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Atomic Structure
How do we know that atoms exist?
Democritus
- Greek philosopher
~ 400 B.C.
- First person to believe
that all matter is
composed of tiny
indivisible particles
- Atomos – not to be cut
Aristotle
- More famous Greek
philosopher
- Did not believe
Democritus was correct
- All matter is continuous
- All matter is made up of
4 elements
- Earth
- Air
- Water
- Fire
Who Was Right?
Neither view was supported by
experimentation.
Greeks settled disagreements by
debate.
Aristotle was a better debater - He won.
His ideas carried through middle ages.
Foundations of Atomic Theory
Late 1700’s – most chemists accepted
the modern definition of an element as a
substance that cannot be broken down
further by ordinary chemical means.
Elements combine to form compounds
whose properties are different from the
elements that form them.
Foundations of Atomic Theory
Great controversy over whether
elements always combine in the same
ratio when forming a particular
compound.
Significant improvements to the
available technologies
Foundations of Atomic Theory
Led to a more quantitative study of
elements, compounds, and chemical
reactions
Led to the discovery of several basic
laws
Law of Conservation of Mass
Mass is neither created nor
destroyed during ordinary chemical
reactions or physical changes.
Antoine Lavoisier – French chemist
http://en.wikipedia.org/wiki/Lavoisier
Law of Definite Proportions
Proust’s Law http://en.wikipedia.org/wiki/Joseph_Proust
Regardless of where or how a pure
chemical compound is prepared, it is
composed of a fixed proportion of.
elements.
It is a ratio by mass.
All salt crystals, NaCl, regardless of
sample size contains exactly 39.34%
sodium and 60.66% chlorine.
Law of Multiple Proportions
If two or more different compounds are
composed of the same two elements,
then the ratio of the masses of the second
element combined with a certain mass of
the first element is always a ratio of small
whole numbers.
Sometimes called Dalton’s Law
Law of Multiple Proportions
1.00g of carbon combines with 2.66g of
oxygen in carbon dioxide and 1.33g of
oxygen in carbon monoxide.
2.66/1.33 = 2 to1
The ratio of the ratios is a whole
number.
John Dalton
- English school teacher
- Proposed an explanation
for the law of conservation
of mass, law of definite
proportions and law of
multiple proportions.
- Elements are composed of
atoms and that only whole
numbers of atoms can
combine to form
compounds.
Dalton’s Atomic Theory
1) All matter is composed of extremely
small particles called atoms.
2) Atoms of a given element are identical
in size, mass, and other properties; atoms
of different elements differ in size, mass,
and other properties.
Dalton’s Atomic Theory
3) Atoms cannot be subdivided, created, or
destroyed.
4) Atoms of different elements combine in
simple whole-number ratios to form
chemical compounds.
5) In chemical reactions, atoms are
combined, separated, or rearranged.
Where Dalton Was Wrong
Atoms are divisible into even smaller
particles!
– We now know that atoms are
composed of subatomic particles.
A given element can have atoms with
different masses!
– Isotopes – atoms with the same
number of protons but different
numbers of neutrons.
The Structure of the Atom
Late 1800’s scientific advances allowed
for a deeper exploration into the nature
of matter.
Atoms are composed of several basic
types of smaller particles.
The number and arrangement of these
particles within an atom determines that
atom’s chemical properties.
J. J. Thomson
English physicist. 1897
Investigated relationship between
electricity and matter.
Made a piece of equipment called a
cathode ray tube.
It is a vacuum tube - all the air has
been pumped out and a limited
amount of other gases are put in
Thomson’s Experiment
Voltage source
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Metal Disks
Thomson’s Experiment
Voltage source
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Passing an electric current makes a beam
appear to move from the negative to the
positive end
Called cathode rays
Thomson’s Experiment
Voltage source
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By adding a magnetic field
Thomson’s Experiment
Voltage source
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By adding a magnetic field he found that
the moving pieces were negative.
