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The Atom
protons, neutrons, and
electrons…oh my!
•What’s an Atom?
•
An atom is the smallest physical particle of an
element that still retains the properties of that
element.
•How Big is an Atom?
At sea level, one cubic centimeter of air (size of a sugar
cube, or marble) will have 45 billion atoms within it.
•
•
45,000,000,000,000,000,000
How many atoms would it take to fill this room? The
universe?
•
•Where Did Atoms First Begin?
First scientist to suggest
atoms:
Democritus
“Everything that is must be
eternal.”
Democritus believed that
there were basic things that
always exist, but can be
rearranged; he called these
“atomos”.
•What They Used to Think About
Atoms
• The
following ideas are now known
as untrue…wrong!
• All
atoms are identical.
• Chemical
reactions occur when
atoms of one element are
changed into atoms of another
element.
•Laws
•
Law of Conservation of Mass
•
Law of Constant Composition
•
Law of Multiple Proportions
Cr2O3
CrO3
What does an atom look like?
•ATOMIC STRUCTURE
•Where’s the Proof?
One century after Dalton,
Viennese scientist Ernst Mach
wrote that “atoms cannot be
perceived by the senses… they
are things of thought.”
•
Many scientists that made
contributions to atomic structure –
John Dalton, J.J. Thomson, Robert
Millikan, Ernest Rutherford, Niels
Bohr, Erwin Schrödinger and
James Chadwick.
•
•John Dalton’s Atomic Theory
1803
1. All elements are composed
of tiny particles called atoms.
2. Atoms of the same element
are identical. Atoms of
different elements are always
different.
3. Atoms of different elements
can chemically combine in
fixed ratios to form
compounds.
•John Dalton
4. Chemical reactions occur when atoms are joined,
separated, or rearranged. Atoms of one element are
NEVER changed into another during a chemical
reaction.
•John Dalton’s Atom
•
Atoms are solid and indivisible.
•Robert Millikan
•
1908
•
Oil Drop Experiment
For more information on how this experiment works go
to
http://www.kentchemistry.com/links/AtomicStructure/
Millikan.htm
•
Measured the charge on an electron as 1.6 𝑥 10−19
coulombs
•J.J. Thompson
•
1897
•
Particles deflected in Cathode Ray tubes
•J.J Thompson
• Thompson passed electric current through
gases in a glass tube. The result was a glowing
beam flowing from the negative side to the
positive side.
• Since the glowing ray (cathode ray) was
attracted to a positive magnet on the side of
the tube the beam must be negatively
charged.
•J.J Thompson
•
Results of the experiment
•
•
Electrons are negatively charged.
Electrons have very small mass. 1/2000 that of the lightest
atom.
•J. J. Thompson’s Atom
Thompson hypothesized that all atoms had such
negative charges within.
•
•
Plum pudding model
• Ernest Rutherford
•
•
•
•
•
1911
Born in New Zealand
Terrible at mathematics
Diligent problem solver
First to discover the power
of the atom, power to make
bombs that could “make
this old world vanish in
smoke”.
•Rutherford’s Experiment
•
•
•
•
Gold Foil Experiment
Shot radioactive alpha particles through thin sheets
of gold foil.
Most went straight through, but other were knocked
off course or bounced back.
“Like shooting a fifteen-inch shell at a sheet of paper
and having it bounce off.”
•Rutherford’s Experiment
•
•
Nucleus - used alpha particles
and discovered a high
concentration of mass and
positive charge at the nucleus
of the atom with the very light
electrons distributed
throughout.
This led to a new model of the
atom.
•Rutherford’s Atom
•
•
Nucleus – compact center
of the atom where protons
and neutrons are located.
Nucleus has very little of
the atom’s volume, but
contains most of the
atom’s mass.
•James Chadwick
•
•
•
•
1932
The mass of an atom is greater
than the mass of protons plus
electrons.
Because of this he was able to
determine a 3rd particle of the
atom with no charge –
neutrons.
Protons, neutrons, and
electrons are called subatomic
particles.
