Download Inside the Atom

Inside the Atom
So far in our Chemistry unit we have looked at some of the basic ideas of
Chemistry, and have started to look at elements, compounds, and the periodic
table. But before we go any further, it is important for us to understand exactly
what an atom is. To do this, we are going to briefly examine the development of
our current model of the atom.
The earliest model of the atom was that of Dalton in 1803. He proposed that
atoms were tiny, indivisible spheres, like a billiard ball. However, experiments
soon showed that this was not the case.
The Electron
Near the end of the 1700’s scientists had discovered electricity. They had made
hydroelectric power, light bulbs, batteries, electric batteries, and many more
inventions. However, they still did not know what electricity actually WAS!
One of the inventions that was being used to investigate electricity was a gas
discharge tube. It consisted of a hollow glass tube with electrodes (metal)
embedded at each end. When a source of electricity was attached to the
electrodes, the gas inside the tube would glow. It was very similar to Neon
lights that we still use today! (The technology is also very similar to how regular
televisions work today).
In 1855, a German mechanic and glass blower named Heinrich Geissler improved
a gas discharge tube by including a vacuum pump (to remove the gas inside the
tube). He discovered that if you remove most of the gas from inside the tube,
the glass wall of the tube would glow green. This happened regardless of what
gas had been inside the tube OR what metals the electrodes had been made of.
The conclusion that was made was that a particle was passing between the
electrodes. This was what made the gas glow AND what made the glass glow
green. Since the particles seemed to be coming from the cathode, they called
them cathode rays. Since they were coming from the cathode (which was
attached to the negative side of the electricity source), they concluded that they
must be negative.
Other experiments (that you will learn about in Grade 11) determined that the
cathode ray particles had very little mass.
Today we call these particles electrons.
The Proton
In the late 1800’s, another scientist named JJ Thompson was doing more
experiments with a gas discharge tube. While thinking about his experiments
(and those of other Scientists that he had read about), he came to some
important conclusions. Basically, he realized:
 The electron seems to come from atoms
 Electrons are negative but atoms themselves are neutral
 Electrons have very little mass, but atoms have lots of mass
 The only way all of the above statements can be true is if there is a
positively charge, much more massive particle, inside the atom also.
He made some modifications to his gas discharge tubes, tested for positive
particles, and discovered that there were also positive particles inside the atom.
Similar to what was done with electrons, he repeated his experiments with
different metals, and found that the positive particles seemed to be present
regardless of what metal was used.
He called the positive particles the proton. It was 2000 times more massive
than the electron. In fact, the proton was as heavy as a hydrogen atom.
Previous experiments had shown that hydrogen had one electron. Since the
proton was as heavy as a hydrogen atom, scientists concluded that hydrogen
was one proton and one electron.
The discoveries of electrons and protons led Thomson to propose a new model
of the atom. In it, the negative electrons are embedded in a framework of
positive particles. It is often called the “plum pudding” or “raisin bread” model.
The Discovery of Radiation: The Next Great Tool
In the late 1800’s (near the same time Thompson discovered the proton), a
scientist named Roentgen discovered X-rays (by accident!) He was working with
cathode rays, using a crystal that was known to glow when struck by ultra violet
(UV) light. He wanted to determine if a cathode ray tube also produced UV light.
Te be able to see the glow better, he turned off the lights in his laboratory.
While working, he realized that his spare crystals (on the other side of the room)
were also glowing! They would even glow if he put them in the next room, and
turned on the cathode ray tube!
He realized that the cathode ray tube was giving off very strong radiation (which
he called Radiation X), which could even pass through walls! The publication of
his results led to many scientists repeating his experiments. Unfortunately, they
had not realized that large doses of x-rays would destroy human tissue, and
many died over the next few years as a result of their own experiments.
What do we do today to prevent overexposure to x-rays in hospitals?
Henri Becquerel also took an interest in these “X-rays”. He experimented with
several types of crystals, including ones containing uranium, to see if they would
emit x-rays when exposed to UV light. While preparing for an experiment, he
placed some of his crystals on top of his detector (photographic films). Imagine
his surprise when these films produced a picture of the uranium containing
crystals, without being exposed to UV light! He had discovered another form of
radiation, one that was spontaneously emitted from uranium.
Marie Curie did many more experiments with Uranium. She discovered that
there were other elements that emitted the particles, and that the strength of
the emissions only depended on the amount of material, not its form. She
coined the term radioactivity, which meant to spontaneously emit particles. She
was also the first woman in France to receive a Ph.D.
The Nucleus
It was the discovery of radiation that allowed a scientist named Rutherford to
conduct experiments which completely changed our understanding of the atom.
