The Atom
• Atom
Unit 3 – Atomic Structure And
Nuclear Chemistry
• What are the basic parts of an atom?
• How is an atom identified?
• What is nuclear chemistry?
• How is a nuclear equation written?
Ancient Ideas of the Atom
• Democritus
– Ancient Greece – 400
B.C.
– First person credited
with idea of atom
Ancient Ideas of the Atom
• Aristotle
– Greek philosopher
– Everything made of 4
elements:
• Fire – Hot
• Air – light
• Earth – cool, heavy
• Water – wet
– Smallest particle of an
element that retains the
properties of that
element
– As small as ~0.5Å
(angstroms)
• 5 x 10-11 m
– Microscopes cannot see
much inside the atom
Ancient Ideas of the Atom
• Democritus’ ideas:
– All matter made of Atoms:
• Move through empty space
• Solid, homogeneous, indestructible, and
indivisible
• Different atoms have different shapes and sizes
• Size, shape, and movement determine
properties
Ancient Ideas of the Atom
• No scientific evidence to back up either
Democritus or Aristotle
• People believed Aristotle based on reputation
• Aristotle’s theory persisted for 2,000 years
– Blend those 4 in
different proportions
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Dalton’s Atomic Theory
• Dalton’s Atomic Theory
– John Dalton –1803
– Summarized results of his
experiments and those of
others
Dalton’s Atomic Theory
• Billiard Ball Model
– 1st model of the atom
• John Dalton
– Atoms are small, solid
spheres
Dalton’s Atomic Theory
• Law of Multiple
Proportions
– John Dalton
– Same atoms can
make different
compounds
– Ex: CO2 vs. CO
Dalton’s Atomic Theory
1. All matter made of tiny indivisible atoms
2. Atoms of same element are identical, atoms of
different elements are different
3. Atoms of different elements combine in whole
number ratios to form compounds
4. Chemical reactions involve rearrangement of
atoms – No atoms created or destroyed
Dalton’s Atomic Theory
• Law of Definite
Proportions
– Joseph Proust
– All compounds contain
the same elements in the
same ratio
Dalton’s Atomic Theory
• Law of Conservation
of Mass
– Antoine Lavoisier
– Matter is neither
created nor
destroyed in
chemical reactions
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Parts of Atoms
• Cathode Ray Tube Experiment
– J. J. Thomson – 1897
– Passed current through
vacuum tube
• All air has been pumped
out
– Discovered Electrons
• Negatively-charged
particles
Parts of Atoms
• Plum Pudding Model
– JJ Thomson – 1904
– Negatively charged
electrons reside in uniform
positive charge
– Like chocolate chips in a
cookie
Uniform Positive Charge
Parts of Atoms
• Proton
– Ernest Rutherford
– 1919
– Positively charged
pieces
– 1840 times
heavier than the
electron
Rutherford’s Experiment
• Gold Foil Experiment
– Ernest Rutherford – 1910
– Tested the plum pudding
model
Electron
Parts of Atoms
• Neutron
– James Chadwick –
1932
– No charge
– Same mass as a
proton
Rutherford’s Experiment
• Gold Foil Experiment
– Shot Alpha Particles at
gold foil
• Positively charged
pieces
• Helium atoms minus
electrons
– Screen glows when
particles hit
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Rutherford’s Experiment
• Gold Foil Experiment
– Hypothesis:
• Particles will pass
through without
much direction
change
• Atom’s positive
charge is spread out
evenly and has little
effect on particles
Rutherford’s Experiment
• Gold Foil Experiment
– Results:
• Most particles passed through
• Some particles moved a lot
–Positive pieces were heavy
– Nucleus
• Dense, positively charged center of
atom
• Later found to have protons and
neutrons
Nuclear Atomic Model
• Nuclear Atomic Model
– Rutherford – 1911
– Mostly empty space
– Dense, positive nucleus at
center
– Surrounded by electrons
Bohr Model
• Bohr’s Ring Model
– AKA: Planetary Model
– Niels Bohr – 1913
– Nucleus at center
– Electrons are in circular
energy levels in the atom
– Calculated energy released
when electrons drop energy
levels
– Only works for hydrogen
The Quantum Mechanical Model
• Quantum Mechanical
Model
– Erwin Schrödinger
– Electron location is
described as
probability
– Current model
– More in Unit #4
Atoms
Relative Location in
Atom
Name Symbol Charge mass
Proton
p+
+1
1
Nucleus
Neutron
n0
0
1
Nucleus
Electron
e-
-1
0
Electron Cloud
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Atoms
• Atomic Number
– Number of protons in nucleus
• All protons are alike
– Same as number of electrons in neutral
atom
– Determines atom’s identity
Atoms
• Mass Number
– Protons + Neutrons
– Essentially all mass of atom
– Note:
• DO NOT ROUND NUMBER ON PERIODIC
TABLE!
