History of the Atom
More than 2000 years ago people believed that matter consisted of four elements:
1. ____________________
2. ____________________
3. ____________________
4. ____________________
Soon people began to realize that matter was made up of tiny particles that could not be
broken down any further. We now call these particles _______________.
Dalton was one of the first men to propose a theory about the atom. His theory had four
main parts.
Dalton’s Atomic Theory
1. All matter is made of ______________ too small to see
2. Each element had its own _____________ with a specific mass.
3. ________________ form when two elements combined.
4. ______________ couldn’t be created, destroyed or subdivided.
In the 1800s people believed that matter contained both positive and negative charges.
They also knew that opposites attracted so they thought this was why elements joined
together to form compounds.
We now called the positive charges __________________ and the negative charges
_____________________.
In 1904 Thomson developed a theory to explain how the positive and negative charges
inside an atom were arranged.
Thomson’s Raisin Bun Theory:
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Rutherford’s Gold Foil Experiment
In 1911 Rutherford decided to test Thomson’s theory and see if he was correct. He took
a piece of gold foil (just like aluminum foil) which would have contained several gold
atoms. At it he fired tiny positive particles. He had a screen set up on the other side to
tell him if the particles went through the gold foil.
If Thomson were right all the particles should go right through the gold foil because
there would be no area with lots of positive charge to repel them.
What he found was that some of the particles did go right through but not all.
There must have been an area with lots of positive charge to repel the positive
particles. (Remember that likes repel likes).
Rutherford thought all the positive protons must be located together in an area. He
called this area the nucleus.
All the negative electrons must be located around the nucleus.
Rutherford’s Model of the Atom looks like this:
He didn’t know anything about the neutron. Later it was found that the neutrons were in
the nucleus too.
The three subatomic particles are arranged in an atom like this:
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Bohr-Rutherford Diagram Quiz
Total = 12 Marks
Using only your periodic table, draw correct Bohr-Rutherford diagrams for each of the
following. Each diagram is valued at 3 marks.
1. Hydrogen
2. Carbon
3. Magnesium
4. Chlorine
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Bohr Rutherford Diagrams
We have learned that the protons and the neutrons of an
atom are both located within the nucleus of that atom and
that the electrons are located outside. A man named Bohr
modified this theory of the atom slightly. He said that the
electrons were indeed outside the nucleus but did not float
randomly around the nucleus. Rather they existed in
certain levels around the nucleus called orbits or spheres.
Rutherford’s Gold Foil Experiment
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There are certain rules we must follow when drawing Bohr
Rutherford diagrams:
• Use your periodic table to determine the numbers of
protons, neutrons, and electrons
• Place the protons and the neutrons in the nucleus
• Place up to two electrons in the first orbit
• Place up to eight remaining electrons in the second
orbit
• Place up to eighteen electrons in the third orbit (we
will only be filling this level up to eight for reasons
which will be explained later in grade 11 chemistry)
• Always fill from the inner most level and move
outward until you run out of electrons
Bohr-Rutherford Diagram
Draw a correct Bohr-Rutherford diagram for
an aluminum atom.
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Bohr-Rutherford Assessment
Draw a correct Bohr-Rutherford diagram for
a sodium atom.
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Bohr-Rutherford Assessment
Draw a correct Bohr-Rutherford diagram for
a sodium atom.
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Are the protons and neutrons located in the nucleus?
Are the electrons located around the outside in neat orbits or
levels?
Does the number of electrons follow the pattern: 2 in the first
level, 8 in the successive levels?
Do the number of protons and electrons equal the atomic number?
Does the number of neutrons equal the difference between the
atomic mass and the atomic number?
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Protons, Electrons, and Neutrons
Remember our two rules for calculating the numbers of protons, electrons and
neutrons:
1. The atomic number = the number of protons and the number of electrons
2. Atomic mass – atomic number = the number of neutrons
Complete the following table using your periodic table.
Symbol
Atomic
Number
Atomic
Mass
# Protons
# Electrons
# Neutrons
O
F
H
Ne
Au
7
23
17
20
1. What do you notice about the numbers of protons and electrons in any atom?
2. Is the number of neutrons always the same as the number of protons and
electrons?
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