He was working at McGill University in Montreal. He placed a sample of uranium
inside a lead block drilled with a very small hole, so that the radiation could only
escape in one direction. He then aimed the radiation at a very thin piece of gold
foil, so thin it was only a couple of atoms thick.
Open Rutherford.htm from the memory key.
Rutherford himself believed the plum pudding model, and expected that most of
the particles would be deflected a bit as they collided with the essentially solid
but loosely packed atoms.
The above diagram shows what we would expect the result of Rutherford's
experiment to be if the "plum pudding" model of the atom is correct.
However, the results were not as he expected. He found that 9999 out of every
10 000 particles when straight through. However, the ones that did not go
straight through were deflected at many different angles. A few even bounced
straight back!
Rutherford described the result as “firing a 15 inch shell at a piece of tissue
paper, expecting it to go straight through, and having it bounce back!”
The above indicates the actual result. Most of the alpha particles are only
slightly deflected, as expected, but occasionally one is deflected back towards
the source.
Based on his results, he proposed a new model of the atom. In it, the atoms
was mostly made of empty space, with a very dense core of matter he called the
nucleus. The nucleus was made up of protons, and the electrons were outside
the nucleus. He did not, however, say exactly where the electrons were.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
http://www.shsu.edu/%7Echm_tgc/sounds/ruther.mov
The Controversy: Was it All Wrong?
Despite all of the evidence in favor of the Rutherford model, when it was
published it caused a lot of controversy. Many scientists were unwilling to
admit they were wrong agreeing with the plum pudding model.
Others argued that if the (negative) electrons were outside the (positive)
nucleus, they should be drawn together because they had opposite
charges.
The Bohr-Rutherford Atom
A scientist named Bohr became interested in the controversy, and
proposed a modification to the Rutherford model that solved all of the
problems. Instead of just saying that the electrons were outside the
nucleus, he proposed that they were circling it. If they were moving at
just the right speed, they would circle it forever without falling into it.
He compared the atom to the solar system. The planets are attracted to
the sun (due to gravity), but are moving at just the right speed so that
they do not fall into it.
He called the distances at which the electrons circled the nucleus shells or
levels. He was able to calculate that the first shell could contain two
electrons and the second 8. The electrons were paired together within the
shells. The Bohr-Rutherford model of a Sodium atom, therefore, looked
like this:
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BohrModel/Flash
/BohrModel.html
The Neutron
While studying the atom, Rutherford discovered that the number of protons he
could detect coming from atoms often did not seem to match the mass of the
atom. This led him to propose that there was another particle inside the atom
which also had mass but no charge. He could find no experimental proof that it
existed, however. In the 1930’s James Chadwick, a student of Rutherford,
proved the existence of the neutron.
We can use what we have learned so far to determine the number of
protons, neutrons and electrons in an atom. The general rules are as
follows:
1. The number of protons is ALWAYS equal to the atomic number
2. The number of electrons for a Neutral atom is equal to the
number of protons. We will learn about charged (not neutral)
atoms later in the unit.
3. You subtract the atomic number from the mass number (or
rounded atomic mass, if you do not have the mass number) to
get the number of neutrons.
Complete this table. The first two lines are done for you. If the mass
number is missing, use the rounded atomic mass from the periodic table.
Atomic
Number
Element
Symbol
1
2
6
8
16
hydrogen
helium
carbon
oxygen
H
He
C
K
gold
Mass
Number
1
12
16
32
Protons
1
2
6
19
79
Electrons
Neutrons
1
2
0
2
20
How many electrons are in each level? Remember:
1. The number of total electrons is equal to the number of protons, which
is equal to the atomic number
2. The first level gets 2 electrons, the second level 8, and the third (well,
you’ll see)
Number of Electrons in the Level
Element
Atomic
Number
Hydrogen
Carbon
Oxygen
Magnesium
Chlorine
Argon
1
6
1st
Level
1
2nd
Level
0
3rd
Level
0
Summary: Parts of the Atom
Part of
the Atom
Nucleus
Proton
Neutron
Electron
Location
Center
of the
atom
inside
the
nucleus
inside
the
nucleus
outside
the
nucleus
Charge
Mass
(AMU)
Depends
on the
atom
Extra Information
Consists of protons and neutrons.
Contains most of the mass of the
atom
They NEVER leave the nucleus. The
number of protons determines the
identity of the atom. They are equal
to the atomic number.
They never leave the nucleus. The
number of neutrons is the mass
minus the atomic number.
They circle the nucleus. They can be
made to leave an atom without
changing the identity of the atom.
There is usually as many electrons as
there are protons.