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Atomic Structure
http://en.wikibooks.org/wiki/High_School_Che
mistry/Further_Understanding_of_the_Atom
Subatomic Particles and the Nuclear
Atom (4.2)
• I will distinguish between the subatomic
particles in terms of relative charge and mass
• I will describe the structure of the nuclear
atom, including the locations of the subatomic
particles
Dalton's Atomic Theory
Held up well to a lot of the different chemical experiments that
scientists performed to test it. In fact, for almost 100 years, it
seemed as if Dalton's Atomic Theory was the whole truth.
As it turns out, Dalton had a lot right.
• He was right in saying matter is made up of atoms
• He was right in saying there are different kinds of atoms with
different mass and other properties
• He was "almost" right in saying atoms of a given element are
identical
• He was right in saying during a chemical reaction, atoms are
merely rearranged
• He was right in saying a given compound always has atoms
present in the same relative numbers.
• But he was wrong in saying atoms were indivisible or
indestructible.
J.J. Thompson
In 1897, a scientist named J. J. Thompson conducted
some research which suggested that Atomic Theory
wasn’t the entire story.
• As it turns out, atoms are divisible. In fact, atoms are
composed of smaller subatomic particles.
Discovering Electrons
Cathode Ray Tube Experiment
In the mid-1800s, scientists were beginning to
realize that the study of chemistry and the
study of electricity were actually related.
• First, a man named Michael Faraday
showed how passing electricity through
mixtures of different chemicals could
cause chemical reactions.
• Shortly after that, scientists found that by
forcing electricity through a tube filled
with gas, the electricity made the gas
glow!
• Scientists didn't, however, understand the
relationship between chemicals and
electricity until a British physicist named J.
J. Thomson began experimenting with
what is known as a cathode ray tube.
Cathode Ray Tube Experiment
Cathode Ray Tube Experiment
J. J. Thomson made a radical proposal:
• Maybe atoms are divisible!
• J. J. Thomson suggested that the small, negatively
charged particles making up the cathode ray were
actually pieces of atoms.
• He called these pieces "corpuscles", although
today we know them as "electrons".
• Thanks to his clever experiments and careful
reasoning J. J. Thomson is credited with the
discovery of the electron.
Thomson's Model of the Atom
• Thomson's "plum-pudding" model for the
atom. Notice how the "plums" are the
negatively charged electrons, while the
positive charge is spread throughout the
entire pudding batter.
Thomson's Model of the Atom
• Negative electrons = Pieces of Fruit-Plums
(chocolate chips)
• Positive material = Pudding (cookie dough batter)
• How this fits Thompson’s Observations:
– Isolated electrons using a cathode ray tube
– Never managed to isolate positive particles
• Result: Thomson theorized that the positive
material in the atom must form something like
the "batter" in a plum pudding, while the
negative electrons must be scattered through this
"batter".
Disproving the "plum pudding" model
• A man named Ernest Rutherford used alpha
particles (Helium atoms that lost their
electrons and had a positive charge)as tiny
bullets to fire at all kinds of different
materials.
• One experiment in particular, however,
surprised Rutherford, and everyone else.
– Fired alpha particles at a very thin piece of
gold foil
– Almost all of the alpha particles went straight
through the foil as if they'd hit nothing at all
– Every so often, though, one of the alpha
particles would be deflected slightly as if it
had bounced off of something hard
Structure of the Atom: The Rutherford Model
Discovering the Nucleus
• Rutherford Observation
– Most alpha particles passed through the gold
foil without hitting anything at all
– Once in a while, though, the alpha particles
would actually collide with a gold nucleus,
causing the alpha particles to be deflected, or
even to bounce right back in the direction they
came from.
• Rutherford Explanation
– The positive matter forming the gold atoms was
not distributed like the batter in plum pudding
– The positive matter was concentrated in one
spot
• forming a small positively charged particle
somewhere in the center of the gold atom.
• We now call this clump of positively charged mass
the nucleus.
What about our “Batter” of electrons?
• Rutherford Suggestion
– Electrons might be circling or "orbiting" the
positively charged nucleus as some type of
negatively charged cloud instead
– Not much evidence at this time to suggest exactly
how the electrons were held in the atom
• Modern Reconstruction of Rutherford's
Experiment
Quick Think
• Without talking to a neighbor, write one fact
about a scientist we recently studied
• Place your sticky note on the board
Quick- Vocab Review
• Electrons
– Negatively charged sub atomic particles that are
part of all forms of matter.
• Nucleus
– Centrally located within the atom (very dense)
– Contains all of an atom’s positive charge
– Contains virtually all of an atom’s mass
Electrons
• 1909 Robert Millikan – American Physicist
– Determined the charge of an electron
• Electrons have a charge of –1
– Used the known charge-to-mass ratio to calculate
the mass of a single electron
• 9.1
X10-28g
=
1
1840
the mass of a hydrogen atom
Completing the Atom-The Discovery of
Protons and Neutrons
• 1920 (8 years after the Gold Foil Experiment)
– Rutherford refines concept of nucleus
• Nucleus contained positively charged particles called
protons
– Proton
» a subatomic particle carrying a charge equal to but
opposite that of an electron
» Protons have a positive charge +1
Completing the Atom-The Discovery of
Protons and Neutrons
• 1932 James Chadwick (Rutherford’s Coworker
and English Physicist)
– Showed that the nucleus also contained another
subatomic particle, a neutral particle called the
neutron
• Neutron
– Has a mass nearly equal to that of a proton
– Carries NO electrical charge
Properties of Subatomic Particles
Particle
Symbol
Location
Relative
Electrical
charge
Relative
Mass
Actual
Mas
(g)
Electron
e-
In the space
surrounding
the nucleus
1-
1
1840
9.11x10-28
Proton
p+
In the
nucleus
1+
1
1.673x10-24
Neutron
n0
In the
nucleus
0
1
1.675x10-24
Atom Summary
• Spherically shaped
• Contains negative electron/s traveling through the empty
space surrounding the nucleus
• Contains dense nucleus (99.97% of atom’s mass)
– Neutral neurons
– Positive protons
• Atom = electrically neutral
– Number of electrons = number of protons
• NOTE: these 3 subatomic particles recently have been
found to also contain quarks (yet smaller subatomic
particles that we don’t understand quite yet if or how they
affect chemical behavior)
Quick Think
• Describe the structure of a typical atom, be sure
to identify where each subatomic particle is
located.
• Briefly describe the experiments that led to the
discovery of the electron and nucleus.
• Compare and Contrast Thompson’s Plum Pudding
Model to Rutherford’s Nuclear Atomic Model
How Atoms Differ (4.3)
• I will explain the role of atomic number in
determining the identity of an atom
• I will define an isotope and explain why
atomic masses are not whole numbers
• I will calculate the number of electrons,
protons, and neutrons in an atom given its
mass number and atomic number
Atomic Number
• English Scientist Henry Moseley
– Atoms of each element contain a unique positive
charge in their nuclei
• Number of protons in an atom identifies it as an atom
of a particular element
• Atomic Number
– Number of protons in an element
– Determines the element’s position on the periodic
table
Quick Think
• How many protons does a gold atom contain?
• A silver atom?
Atomic Number
• Remember ALL atoms are neutral
• Atomic number = number of protons = number of electrons