Transcript The Development of Atomic Theory

The Development of Atomic Theory
Objectives: Students will be able to:
• Make inferences based on observations in
order to make conclusions about atoms.
• Research a specific atomic model in order
to make a presentation to another group.
Warm Up:
What do dinosaurs and atoms
have in common?
What do Dinosaurs and Atoms
have in Common?
No one has seen an atom or a dinosaur directly.
We know of their existence only by indirect
evidence. Our theories of both dinosaurs and
atoms has changed over time based on this
indirect evidence.
What Do You Know About Atoms?
Using a white board you may:
–Draw your representation of an atom.
–If you are not sure, draw what you
think an atom might look like.
–Describe what you know about
atoms.
Exploration Activity
• In your team, make and record observations about
the contents of two sealed bags. DO NOT open
bags. Use your senses of touch, smell, and
hearing to help you make your observations.
• You will make a prediction of what is in the bags.
• Be prepared to share your findings and explain
your reasoning behind your predictions.
You have 5 minutes to fill in chart and answer 1st question.
Activity (continued)
• After hearing the presentations, answer the
questions on the back of the hand-out
individually.
• What did you learn from the activity to
explain how scientists study atoms.
Scale of the Universe
• Now that we know atoms are small, how
small are they?
Do Theories in Science Stay the
Same?
• Ideas and theories in Science change as new
information is gathered.
Our theory about the atom has changed over time as
new studies are done. Even though no one has
ever seen an atom up close, we are still able to
make new discoveries – just like we have made
new discoveries about dinosaurs.
This fossil evidence shows us that some dinosaurs may
evolved into birds.
Who was Democritus?
Democritus was an ancient
Greek philosopher who lived
from 460 - 370 B.C.
What did Democritus conclude about
cutting matter in half? There was a limit
to how far you could divide matter. You
would eventually end up with a piece of
matter that could not be cut.
He thought matter is like motion. It cannot
be divided in half forever. The tortoise and
hare would never finish the race if you could
keep dividing the distance to the finish line
in half forever.
What does the Greek word
atomos mean?
• The Greek word “atomos”
means not able to be divided
or
“indivisible.”
Why was Democritus’s ideas not accepted?
• Aristotle was a very
famous Greek philosopher
who believed that matter
could be divided into
smaller and smaller pieces
forever. He held a very
strong influence on
popular belief and his
views on this were
accepted for two thousand
years.
What is an Atom?
• An atom is the smallest
particle that an
element can be divided
and still be that
element.
• For example the smallest
particle of carbon is a
single atom of carbon. If
you divide it is no longer
carbon anymore.
PIECES
OF
CARBON
CARBON
ATOM
John Dalton 1776-1844
• Two thousand years later a
British chemist and
schoolteacher brings back
Democritus’s idea of the
atom
• He performed many
experiments to study how
elements join together to
form new substances
• He found that they
combine in specific ratios
(remember the electrolysis
of water) and he supposed
it was because the
elements are made of
atoms.
What 3 new ideas did John Dalton
propose about the atom?
• All substances are made up of atoms which
are small particles that cannot be created,
divided, or destroyed.
• Atoms of the same element are exactly alike
and atoms of different elements are
different.
• Atoms join with other atoms to form
different substances
Dalton’s Atomic Theory
When magnesium burns, it
combines with oxygen. In
magnesium oxide, the ratio of
the mass of magnesium to the
mass of oxygen is always about
3 : 2. Magnesium dioxide has a
fixed composition.
Dalton’s Atomic Theory
Dalton made these wooden spheres as a model to
represent the atoms of different elements. A tiny,
solid sphere with a different mass represents each
type of atom.
Dalton’s Atomic Theory
A theory must explain the data from many
experiments. Because Dalton’s atomic theory met
that goal, the theory became widely accepted.
Over time, scientists found that not all of Dalton’s
ideas about atoms were completely correct. They
revised the theory to take into account new
discoveries.
J.J. Thomson 1856-1940
• What particle did
Thomson discover?
• J.J. Thomson discovered
that atoms are made of
smaller negativelycharged particles called
electrons.
• Thomson’s discovery was
the result of doing
experiments with “cathode
ray tubes”
Thomson’s Cathode Ray
Experiment
Stream of electrons is attracted to positively
charged plate here.
"What are these particles?
are they atoms, or
molecules, or matter in a still
finer state of subdivision?“ quote by Thomson
Thomson’s Model of the Atom
When the current was turned on, the disks
became charged, and a glowing beam appeared in
the tube.
• Thomson hypothesized that the beam was a stream
of charged particles that interacted with the air in
the tube and caused the air to glow.
• Thomson observed that the beam was repelled by
the negatively charged plate and attracted by the
positively charged plate.
Thomson’s Model of the Atom
Evidence for Subatomic Particles
Thomson concluded that the particles in the beam
had a negative charge because they were attracted
to the positive plate. He hypothesized that the
particles came from inside atoms because
• no matter what metal Thomson used for the disk, the
particles produced were identical.
• the particles had about 1/2000 the mass of a
hydrogen atom, the lightest atom.