Thomson’s Experiment
Used many different metals and gases
Beam was always the same
By the amount it bent he could find the
ratio of charge to mass
He found the ratio was the same with
every material
He concluded that the rays were
composed of identical negatively
charged particles.
Thomsom’s Plum Pudding
Model
Said the atom
was like plum
pudding.
For us, like seeds
in a watermelon
A bunch of
positive stuff, with
the electrons able
to be removed.
Robert Millkan
- 1909 Robert Millikan
measured the charge of an
electron.
- Scientists used this information
and the charge-to-mass ratio to
determine the mass of an
electron.
Millikan’s Experiment
Atomizer
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Oil
Microscope
Metal Plates
Millikan’s Experiment
Atomizer
Oil droplets
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Oil
Microscope
Millikan’s Experiment
X-rays
X-rays give some drops a charge by knocking off
electrons
Millikan’s Experiment
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Millikan’s Experiment
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They put an electric charge on the plates
Millikan’s Experiment
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Some drops would hover
Millikan’s Experiment
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Some drops would hover
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Millikan’s Experiment
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From the mass of the drop and the charge on
the plates, he calculated the charge on an electron
Inferences About Atomic
Structure
Atoms are electrically neutral, therefore,
they must contain a positive charge to
balance the negative electrons.
Electrons have so much less mass than
atoms, atoms must contain other
particles that account for most of the
atoms mass.
Lord Ernest Rutherford
- English Physicist 1911
- Worked with
Hans Geiger and
Ernest Marsden
- Student of J.J. Thomson
- Believed the plum
pudding model of the
atom was correct
- Set out to prove it!
Rutherford’s Experiment
Alpha Particle Scattering Experiment
– Bombarded a thin piece of gold foil
with fast-moving alpha particles,
positively charged particle 4 times the
mass of a hydrogen atom.
– Expected the particles to go through
the gold atoms with only slight
deflection.
Rutherford’s Experiment
Most did what was expected but 1 in
8000 was deflected back toward the
source.
http://www.youtube.com/watch?v=XBqH
kraf8iE
Lead
block
Uranium
Flourescent
Screen
Gold Foil
What he expected
Because, he thought the mass was
evenly distributed in the atom
What he got
Rutherford’s Conclusion
When the alpha particles hit a florescent
screen, it glows.
The deflected particles experienced a
powerful force within the atom.
He figured this force occupied a very
small space because most of the
particles went through.
Rutherford’s Conclusion
Atom is mostly
empty space.
Small densely
packed bundle of
matter with a
positive electric
charge.
Alpha particles are
deflected by it if
they get close
enough.
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What We Know So Far
Except for the simplest hydrogen atom,
all nuclei are composed of two kinds of
particles
– positive Protons
– neutral Neutrons
Atoms are electrically neutral, they
contain the same number of protons
and electrons.
What We Know So Far
The nuclei of atoms of different
elements differ in their number of
protons and therefore in the
amount of positive charge.
The number of protons determines
the atoms identity.
What We Don’t Know Yet
Where are the electrons?
Forces In The Nucleus
Like charges repel each other
– Except in the nucleus
» When like charges are extremely close
together there is a strong attraction between
them.
» More than 100 protons can exist together to
help form a nucleus.
These short range forces hold the
nuclear particles together, they are
called nuclear forces.
Subatomic Particles
Proton - positively charged pieces 1836
times heavier than the electron.
Neutron - no charge but the slightly
larger than the mass of a proton.
Electron – negatively charged pieces
~1/1837 times the mass of the
hydrogen atom.
Where are the pieces?
Subatomic particles
Relative Actual
mass (g)
Name Symbol Charge mass
Electron
e-
-1
1/1837 9.109 x 10-28
Proton
p+
+1
1
1.673 x 10-24
Neutron
n0
0
1
1.675 x 10-24
The Structure of the Atom
An atom is the smallest particle of an
element that retains the chemical
properties of that element.
