Download Chemistry 11 Early Models of the Atom

Models of the Atom
a Historical Perspective
Ancient Greeks were the first to come
up with the idea of atoms.
Democritus
suggested that all
matter was made of
tiny indivisible
particles called
atoms. (Greek
“atomos”)
• This led to the idea of
atoms in a void.
Democritus
What is an atom?
• Definition:
An atom is the smallest particle of an
element that still retains the identity and
properties of the element.
• The average size of an atom is 10-10 m in
diameter
Early Greek Theories
fire
earth
air
water
• 350 B.C - Aristotle modified an earlier theory
that matter was made of four “elements”:
earth, fire, water, air.
• Aristotle was wrong. However, his
theory persisted for 2000 years.
Aristotle
In the dark ages, the
idea of atoms was
frowned upon. Not
much progress was
made.
ATOMS?
What’s that?
OFF with your
HEAD!
In the early 1800’s,
John Dalton came
up with the ATOMIC
THEORY.
(1800-1900)
Dalton’s Atomic Theory
1)
All matter is made of atoms. Atoms
cannot be created nor destroyed into smaller
particles.
2) The atoms of one element cannot be converted
into the atoms of another element.
3)
All the atoms of one element have the same
properties (mass and size). These properties
are different from atoms of another element.
4)
Atoms of different elements combine in specific
proportions to form compounds.
Ha Ha! You
can’t break
me!!!!
An indestructible
“Dalton” atom
J.J. Thomson's Experiments
“Plum pudding” model (1900)
•Materials, when rubbed, can develop a
charge difference. This electricity is
called “cathode rays” when passed
through an evacuated tube. These rays
have a small mass and are negative.
•Thomson noted that these negative
subatomic particles were a fundamental
part of all atoms.
Later, “e” was found and the mass of
an electron was found to be 9.11 × 1028 g (much lighter than H)
Thomson’s Plum
Pudding Model
Ernest Rutherford
Rutherford’s Scattering Experiment
Ernest Rutherford (movie: 10 min.)
• Rutherford shot alpha () particles at gold foil.
ZnS screen
Thin gold foil
Lead block
Radioactive
path of invisible
substance
-particles
Most particles passed through.
So, atoms are mostly empty.
Some positive -particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
An Interactive Model of
Rutherford’s Gold Foil
Experiment
http://micro.magnet.fsu.edu/electro
mag/java/rutherford
Click here
Rutherford Gold Foil Experiment - Backstage Science
http://www.youtube.com/watch?v=XBqHkraf8iE
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 .
Why is the head of a pin
compared to the diameter of a
stadium like an atom?
The diameter of a pinhead is 100,000 times smaller than the
diameter of a stadium.
Likewise the diameter of the nucleus of an atom is 100,000 times
smaller than the diameter of an atom.
Rutherford could not explain why the
electron didn’t fall into the nucleus and
destroy the atom.
?
Neils Bohr
Each “jump” would give off light of a
particular wavelength or colour. This
gave rise to hydrogen’s spectrum.
According to Bohr, each energy “level”
corresponded to a different “orbit” of an
electron around the atom. (Like planets
around the sun.)
I’m so
SMART &
Handsome!!
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.
Bohr's Atom: Quantum Behavior in Hydrogen (ONLINE)
http://www.colorado.edu/physics/2000/quantumzone/bohr.html
SPECTRAL LINES: (ONLINE)
http://www.colorado.edu/physics/2000/quantumzone/index.html
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)
Element 107 on the
Periodic Table is
called Bohrium (Bh)
In the 1920’s things changed!
Although Bohr’s idea of energy levels
was still accepted, his idea of planetary
orbits for electrons was rejected!
REJECT ! !
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
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!