Thomson’s Model of the Atom
Thomson’s Model
Thomson revised Dalton’s model to account for
these subatomic particles.
• The atom has neither a positive nor a negative
charge, but there must always be some positive
charge in the atom.
• The atom is filled with a positively charged mass of
matter that has negative charges evenly scattered
throughout it.
The Plum Pudding Model
• Thomson did not know how the
electrons in an atom were
arranged. He believed they were
mixed throughout an atom.
• He proposed that the atom was a
sphere of positively charged
material. Spread throughout the
atom were the negatively charged
electrons similar to plums in a
pudding or chocolate chips in
ice cream.
Ernest Rutherford
(1871 - 1937)
• Awarded the Nobel Prize
in Chemistry for his
discovery of alpha
particles, positively
charged particles emitted
from radioactive elements
• Was a student of J.J.
Thomson but disagreed
with the “Plum Pudding
Model”
• Devised an experiment to
investigate the structure of
positive and negative
charges in the atom.
An Interactive Model of
Rutherford’s Gold Foil
Experiment
http://micro.magnet.fsu.edu/electromag/java/rutherford
Click here
4.1 Studying Atoms
Rutherford’s Atomic Theory
The Gold Foil Experiment
Deflected
particle
Undeflected
particle
Alpha
particles
Gold atoms
Slit
Beam of alpha
particles
Alpha
particles
Screen
Source of
alpha particles
Nucleus
4.1 Studying Atoms
Rutherford’s Atomic Theory
Discovery of the Nucleus
The alpha particles whose paths were
deflected must have come close to
another charged object. The closer
they came, the greater the deflection.
However, many alpha particles passed
through the gold without being
deflected. These particles did not pass
close to a charged object.
What did most of the particles
shot at the gold foil do?
• Most of the particles traveled straight
through the gold foil
What was the surprising behavior of
a few of the particles?
• A few
of the particles were deflected
and some even bounced back
Rutherford’s Revised Atomic Theory (1911)
Result: Most of the positively charged particles went straight through
the gold foil.
Atomic Theory: Most of the matter of the atom is found in a very small
part of the atom. This is called the nucleus of the atom. It is very tiny
and extremely dense.
Result: Some of the positively charged particles were deflected or even
bounced back.
Atomic Theory: Like charges repel so the nucleus must have a positive
charge. If electrons have a negative charge they could not be in a
positively charged nucleus. Electrons must surround the nucleus at
a distance.
Result: The diameter of the nucleus is 100,000 times smaller than the
diameter of the entire gold atom.
Atomic Theory: Atoms are mostly empty space with a tiny, massive
nucleus at the center .
Rutherford’s Atomic Theory
Thomson’s model did not explain all of the
evidence from Rutherford's experiment.
Rutherford proposed a new model.
• The positive charge of an atom is not evenly
spread throughout the atom.
• Positive charge is concentrated in a very small,
central area.
• The nucleus of the atom is a dense, positively
charged mass located in the center of the atom.
Rutherford’s Atomic Theory
The Houston Astrodome
occupies more than nine
acres and seats 60,000
people. If the stadium
were a model for an atom,
a marble could represent
its nucleus.
The total volume of an
atom is about a trillion
(1012) times the volume of
its nucleus.
The Bohr Model of the Atom
What did Bohr learn about electron movement?
• Bohr proposed that
electrons move in paths at
certain distances around
the nucleus.
• Electrons can jump from a
path on one level to a path
on another level.
• Click here
http://www.colorado.edu/p
hysics/2000/quantumzone/
bohr.html
The Modern Theory of the Atom
• Electrons
travel in
regions
called
“electron
clouds”
• You cannot
predict
exactly
where an
electron will
be found
http://www.fearofphysics.com/Atom/atom3.html
Energy Levels
The energy that an electron
has is based on its location
around the nucleus.
(Electrons that are closer to
the nucleus have less
energy than those that are
farther away from the
nucleus)
How can bookshelves help you understand
the movement of electrons?
•Each shelf represents an energy level
•Each book represents an electron
•You can move a book to a higher or
lower shelf with the correct amount of
energy.
•A book cannot be between shelves
(An electron can move by gaining or
losing energy but can never be
between energy levels)
How small are atoms?
THERE ARE 2 X 1022 ATOMS IN A PENNY. If all the
atoms in a penny were blown up to the size of a grain of
sand they would cover the entire state of California
What can a scanning
tunneling electron
microscope show us?
• These images do not
show an actual
picture of an atom.
They show a colorenhanced image of
the surface of a
material at the atomic
level.
Image of hydrogen atom
The Universe and Atoms start
here
• A Star Turned Inside Out
Large Hadron Collider
• LHC
• CERN – Interactive Game
Formative Assessment
• Kahoot on Atomic Structure 4-1
Closure
• Do theories in science stay the same?
Explain.
• What do you now know about current
atomic theory and how it developed?
Homework
• p. 105 Writing to Persuade Imagine you
live in ancient Greece. Assume all you
know about matter is what you can observe
with your five senses. You have heard the
views of both Democritus and Aristotle
about matter. Write a paragraph
supporting one of their views.