The Atom

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(No Transcript)
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The Atom Basic Structure The Nucleus
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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 near 10-15m.
• Density near 1014 g/cm

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Structure of the Atom

• The Nucleus
• With protons and neutrons
• Positive charge
• Almost all the mass
• Electron Cloud
• Most of the volume of an atom
• The region where electrons can be found

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Subatomic Particles
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Counting the Pieces . . .

• Atomic Number of protons
• of protons determines kind of atom
• of protons of electrons for a neutral atom
• Mass Number protons neutrons
• All the things with mass

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Elemental Symbols
X
Mass Number
Charge (as required)
Atomic Number
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Elemental Symbols
C
C
12
14
6
6
9
If an atom looses one electron . . .
1
Na
22
11
10
If an atom looses two electrons . . .
2
Mg
25
12
11
If an atom gains an electron . . .
-1
Cl
37
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Isotopes

• Isotopes Atoms of the same element that have
  the same atomic number ( of protons) but
  different atomic masses (due to a different
  number of neutrons)
• Example C-12, C-13, C-14
• Example O-16, O-17, O-18

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Atomic Mass Units (AMUs)

• Mass of a proton 1.67267 x 10-27 kg
• Mass of a neutron 1.6750 x 10-27 kg
• Inconvenient numbers
• Instead, we define a new unit and reference it to
  the mass of a specific isotope of carbon
  carbon-12
• carbon-12 has 6 neutrons and 6 protons. By
  definition, we say carbon-12 as a mass of
  12.00000 atomic mass units (AMU).
• 1 AMU 1.6606 x 10-27 kg.
• Then, 1 proton and 1 neutron have a mass of
  approximately 1 AMU.

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Why arent the atomic masses reported on the
Periodic Table close to whole numbers of AMUs?

• The reported numbers are average atomic mass
  units, reflecting the existence and relative
  abundance of isotopes for any given atom.
• IN NATURE, MOST ELEMENTS OCCUR AS A MIXTURE OF
  TWO OR MORE ISOTOPES.

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Atomic Masses

• Many atomic masses on PT are close to a whole
  number of AMUs sodium - 22.990 phosphorous -
  30.974 gold - 196.97
• But some are not chlorine 35.453.
• There are two naturally occurring isotopes of
  chlorine Chlorine-35 and Chlorine-37.
• If equal numbers were found in nature, we would
  expect chlorine to have an average atomic mass
  near 36 AMU. However, we do not get this because
  75 of chlorine is Cl-35. So the average is
  weighted toward the atomic mass of Cl-35.

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ATOMIC MASS

• ATOMIC MASS the atomic mass of an element is a
  weighted average mass of the atoms in a naturally
  occurring sample of the element. It reflects
  both the mass and the relative abundance of the
  isotopes occurring in nature.

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For Chlorine

• Cl-35 34.968853 75.0
• Cl-37 36.969033 25.0
• (34.968853) x (75/100) 26.22663975
• (36.969033) x (25/100) 9.24225825
• 35.468898 AMU
• 35.5 AMU

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To determine the average atomic mass, do the
following

• Obtain the atomic mass and relative abundance of
  each isotope.
• Convert the abundance to a multiplication
  factor by dividing by 100.
• Multiply each isotopes atomic mass by its
  multiplication factor.
• Sum the products of Step 3.

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Finding an Average Atomic Mass

• Naturally occurring Sulfur has four isotopes
• S-32 31.972 AMU 95.00
• S-33 32.971 AMU .7600
• S-34 33.967 AMU 4.220
• S-36 35.967 AMU .01400

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For Sulfur

• 31.972 AMU x .9500
• 32.971 AMU x .007600
• 33.967 AMU x .0422
• 35.967 AMU x .0001400
• 32.06242 AMU
• Or 32.06 AMU using the proper sig. figs.