All atoms are composed of two regions:
– Nucleus: very small dense region
located in the center of the atom.
»Made up of at least one positive
particle, proton, and usually one or
more neutral particles called neutrons.
The Structure of the Atom
Surrounding the nucleus is a large
region occupied by negatively charged
particles called electrons.
This region is called the electron cloud.
Protons, Neutrons, and Electrons are
referred to as subatomic particles.
Structure of the Atom
There are two regions.
The nucleus.
With protons and neutrons.
Positive charge.
Almost all the mass.
Electron cloud- most of the volume of
an atom.
The region where the electron can be
found.
Size of an atom
Atoms are small.
Measured in picometers, 10-12 meters.
Hydrogen atom, 32 pm radius.
Nucleus tiny compared to atom.
IF the atom was the size of a stadium, the
nucleus would be the size of a marble.
Radius of the nucleus is near 10-15m.
Density near 1014 g/cm3.
Counting the Pieces
Atomic Number = number of protons
# of protons determines kind of atom.
the same as the number of electrons in
the neutral atom.
Mass Number = the number of protons
+ neutrons.
All the things with mass.
NOT on the periodic table
Isotopes
Dalton was wrong.
Atoms of the same element can have
different numbers of neutrons.
different mass numbers.
called isotopes.
Symbols
Contain the symbol of the element, the
mass number and the atomic number.
Symbols
Contain the symbol of the element, the
mass number and the atomic number.
Mass
number
Atomic
number
X
Naming Isotopes
Put the mass number after the name of
the element.
carbon- 12
carbon -14
uranium-235
Symbols
Find the
– number of protons
– number of neutrons
– number of electrons
– Atomic number
– Mass Number
– Name
24
11
Na
Symbols
Find the
–number of protons
–number of neutrons
–number of electrons
–Atomic number
–Mass Number
– Name
80
35
Br
Symbols
if an element has an atomic
number of 34 and a mass number
of 78 what is the
–number of protons
–number of neutrons
–number of electrons
–Complete symbol
– Name
Symbols
if an element has 91 protons and
140 neutrons what is the
–Atomic number
–Mass number
–number of electrons
–Complete symbol
– Name
Symbols
if an element has 78 electrons and
117 neutrons what is the
–Atomic number
–Mass number
–number of protons
–Complete symbol
– Name
Atomic Mass
How heavy is an atom of oxygen?
There are different kinds of oxygen atoms.
More concerned with average atomic mass.
Based on abundance of each element in
nature.
Don’t use grams because the numbers
would be too small.
Measuring Atomic Mass
Unit is the Atomic Mass Unit (amu)
One twelfth the mass of a carbon-12
atom.
6 p+ and 6 n0
Each isotope has its own atomic mass
we get the average using percent
abundance.
Calculating averages
You have five rocks, four with a mass of 50
g, and one with a mass of 60 g. What is the
average mass of the rocks?
Total mass = 4 x 50 + 1 x 60 = 260 g
Average mass = 4 x 50 + 1 x 60 = 260 g
5
5
Average mass = 4 x 50 + 1 x 60 = 260 g
5
5
5
Calculating averages
Average mass = 4 x 50 + 1 x 60 = 260 g
5
5
5
Average mass = .8 x 50 + .2 x 60
80% of the rocks were 50 grams
20% of the rocks were 60 grams
Average = % as decimal x mass +
% as decimal x mass +
% as decimal x mass +
Atomic Mass
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
Magnesium has three isotopes. 78.99%
magnesium 24 with a mass of 23.9850
amu, 10.00% magnesium 25 with a mass of
24.9858 amu, and the rest magnesium 25
with a mass of 25.9826 amu. What is the
atomic mass of magnesium?
If not told otherwise, the mass of the
isotope is the mass number in amu
Atomic Mass
Is not a whole number because it is an
average.
are the decimal numbers on the periodic
table.