• Will be given to you or determined from
problem
Atoms
Isotopes
• Isotopes
• Protons
– Equal to Atomic Number
• Neutrons
– Mass Number – Atomic Number
• Electrons
– Equal to atomic number in neutral atom
Isotopes
• Isotopes
– To name:
• Always include mass number
• Put mass number after name of element
– Carbon–12, Carbon–14, etc.
–Atoms of the same element with:
• Different numbers of neutrons
• Different mass numbers
–Describe one specific atom with a
specific mass
Isotopes
• Element Symbols
– Symbol of the element
from table
• Note: second letter is
ALWAYS lowercase
– Mass number on top
– Atomic number on
bottom
• Optional unless
“complete symbol”
Mass
Number
Atomic
Number
X
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Ions
• What if atoms aren’t neutral?
• Ions
– Charged atoms
– Result from loss or gain of electrons
– Charge = p+ – e-
Ions
• Cation
Ions
• Anion
– Negatively charged ion
– Result from gaining electrons
– Add charge to electron total (ignore negative sign)
Atomic Mass
• Atomic Mass
– Positively charged ion
– Result from loss of electrons
– Subtract charge from electron total
Atomic Mass
• Atomic Mass
– Too small to be measured in grams
• Atomic Mass Unit (amu)
– Mass of 1 proton or 1 neutron
– 1/12th the mass of a 12C atom
– Decimal numbers on the periodic table
– Weighted average of all isotopes of an
element
– Based on abundance of each isotope in
nature
Atomic Mass
• Atomic Mass - Calculating
– Unless told otherwise, mass of isotope is
mass number in amu
– Convert percent abundance to decimal
• Divide by 100
– Multiply isotope mass by decimal for each
isotope, then add the results
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Radioactivity
Radioactivity
• Radioisotope
• Isotopes
– Different numbers of
NEUTRONS
– Some isotopes more
stable than others
• Band of Stability
– Shows ratios of
protons to neutrons in
a stable nucleus
Radioactivity
• Radioactivity/Radioactive Decay
– Atoms release particle and/or electromagnetic
radiation (EMR)
– ALWAYS results in a more stable nucleus
– ALWAYS results in a new element
• Transmutation
– Isotope with unstable nucleus
• Radioactivity/Radioactive Decay
– Spontaneous change radioisotopes undergo to
become stable
– Shown in nuclear equation
• Mass and atomic numbers MUST balance on
both sides
• Parent is original atom, daughter atom is
result
Radioactivity
• Alpha particle (α)
– Helium nucleus
• 2 protons, 2 neutrons
– Release makes nucleus smaller
– Relatively low energy, heavy particles
– Easily shielded by paper or clothing
– The change in identity of the element after it
undergoes radioactive decay
Radioactivity
• Beta Particle (β)
– Result of a neutron breaking down
• 1 neutron is converted to 1 proton and 1
electron
– Proton stays in nucleus
– Beta particle is the electron
– More energy than alpha, but still easily shielded
by Al foil or wood
Radioactivity
• Gamma Radiation (γ)
– High energy radiation
– No mass or charge
– Released along with alpha or beta particles
– Must be shielded using lead or concrete
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Half–Life
• All radioisotopes decay at a constant rate
• No two radioisotopes decay at the same rate
• Half Life (t1/2)
– Time required for half the atoms of radioisotope
to decay
– Can be a few seconds to billions of years
Radiation Detection
• Film Badges
– Exposure of film measures radiation exposure
• Geiger Counters
– Detect radiation through electric pulses in
ionized gas
• Scintillation Counters
– Measure radiation from substances that emit
visible light when energy is absorbed
Uses of Radioactivity
• Radioactive Dating
– Determine approximate age of a fossil
• Medical Field
– Detect and kill cancerous cells
– X-Rays
• Disinfect Foods (Food Irradiation)
• Smoke Detectors
Nuclear Fission
• Fission
– Radioisotope bombarded by neutrons
– Splits into smaller pieces
– Releases large amount of energy
• Less than fusion
– Can result in a chain reaction
• Criticality
Nuclear Fusion
• Fusion
– Nuclei combine to make nucleus with
greater mass
– Releases a LOT of energy
– Energy produced by the sun
Nuclear Energy
• Nuclear Power Plant
– Fission creates heat, which boils water
– Steam spins turbines, creating electricity
– Water is then cooled off
• Same process in nuclear submarines
• Nuclear Fission produces about 20% of energy
in US
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