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You try it

• Naturally occurring zinc is comprised of five
  isotopes. Determine its average atomic mass
  given the following

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Answer

• 65.38682 AMU, or 65.39 AMU when limited to the
  proper number of sig. figs.

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The Nucleus Radioactivity

What holds a nucleus together? What drives
radioactive decay? What sets the timescale for
radioactive decay? What happens during
radioactive decay?
a particles in a cloud chamber
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The Four Forces of Nature
Force Strength Range Occurrence Strong
nuclear 1 ltlt1/r2 (finite, v. short) inter-nucleon
Electromagnetic 10-2 1/r2 (infinite, but
shielded) nucleus, atom Weak nuclear 10-13 ltlt1/r
2 (finite, v. short) B-decay, neutrinos G
ravity 10-39 1/r2 (infinite) everywhere
Four Tenets of Nuclear Physics
1) mass-energy equivalence (Emc2) 2)
wave-particle duality (particles are waves, and
waves are particles) 3) conservation of energy,
mass, momentum 4) symmetry
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The Discovery of Radioactivity

• Henri Becquerel (1852-1908) by accident
• Pierre and Marie Curie continue investigation

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What is Radioactivity?

• Natures way of making unstable nuclei (having
  more energy) stable (having less energy)

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Forces In the Nucleus Determine the Energy State
of the Nucleus

• The Strong Nuclear Force
• Holds neutrons and protons together
• Glues nucleus together
• Very short range (10-15 m)
• The Electrostatic Force
• Like charges (protons) repel one another
• 100 times weaker than strong nuclear force
• Long range

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Nuclear Stability

• Requires the presence of neutrons (and the strong
  force)
• Large nuclei become unstable because protons on
  opposites side of the nucleus repel one another
  without the glue of the strong force acting
  between them

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Unstable Nuclei . . .

• Have too many protons for the given number of
  neutrons
• Too few neutrons to glue the nucleus together
• May simply be too big allowing the
  electrostatic force to pull the nucleus apart
• May have absorbed extra energy by some means and
  needs to release it

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Radioisotopes

• Certain isotopes that have unstable nuclei
• Unstable nuclei lose energy by emitting radiation
  during radioactive decay
• Radioactive decay spontaneous, requires no
  energy input nature naturally tends to move
  from states of high energy to low energy

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Four types of radioactive decay
1) alpha (a) decay - 4He nucleus (2p 2n)
ejected 2) beta (?) decay - change of nucleus
charge, conserves mass 3) gamma (g) decay -
photon emission, no change in A or Z 4)
spontaneous fission - for Z92 and above,
generates two smaller nuclei
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a decay
- involves strong and coloumbic forces - alpha
particle and daughter nucleus have equal and
opposite momentums (i.e. daughter experiences
recoil)
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? decay - three types
1) ?- decay
- converts one neutron into a proton and
electron - no change in mass number, but
different element - release of anti-neutrino (no
charge, no mass)
2) ? decay
- converts one proton into a neutron and
electron - no change in mass number, but
different element - release of neutrino
3) Electron capture
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g decay
- conversion of strong to coulombic E - no change
of A or Z (element) - release of photon - usually
occurs in conjunction with other decay
Spontaneous fission
- heavy nuclides split into two daughters and
neutrons - U most common (fission-track dating)
Fission tracks from 238U fission in old zircon
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Nuclear Stability

• Instability leads to decay
• Band of stability (p. 763)
• Too many nucleons alpha decay
• Too many protons beta decay (electron capture)
• Too many neutrons beta decay (emission of
  electron to make a proton)
• Too much energy gamma decay

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Working with Nuclear Reactions

• Nucleon number is conserved.
• The atomic number (charge) is conserved.
• Alpha particle

4 2
4 2
He
or
a
0 -1
e

• (-) Beta Decay emits an electron
• () Beta Decay emits a positron

0 1
e
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Which nuclei are radioactive?

• All elements with atomic numbers greater than 83
  (bismuth)
• These usually undergo alpha decay
• Some lighter isotopes as well.