Download Models of the Atom A Brief History First Thoughts For over 2500

Models of the Atom
A Brief History
First Thoughts
For over 2500 years the model of the atom has been
refined. It started with the Greek philosopher Democritus,
who reasoned that you could take a piece of matter, cut it
in half, cut the half piece in half again, and continue to cut
again and again. Eventually, you wouldn’t be able to cut
any more. You would have only one particle left, and that
particle would be called an atom, a term that means
“cannot be divided”.
Greek philosophers such as Democritus did
not try to prove their theories by doing
experiments as scientists now do. Their
theories were the result of reasoning, debating
and discussion – not of evidence or proof.
Today scientists will not accept a theory that is
not supported by experimental evidence. But
even if Greek philosophers had experimented,
they lacked the equipment to prove the
existence of atoms. Even as recently as 500
years ago, atoms were still a mystery.
A Model of the Atom
A long period passed before the theories about the atom developed further. Finally,
during the 18th century scientists began debating about the existence of atoms once
again. In simple laboratories they were putting
substances together to form new substances and
taking substances apart to find out what they were
made of. They found that certain substances could not
be broken down into simpler substances. Scientists
came to realize all matter is made up of elements. An
element is made of atoms of only one kind. For
example, iron is an element made of iron atoms. Silver,
another element, is made of silver atoms.
Dalton’s concept John Dalton, an English schoolteacher in the early nineteenth
century, combined the ideas of elements with the Greek theory of the atom. He
proposed the following ideas about matter:
1. Matter is made up of atoms
2. Atoms cannot be divided into smaller pieces
3. All the atoms of an element are exactly alike
4. Different elements are made of different kinds of atoms
Dalton pictured the atom as a hard sphere that was the same throughout, something
like a tiny marble.
Testing the Model
Dalton’s theory of the atom was tested in the second half of the nineteenth century. In
1897 a scientist named J.J. Thomson determined that an atom was not the same
throughout, but was made up of charged particles smaller than the atom, later these
were called electrons. His electron discovering experiment is shown below. Basically,
a battery was hooked up to two pieces of metal (called electrodes) inside a gas-filled
tube. When turned on a ray of particles streaks from the cathode (negative electrode)
to the anode (positive electrode). If a magnet is placed near the stream of particles it
deflects them toward it (or away, depending on the magnetic pole). Perhaps the
biggest surprise that came from Thomson’s experiment was that atoms were not
uniformly solid throughout; inside of it were particles smaller than the atom.
Thomson’s Atomic Model
Because Dalton’s model did not agree with experimental evidence, it was necessary to
change the model of an atom. Thomson revised Dalton’s model from a uniform solid
ball to a sphere of positive charge with electrons spread evenly among the positive
charge. Below is an example of Thomson’s model – the blue clay is the positive
charge and the metal ball bearings represent electrons. The positive clay is equal to
the negative charge of the electrons, and so overall the atom is electrically neutral.
Rutherford’s Experiments
A model is not accepted in the scientific community until it has been tested and the
tests support previous observations. In 1906, Ernest Rutherford began an experiment
to find out if Thomson’s model of the atom was correct. His experiment involved
shooting a stream of particles toward the atoms in a piece of gold. If Thomson’s model
was correct, most of the particles should be only slightly deflected because the matter
in atoms is spread out over a sphere. But this did not happen. What did happen is
that most of the time the stream of particles hit nothing and every once in a while a
particle was deflected right back at the source. See the picture of Rutherford’s
experiment below.
Thomson’s Model Fails The uniform mix of mass in Thomson’s model of the atom did
not allow for most particles to pass through unscathed and certainly not for some
particles to be deflected straight back. Below are two figures – what scientists thought
would happen and what actually happened.
Thomson’s model – what was expected
What actually happened
A Model with a Nucleus
The Proton From his experiment Rutherford concluded that almost all of the mass of
the atom and all of the positive charge must be crammed into an incredibly small
region of space at the center of the atom. He made a new model of the atom and
placed protons at the nucleus and electrons, with very little mass, orbiting the nucleus.
Rutherford’s model explained not only his experimental results, but also past
experimental results too.
The Neutron Rutherford’s nuclear model was applauded by other scientists; however
some of the data did not fit. For instance, an atom’s electrons have almost no mass.
This means according to Rutherford’s model, the only other particle around was the
proton, and hence the mass of an atom should be approximately equal to the mass of
its protons. The problem was most atoms were twice as massive, sometime more,
than the protons present.
From this discrepancy it was proposed that there must
be another particle in the nucleus, one without an
electric charge, and this particle would be called the
neutron.
Proving the existence of neutrons was not easy
because they have no electric charge and they do not
react to magnets. It took another 20 years before
scientists were able to confirm that atoms contain
neutrons. When this occurred in 1926 the model of
the atom was yet again revised.
Electron Orbits to Clouds
Bohr Model With the protons and neutrons
firmly tucked away in the nucleus, scientists
set out to map how electrons orbit the
nucleus. From experiments, it was known
that electrons only emit certain types of
energy. One of the first successful attempts
to model electrons capable of emitting only
select energies was done so by Niels Bohr.
He suggested that electrons orbit the
nucleus in perfect circular orbits. Bohr’s
model also stated electrons could only orbit
the nucleus in set pathways and they could
leap from orbit to orbit, but they were not
permitted in the spaces in between.
Cloud Model Further experimentation in the 20th century showed that electrons really
did not act as particles in simple orbits circling the nucleus. As a matter of fact, it
turned out that predicting exactly where an electron is in the atom was a very difficult
task – as electrons tend to spread out over an entire space, much like an ocean wave
spreads out over a stretch of beach. Scientists determined that it was impossible to
know the precise location of an electron at any particular moment, because electrons
would often act like waves. The current model of the atom allows for the somewhat
unpredictable wave nature of electrons by defining where an electron is most likely to
be, but it doesn’t mean the electron is actually there. Electrons travel in a region
surrounding the nucleus called the electron cloud.
Questions about the reading
1. According to Democritus’ reasoning, what should eventually happen if you cut a
piece of paper in half again and again?
2. Starting in the 18th century scientists in simple laboratories started breaking
substances apart. What did they discover?
3. Which of Dalton’s ideas about matter are still true today? Which of Dalton’s
ideas about matter are false, and why are they false?
4. Why was it such a big deal that Thomson found electrons inside the atom?
5. What particle did Rutherford discover, and how was his model of the atom
different from Thomson’s model?
6. Rutherford’s model of the atom simply wasn’t massive enough. What was his
model missing?
7. Describe Bohr’s model of how electrons go around the nucleus.
8. As best you can, describe the current model of the atom.