Transcript 2.1 Modern Atomic Theory pptx

Do Now 8/29/14
A student in the lab wanted to
classify an unknown substance as an
element, compound, homogeneous
mixture, or heterogeneous mixture.
The student took the liquid substance
and put it on top of a Bunsen burner.
As the liquid heated up, she was able
to separate two substances from
each other through evaporation.
How would you classify this
substance? Why?
Objective
By the end of this class period I
will be able to….
describe the modern structure
of the atom and summarize the
experiments that led to its
development.
This has a few pieces to it…
 SWBAT describe and summarize Dalton’s 5
postulates
 SWBAT describe and summarize the design of: J.J.
Thomson’s cathode-ray experiment, Rutherford’s
alpha particle experiment and the discovery of the
atomic nucleus.
 SWBAT describe and draw a representation of
Bohr’s nuclear atom.
Modern Atomic Theory
4 Dead White Guys
Some people just like proving they’re better than you…
Turn and Talk: 2 min.
Why might this peach have been used to
explain the structure of the atom to kids?
What do the parts of the peach
represent?
How would you describe an atom, what is
it, what’s in it, how does it work?
Oldest mention of the atom…
Democritus suggested that if you
kept splitting an object in half, you
would eventually get to a tiny,
fundamental piece that can’t be
broken down any further.
Atom
The smallest particle of an
element that has the
chemical properties of
that element
Key Players in Modern Atomic
Theory
John Dalton
• English school
teacher turned
Chemist
• Proposed his
atomic theory in
1808
Reading
 Read the following passage about John
Dalton and his ideas about the atom.
 What were his postulates?
 After you have found them all, decide:
which are still accepted views and
which are outdated?
All matter is
composed of tiny,
indivisible particles
called atoms
Postulate 1
Atoms of a given
element are
identical
Postulate 2
Atoms of a given
element are
different from those
of any other
element
Postulate 3
Atoms of different
elements combine in
simple whole-number
ratios to form
chemical compounds
Postulate 4
Examples:
CO2
Cu(NO3)2
H2O
H2O
2 Hydrogen
1 Oxygen
Fe(NO3)2
Fe(NO3)2
1 Iron
2 Nitrogen
6 Oxygen
Let’s do our last postulate…
Atoms cannot be
created or destroyed. In
chemical reactions,
atoms are combined,
separated, or
rearranged
Postulate 5
Where Dalton was Wrong!
2 of Dalton’s postulates
are not exactly correct.
Which are they?
Turn and Talk (2 min.)
Where Dalton’s Wrong and
Right
Dalton was wrong
about…
But, He was right about…
Every atom of a given element
always has one thing that is the
same. What is it?
The same
number of
protons
J.J. Thomson’s Cathode Ray
Experiment
• British Physicist
• Performed his
experiments in the late
1800s
• Until this time, it was not
believed that the atom
was composed of
various parts
J.J. Thomson
• Discovered atoms
contain electrons
in 1897 by using a
cathode ray tube
Cathode Ray Tube
http://www.youtube.com/watch?v=O9Goyscbazk
Plum Pudding Model
Thomson believed that electrons were like
raisins embedded in a positively charged
‘pudding’ (plum pudding model)
Plum Pudding Model
Thomson believed
the mass of the
atom was uniformly
distributed
throughout the
atom
Stop, Read and Jot
Alternate reading in pairs
the short selection on your
table. When done, discuss
and answer the following
question in your notes:
JJ Thomson discovered the
Postulate in
#1:1897,
Matterwhich
is made of
of
electron
tiny, indivisible particles
Dalton’s postulates did this
discovery
ACTUALLY, change?
electrons are smaller
than atoms and are in every
atom
What tool did JJ
Thomson use to discover
the electron?
How did Thomson know that atoms
must include a negatively charged
particle?
Dear John,
ROLE:
J.J. Thomson
AUDIENCE: John Dalton
FORMAT:
Letter
TOPIC: You just discovered the electron!
Write a letter to John Dalton telling him how
you have just updated his theory
Quick Break…
Model of the atom in 1897
With the person next to you, brainstorm a
way to test if this is the correct structure of
the atom.
Ernest Rutherford
• New Zealander
• In 1911
performed his
gold foil
experiment
The Gold Foil
Experiment
Observations from gold foil
experiment:
1)Most of the alpha particles
passed through the gold foil undeflected
2)Very few bounced back (great
deflection)
Modern Atomic Theory
Actual
Results
Expected
Results
Rutherford’s Conclusion:
1) The nucleus is small,
dense, and has a
positive charge
2) The nucleus is
composed of protons
and neutrons
Summarize the gold foil
experiment. What model did
Rutherford’s experiment
disprove?
Niels Bohr
1912
Convinced atom
was small positive
nucleus with
electrons orbiting
around it
• Constructed model of
the hydrogen
atom
Bohr Model
with quantized energy
levels (electrons can
only exist on certain
energy levels/rings)
Pros and Cons of the Bohr Model
Pros:
+The model
gives us a clear
visual of the
atom
+Accurate
model for
Hydrogen
Con:
-Electrons DO NOT
move around the
nucleus in circular
orbits like planets
orbiting the sun
Rules for drawing Bohr Models
1) We will be working with neutral atoms, so we can
expect the number of electrons in each element to
be equal to that element’s number of protons!
(#electrons = #protons)
2) Draw each electron energy level with a ring.
3) Electrons fill energy levels in the following way:
2
8
18
Bohr Model
Carbon
6 Protons
6 Electrons
Bohr Model
Nitrogen
7 protons
7 electrons
Bohr Model
Sodium
? protons
? electrons
Independent Practice
Create/complete
the timeline of the
atomic theory
What must be included?
 All 4 Scientists associated with the atomic theory
 The atomic model each scientist developed (include dates)
 The subatomic particles each scientist discovered
 The experiments that led to each discovery/ model