about it, try to draw
what you think it would look like.
Station 5: The quantum electron cloud and Edwin Schrodinger
More recent research has established that electrons do not orbit the nucleus
like planets orbit the sun. Rather, electrons are most likely to be found in certain
areas around the nucleus, called “electron clouds.” The electron cloud is defined
as the area where an electron spends 95% of its time. Sometimes (5% of the time)
the electron will wander off somewhere else, which could be closer to or further
from the nucleus. The probability that an electron is in a particular location is
described by a very complicated set of equations, which were developed by Edwin
Schrödinger. Many other scientists were involved in the development of this
model, including German physicist Werner Heisenberg, who showed that is
impossible to know both an electron’s location and speed at the same time. If you
know one of those two things, it is impossible to know the other.
The two electrons that are closest to the nucleus are usually found in a
spherical cloud. The electrons that tend to be further from the nucleus have clouds
with more complicated shapes.
We call this model the “electron cloud” because the electrons surround the
nucleus in a disorderly fashion but are more likely to be in one certain spot. The
electrons surround the nucleus like a cloud.
Think about what “the electron cloud” might look like and think about
how Schrodinger’s model might look to you. When you finish thinking about it, try
to draw what you think it would look like.