•An Overview of Subatomic Particles
• Proton
•
•
•
Mass equal to H
Positive one charge
Found in the nucleus
• Neutron
•
•
•
Mass equal to H
No charge
Found in the nucleus
• Electron
•
•
•
Mass 1/2000 of H
Negative one charge
“orbits” the nucleus
amu
•Niel Bohr’s Atom
• 1922
• Electrons orbit the nucleus in orbits, like a
solar system.
Planetary Model
or
Bohr’s Model
•Erwin Schrödinger
•
•
•
•
1930
Born in Vienna, Austria
University of Berlin professor
Nobel Prize 1933
•
Work based on waves of light and probability
•
Quantum Mechanical Model – like a shot gun blast
How do Atoms Differ?
•ATOMIC PROPERTIES
• Atomic Number
•
Atoms of different elements have different structures.
Elements differ because their atoms contain different
numbers of protons!
•
Atomic Number – number of protons or electrons in
the atom
•
•Mass Number
•
atomic mass
Electrons have negligible
mass compared to protons and
neutrons.
•
Mass of a neutron is
approximately equal to mass of
a proton.
•
• Mass Number – number of protons plus number of
neutrons in an atom.
• Atomic mass – average mass number of atoms of
the same element.
• measured in amu (atomic mass units) not grams!
• Using the Periodic Table
•Nuclear Symbol
When not given
the number of
neutrons round
the amu to the
nearest whole
number to find
mass number of
the average
isotope.
•Let’s Practice! Find the mass number
and write the nuclear symbol!
•
Beryllium – 5 neutrons
•
Sodium – 12 neutrons
•
Gold – 118 neutrons
•
•
•
•
•
•
Mass Number =
Atomic Number =
Mass Number =
Atomic Number =
Mass Number =
Atomic Number =
•IONS AND ISOTOPES
•Vocabulary
• Ion:
same number of protons,
different number of electrons.
• Cation:
• Anion:
positively charged ion
Negatively charged ion
• Isotope:
Same number of protons,
different number of neutrons
Ions
•
•
Ions – is an atom whose
electrons do not equal the
total number of protons.
This gives the atom a
positive or negative
electrical charge.
Cation - Positive charge
(more protons than electrons)
Anion – Negative charge
(more electrons than protons)
•Let’s Practice
•
How would I write the following using the nuclear symbol?
•
Lithium atom with 3 protons, 4 neutrons and 2 electrons.
•
Oxygen atom with 8 protons, 7 neutrons and 10 electrons.
•
Carbon atom with 6 protons, 5 neutrons, and 6 electrons.
•Isotopes
Atoms of the same
element (same number
of protons) with
different number of
neutrons
•
Chemically
equivalent (same
element)
•
Most elements occur
as a mixture of isotopes
•
Isotopes of Hydrogen – Write the
correct nuclear symbol for each!
•Calculating Atomic Mass –
in a Mass Spectrometer
Mass of a proton is more than
1800 times the mass of an
electron
•
A Mass Spectrometer is used
to find masses of atoms
•
Masses of subatomic particles
too small to work with, so a
reference isotope is used
•
•How a mass Spectrometer Works
Charged atoms fly towards a
magnet.
• Where they hit the wall
(detection screen) determines
their mass.
• How many hit a certain spot
determines the abundance
(percent) of each isotope.
•
magnet
WALL
•Calculating Atomic Mass
Atomic masses found
on periodic table are
averages of all isotopes
of that element based
on abundance
To calculate the atomic mass
of an element, multiply the
mass of each isotope by its
natural abundance
(expressed as a decimal),
and then add the products.
•Calculating Atomic Mass
Chlorine is a mixture of Cl-35
and Cl-37.
75.77% Cl-35 (35)
24.23% Cl-37 (37)
•
0.7577 x 35 amu = 26.50 amu
+
 0.2423 x 37 amu = 8.957 amu

Added together… 35.45 amu
•Example
• Silicon-28
93.37%
• Silicon-29
4.63%
• Silicon-30
2.00%
• Atomic Mass?
How do they get that
atomic mass?
ISOTOPE LAB!