Download Atomic Structure

What is an Atom?
ATOMIC STRUCTURE,
NUCLEAR CHEMISTRY &
UNIT 6
Democritus
He asked this question: If you break a piece of matter in
half, and then break it in half again, how many breaks
will you have to make before you can break it no
further?
Democritus thought that it ended at some point, a
smallest possible bit of matter. He called these basic
matter particles, atoms.
History of the Atom
Aristotle – famous philosopher
Believed all substances made of 4
elements
Blending these in different proportions to
get all substances
Fire – hot
air – light
Earth – cool, heavy
Water – wet
History of the Atom
Late 1700s – John Dalton
Teacher who summarized results of his
experiments and those of others
Combined idea of elements with that of
atoms
Dalton’s Atomic
Theory
1. All matter is made of tiny, indivisible
particles call atoms
2. Atoms of the same element are identical
3. Atoms of different elements combine in
whole number ratios to form compounds
4. Chemical reactions involve the
rearrangement of atoms. No atoms are
created or destroyed.
Atoms
Smallest particle of an element that still
retains the properties of that element
JJ Thompson
In 1897, Thompson discovered the first
component part of the atom: the electron, a
particle with a negative electric charge.
In 1904, he proposed an initial model of an atom,
since nicknamed "Thompsons pudding".
He imagined the atom as a sphere full of an
electrically positive substance mixed with
negative electrons "like the raisins in a cake".
Thomson’s
Experiment
Voltage source
Vacuum tube
Metal Disks
Thomson’s
Experiment
Voltage source
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Vacuum tube
Metal Disks
Thomson’s
Experiment
Voltage source
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Thomson’s
Experiment
Voltage source
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Thomson’s
Experiment
Voltage source
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Thomson’s
Experiment
Voltage source
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Passing an electric current makes a beam
appear to move from the negative end to
the positive end
Thomson’s
Experiment
Voltage source
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+
Passing an electric current makes a beam
appear to move from the negative end to
the positive end
Thomson’s
Experiment
Voltage source
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+
Passing an electric current makes a beam
appear to move from the negative end to
the positive end
Thomson’s
Experiment
Voltage source
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+
Passing an electric current makes a beam
appear to move from the negative end to
the positive end
Thomson’s
Experiment
Voltage source
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+
By adding an electric field…
Thomson’s
Experiment
Voltage source
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By adding an electric field…
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Thomson’s
Experiment
Voltage source
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By adding an electric field…
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Thomson’s
Experiment
Voltage source
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By adding an electric field he found that
the moving pieces were negative
Millikan’s
Experiment
Atomizer
Oil droplets
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Oil
Telescope
Millikan’s
Experiment
X-rays
X-rays give some electrons a charge.
Millikan’s
Experiment
From the mass of the drop and the charge on
the plates, he calculated the mass of an electron
Other Particles
Proton – positively charged pieces 1840
times heavier than electrons, discovered
by E. Goldstein
Neutron – no charge but the same mass
as a proton, by J. Chadwick
Where are these pieces?
Rutherford’s
Experiment
Ernest Rutherford – English physicist
Believed in the plum pudding model of the
atom
Rutherford’s
Experiment
Wanted to see how large the positive
particles are
Used radioactivity
Shot alpha particles at gold foil
Alpha particles – positively charged pieces,
Rutherford’s
Experiment
When an alpha particle hits a fluorescent
screen it glows
Lead
block
Uranium
Fluorescent
Screen
Gold Foil
Rutherford’s
Experiment
He expected the alpha particles to pass
through without changing direction very
much
Because…
The positive charges were thought to be
spread out evenly
Not enough to stop the alpha particles
What he expected
Because
He thought the mass was evenly
distributed in the atom
Since he thought the
mass was evenly
distributed in the atom
What he got
How he explained it
Atom is mostly empty.
Small dense, positive
piece at center.
Alpha particles
are deflected by it
if they get close enough.
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Density and the
Atom
Since most of the particles went through,
it was mostly empty space
Because the alpha particles turned so
much, the nucleus was heavy
Small volume, big mass, big density
The small dense positive area is the
nucleus
The Nucleus
Unit 6 – Part 2
STRUCTURE OF THE
NUCLEAR ATOM
Subatomic Particles
Name
Symbol Charge Relative Actual
Mass
Mass (g)
Electron e-
-1
1/1840
9.11 x 10-28
Proton
+1
1
1.67 x 10-24
0
1
1.67 x 10-24
p+
Neutron n0
Counting the Pieces
Atomic Number – the number of protons
in the nucleus
All protons are alike so…
# of protons determines kind of atom
The same as the number of electrons in a
neutral atom
Counting the Pieces
Mass number = # of protons + # of
neutrons
Protons and neutrons account for most of
the mass of an atom
Symbols
Contain the element symbol, mass
number, and atomic number
Mass
number
Atomic
number
X
Symbols
Find:
# of protons
# of neutrons
# of electrons
Atomic #
Mass #
19
9
F
Symbols
Find:
# of protons
# of neutrons
# of electrons
Atomic #
Mass #
80
35
Br
Symbols
If an element has 91 protons and 140
neutrons, what is the
Atomic #
Mass #
# of electrons
Complete symbol
Isotopes
Dalton was wrong
Atoms of the same element can have
different numbers of neutrons
This means different mass numbers
Called isotopes
Naming Isotopes
Can also put mass # after name of the
element
Carbon-12
Carbon-14
Uranium-235
Atomic Mass
There are different kinds of oxygen atoms.
More concerned with the average atomic
mass
Atomic Mass
Based on the abundance of each in nature
Do not use grams because numbers would
be too small
Measuring Atomic
Mass
Use the atomic mass unit (amu)
One twelfth of a carbon-12 atom
Each isotope has its own atomic mass
Determine average from % abundance
Calculating
Averages
Multiply atomic mass by % abundance
then add results
Atomic Mass Example
Calculate the atomic mass of copper if
copper has two isotopes. 69.1% has a
mass of 62.93 amu and the rest has a
mass of 64.93 amu.
Atomic Mass –
Example 2
Magnesium has 3 isotopes. 78.99% has a
mass of 23.9850. 10.00% has a mass of
24.9858. The rest has a mass of 25.9826
amu. What is the atomic mass?
Atomic Mass
Is not a whole number because it is an
average
The number on the periodic table