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Chapter 7
1. Definitions:
- electron
- proton
- neutron
- subatomic particle
- radioactivity
- nucleus
- electron cloud
- electron shell
- Bohr-Rutherford Model
- Atomic number
- Atomic mass unit
- Mass number
- Isotope
2.
3.
4.
5.
Note- Models of atomic structure
read pg 234- Thomson- and questions
pg 240-244 Bohr-Rutherford- and questions
246- 248 and questions ( Atomic number…etc)
Chapter 7: Models of Atomic Structure
Investigating the Atom
During the 19th century, when many chemists were studying elements,
other scientists were interested in the way electricity interacts with matter.
Because they knew that air conducts electricity better if it is trapped and its
pressure is reduced, they used gas discharge tubes in their investigations.
Gas discharge tubes are made of glass and can trap air and other
gases. They are equipped with electrodes at either end: The positive charged
electrode is called the anode, and the negatively charged electrode is called
the cathode. The pressure inside the tube can be reduced by pumping air out
of the tube. (Modern gas discharge tubes are used in television picture
tubes.)
Scientists found that when they reduced the pressure in the tube, the
gases inside began to glow! The reason was that cathode rays from the
cathode were traveling through the tube toward the anode and were carrying
a negative charge. The search for the electron began! (Electron is the word
they used to describe tiny, negatively charged bits of matter).
Scientists were puzzled. If atoms contain negatively charged
electrons, why did they usually have no charge?
Joseph John Thomson (1856-1940) inferred that atoms must possess
something with a positive charge to balance the negatively charged
electrons. He experimented with tubes of hydrogen gas and concluded that
anode rays must be made up of positive particles. Thomson called them
protons.
Thomson concluded the following:
-all atoms contain both protons and electrons
-all protons are identical, and all electrons are identical.
Electrons differ from protons, however.
-an electron has a negative charge. A proton has a positive
charge. A proton has a positive charge.
-an electron has the same amount of charge as a proton, even
though the charges are opposite in kind.
-a proton has much more mass than an electron.
1938-The first electron microscope was made by Albert Prebus and James
Hillier at the University of Toronto. This gave scientists more knowledge
about the structure of the atom.
In 1803, Dalton had said that atoms were indivisible. By Thomson’s
time (1900’s), it was evident that this was not true! Atoms could be torn
apart by high-energy electricity. Protons and electrons were known to be
sub-atomic particles
Thomson’s view of subatomic particles:
Negative electrons were like raisins in a muffin:
they could be pulled out of the positively charged dough
In 1895, x-rays were discovered. This led to the study of radioactivity.
During that time, a student of Thomson’s named Ernest Rutherford was
teaching at McGill University. He won the Nobel Peace Prize in Chemistry
for his work in radiation.
In 1909, Rutherford probed atoms with radioactive particles. He discovered
that atoms must also contain:
-a nucleus: a tiny core that is very small in volume, dense compared
to the rest of the atom, and intensely positive.
-an electron cloud: an “envelope” that is very large in volume, light
compared to the nucleus, and negatively charged.
Because the protons in an atom’s nucleus account for less than half the mass
calculated for the nucleus, Rutherford inferred that the nucleus must also
contain additional, uncharged (neutral) particles. He called them neutrons
and concluded that each neutron would have roughly the same mass as a
proton. His ideas would not be confirmed until the 1930’s!
In 1912, a Danish scientist, Niels Bohr, began working with Rutherford in
Manchester, England. He was trying to explain the question put forth by
doubters of Rutherford’s ideas: if positive and negative charges attract, why
didn’t the negatively charged electrons crash into the positively charged
nucleus?
Bohr came up with a model that compared the nucleus to the sun and the
electrons to the planets. Why doesn’t the gravitational pull of the sun cause
the planets to spiral inward and crash? Because they revolve at just the right
speed to remain in orbit. Similarly, the atom’s positively charged nucleus
exerts a strong force of attraction on negative electrons. The electrons don’t
spiral inward and crash because they are moving rapidly in fixed regions
around the nucleus. These 3-D, sphere-shaped regions are called electron
shells. Bohr concluded that the more energy an electron had the more
distant shell (energy level) it could occupy.
