Chapter 8 Composition of the Atom
8-1 Scientific Modelling
Scientists use models to help explain things that cannot be observed.
This happens all the time; consider a globe for example….
Scientific models are only representations of how we perceive things
to be – and frequently our models change to fit new information.
Good models can be used to make good predictions, propose
experiments or bring understanding.
8-2 The Discovery of the Electron – Crookes’ and Thomson’s
Cathode Ray Tubes
Crookes’ Tube
Thomson’s Tube
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Question:
What did these experiments show?
Answer:
There is a component of the atom which has a negative electrical
charge.
Why?
8-3 The Proton – Thomson’s 2nd Tube
Question:
What did this experiment show?
Answer:
There is a component of the atom which has a positive electrical
charge.
Why?
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8-4 (A) Discovering Relative Sizes
Thomson’s experiments lead to a significant conclusion:
Since the amount of deflection of a beam of particles is proportional
to the ratio of it’s charge (e) to mass (m), then the experimental
value should give you some idea of how the two values compare.
The experimental value was found to be:
__e__
m
=
___1.76 X 108 C (coulombs)____
gram
What does this tell us? Why?
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8-4 (B) Millikan’s Experiment
Questions:
What did the ionizing radiation do?
How would the charged metal plates affect a positively charged oil
drop?
How would the charged metal plates affect a negatively charged oil
drop?
What do you think would happen to a negatively charged oil droplet
as you increased and decreased the charges on the plates (made
them more or les +/-)?
Okay, the beauty of the experiment…
If you know the rate of the rise in the oil droplet, it’s mass and the
charge on the plates, you can calculate the mass of an electron!
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This turned out to be 9.11 X 10-28 g (~ 1840 electrons = 1 proton)
It was also determined that this value held for all gases which implied
that the electrons from all gases were the same.
Okay! One more question:
How would you use this same apparatus to determine the mass of a
proton?
Turns out that all gases did not behave the same, that Hydrogen was
the lightest with a value of 1.67 X 10-24 g.
Turns out that ionized hydrogen IS a proton!
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8-6 / 8-7 Sir Earnest Rutherford’s Contribution
see handout
Question:
What is an alpha particle? What kind of charge does it have?
Run simulation
Question:
What does the fact that most of the α – particles went straight
through tell us about the nature of the atom?
What does the fact that some of the α – particles were deflected tell
us about the nature of the atom?
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What does the fact that some of the α – particles were deflected
directly back at the source tell us about the nature of the atom?
8-8 The Neutron
1932. Irene Curie & Frederic Joliet bombard Be with α – particles. A
high penetrating powered beam is made. Only thing is…the beam is
not affected by electrical charges.
Question:
What does this tell us about the nature of the beam?
The ‘Evolution of the Atomic Model’
1807 – The Dalton Model. Solid Sphere
1903 – The Thomson Model. Plum pudding
1911 – The Rutherford Model. Planetary
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Atoms –
• positively charged dense nuclei composed of protons (+) and
neutrons (0).
• negatively charged electrons (-) in cloud-like orbits or energy
levels.
proton
neutron
electrons
amu
1
1
1/1820*
charge
(+)
(0)
(-)
symbol
p
n
e-
* do not significantly add to the atomic mass of an
element
The Nucleus and Isotopes
figure
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Atomic Number.
The identity of an element is determined by its atomic number - (THE
NUMBER OF PROTONS)
Atomic Mass.
The atomic weight/mass of an element is determined by the - SUM
OF PROTONS AND NEUTRONS in the nucleus.
Isotopes
are variations of an element having the same atomic number with a
different atomic mass
…think Æ (otherwise it would be a different element)
* isotopes are “produced” by varying the number of neutrons
Three isotopes of carbon
figure
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Why are there neutrons?
figure
figure
without n - strong
(+) - (+) repulsive force
would blow the nucleus
apart
n - shield repulsive effects
and act like nuclear
“glue” (aka - nuclear
force)
As the number of protons increases, the number of neutrons does as
well, but at a greater rate. When you get many protons in a nucleus ,
the number of possible (+) - (+) interactions increases
dramatically…so you need much more glue.
Calculating atomic masses from isotopic data
Atomic masses are commonly quoted in decimal numbers. This is
because these numbers refer to the average atomic masses of all the
isotopes as they occur in nature. Therefore, if a whole number is
quoted on you periodic table, this means that there is only one
version of this element: this means that it was manufactured…it
does not occur in nature!!!
Suppose you were told that chlorine has two isotopes and is present
in nature in the following proportions:
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Chlorine-35 (75.77%)
Chlorine-37 (24.23%)
Find the average atomic mass (as would be expected on the Periodic
Table).
Calculate: Carbon and oxygen using the data on p237 of Heath.
Electrons
Electrons are important for two main reasons:
1) They balance the charge of the nucleus
2) They determine the reactivity of an element since chemical bonds
are formed at the electron cloud.
- They do not exist in “planetary” orbits, but in “probability”
clouds at different levels (energy levels) from the nucleus.
These energy levels are further sub-divided into “sub-orbits”
each of which is occupied by a maximum of two electrons.
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- Electrons “load” at the lowest energy level (shell) first and then
into increasing energy levels in the most stable configuration
possible.
See handouts
Mr. H’s Handy-Dandy Electron Loading Memory Aid (phew!)
7s
6s
5s
4s
3s
2s
1s
7p
6p
5p
4p
3p
2p
7d
6d
5d
4d
3d
7f
6f
5f
4f
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Example
Isoelectronics - when one element (ionized) has the same electron
configuration as another element. (Usually a step up or down on the
periodic table).
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