Bohr-Rutherford Model of Atomic Structure
1st
shell
2nd
shell
3rd
shell
This was the first modern view of the atom!
In 1913, Henry Mosely began working in Rutherford’s laboratory. He
discovered a very regular pattern in the way different elements responded to
x-rays. This pattern provided information about the atomic nucleus of each
atom. He determined an increasing number of protons as you move through
Mendeleeve’s table. #of protons=Atomic #
Atomic Number: number of protons in an atom.
Example: Fluorine (9) –has nine protons in the nucleus
Therefore, it also has 9 electrons
How many neutrons does it have? You cannot tell by the atomic number.
You must look at the mass number (based on atomic mass): gives the total
# of protons and neutrons.
Ex: the atomic mass of F = 19
Atomic #
- = 9
____
10 neutrons
Proper notation:
F
Atomic Mass Units: because atomic mass is not always a whole number, it
is stated in atomic mass units (u).
Atomic mass is not the same as the mass expressed in grams. It is a ratio.
All atomic masses are defined relative to the mass of carbon12.
Thomson’s Inferences about the Proton (P234)
Questions:
1. Who is JJ Thomson referred to as?
2. What did he infer about the protons and electrons of
the atom?
3. What is meant by the term subatomic particle?
4. What was one idea about the atom that was changed in
Dalton’s Theory by Thomson’s time?
5. Draw Thomson’s model of the atom.
6. What did he compare his model to?
Answers:
1. The Father of the Electron.
2. The atom must have something with a positive charge to balance the
negatively charged electron.
3. Particles smaller then an atom
4. By Thomson’s time it was clear that atoms could be torn apart by
high-energy electricity; therefore atoms were divisible after all.
5. Diagram:
6. Raisin bread/raisin muffin
The Bohr- Rutherford Model
Page 238 to 244
Questions:
1.
2.
3.
4.
What does Rutherford conclude about the model of the atom?
How did Bohr contribute to Rutherford’s findings?
What is the Bohr- Rutherford model?
Draw the diagram of the atom that came from the conclusions of
Bohr- Rutherford. ( Fig.7.20)
What is an electron shell?
Do the activity on “Summarizing Atomic Structure” of Scientific
Inquiry on page 244.
5.
6.
Answers:
1.
Rutherford concluded that:
a. An atom contains a nucleus that is a tiny core, very small in
volume, dense compared to the rest of the atom and intensely
positive.
b. The atom has an electron cloud that is an envelop very large in
volume, light compared to the nucleus and negatively charged.
c. He also inferred that the protons were not alone in the nucleus
and realized the nucleus must contain additional (uncharged)
particles. These particles were identified as neutrons which
have roughly the same mass as a proton but no charge.
2.
Borh’s contribution to the model states the atom’s positive nucleus
exerts a strong force of attraction on the negative electrons. This
fixed positioning of the electrons was identified as an electron shell.
3.
The Bohr-Rutherford model has a central nucleus and electron shells.
It can be called the first “modern” view of the atom.
Diagram page 243
Electron shell: In Bohr’s model of an atom, a fixed, three dimensional
sphere-like region around the atom’s nucleus; electrons move rapidly
within the shell.
4.
5.
Atomic Number, Mass Number and Atomic Structure: Page 246Isotopes pg 248
Questions:
1.
2.
3.
4.
5.
6.
7.
What is the role of a neutron?
What does the mass number represent?
How do you find the number of neutrons in an atom?
Is the atomic mass and the mass number the same? Why or why
not.
Define atomic mass unit.
What is an isotope?
Due to the different mass numbers of certain elements, why is
the atomic mass of an element on the periodic table constant?
Answers:
1. Neutrons hold the nucleus together and keep the protons from flying
apart.
2. Mass number is the total number of protons and neutrons in the
nucleus.
3. Subtracting the atomic number from the mass number.
4. No, mass number is the number of protons and neutrons in the
nucleus; atomic mass is an average mass of an element.
5. The unit used to measure the mass of atoms relative to 1/12 the mass
of the atom of carbon 12.
6. Any of two forms of an element that have the same number of
protons but a different number of neutrons.
7. The atomic mass in the periodic table is an average of the isotopes.