The Nuclear Atom

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The Nuclear Atom
Atomic Scientists Song 252
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Aristotle (460 B.C. 370 B.C.)

• emphasized that nature consisted of four
  elements air, earth, fire, and water
• did not believe in discontinuous or separate
  atoms, but felt that matter was continuous

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Democritus (460 B.C. 370 B.C.)

• first to suggest the existence of atoms (Greek
  word atomos indivisible)
• atoms are indivisible and indestructible
• no experimental support

http//www.stenudd.com/myth/Greek/images/democritu
s_1628_Brugghen.jpg
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John Dalton (1766-1844)

• used scientific method to test Democrituss ideas
• Daltons atomic theory
• elements composed of atoms
• atoms of the same element are alike
• different atoms can combine in ratios to form
  compounds
• chemical reactions can occur when atoms are
  separated, joined, or rearranged (but atoms are
  not created nor destroyed)

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J.J. Thompson (1856-1940)(need to know this guy)

• discovered the electron
• thought atom was negative charges stuck in a
  positive charged lump
• referred to as the plumb pudding model

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Robert A. Millikan (1868-1953)

• found the quantity of charge carried by an
  electron (one unit of negative charge)
• calculated the mass of an electron (1/1840th the
  mass of a hydrogen atom)

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Ernest Rutherford (1871-1937)(need to know this
guy)
Like howitzer shells bouncing off of tissue
paper!

• proposed that the atom is mostly empty space
• positive charges and almost of the mass are in a
  small, centralized region called the nucleus

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Rutherford Flash Animation
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Try it Yourself!
In the following pictures, there is a target
hidden by a cloud. To figure out the shape of the
target, we shot some beams into the cloud and
recorded where the beams came out. Can you figure
out the shape of the target?
?
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The Answers
Target 1
Target 2
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Niels Bohr (1855-1962)(need to know this guy)

• electrons found only in specific circular paths
  (orbits) around the nucleus
• based on information about how the energy of an
  atom changes when it absorbs and emits light
• called these fixed energies energy levels

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Erwin Schrodinger (1926)

• quantum mechanical model
• probability of electron locations around the
  nucleus
• not an exact orbit
• eventually became the electron cloud model

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Schrödinger's Cat video 141
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Werner Heisenberg (1927)(need to know this guy)

• Heisenberg Uncertainty Principle impossible to
  know the exact position and momentum of an
  electron at the same time
• the observer affects the observed

http//www.deutsches-museum-bonn.de/ausstellungen/
heisenberg/bilder/heisenberg_2.jpg
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Structure of the Atom
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Particle Charge Mass (atomic mass units) Location
Proton 1 1 nucleus
Neutron Ø 1 nucleus
Electron - 1 5.0 x 10-4 (considered negligible) orbit, level, cloud
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Carbon- 12 as a standard

• carbon- 12
• ALL masses on the periodic table are based on
  their relationship to carbon-12
• the carbon- 12 isotope has been given the atomic
  weight of exactly 12.000000000 and is used as the
  basis upon which the atomic weight of other
  isotopes is determined

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Even smaller particles

• quarks
• make protons neutrons
• 6 types

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Learning Check

• An atom has 14 protons and 20 neutrons.
• A. Its atomic number is
• 1) 14 2) 16 3) 34
• B. Its mass number is
• 1) 14 2) 16 3) 34
• C. The element is
• 1) Si 2) Ca 3) Se
• D. The number of electrons in a neutral atom
  is
• 1) 14 2) 6 3) 20

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Isotopes

• same element but differ in their number of
  neutrons
• the atomic mass on periodic table is the WEIGHTED
  AVERAGE MASS of all the isotopes of that
  element
• this is based on an isotopes natural abundance
• the percentage of each isotope of an element that
  occurs in nature
• have the same chemical properties (reactivity)
  but different physical properties (density,
  melting/boiling point)

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2.3
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Chemical symbols for isotopes

• two different ways to write isotopes
• example for sodium
• sodium- 23
• only shows mass number (23) of the sodium isotope
• 23 Na
• shows the mass number (23) and the atomic (11)
  of the sodium isotope

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A
Atomic Number
Z
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Isotopes?

• Which of the following represent isotopes of the
  same element? Which element?
• 234 X 234 X 235 X 238 X
• 92 93 92 92
• 92 is the element
  uranium

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Do You Understand Isotopes?
6 protons, 8 (14 - 6) neutrons, 6 electrons
6 protons, 5 (11 - 6) neutrons, 6 electrons
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Radioactive Isotopes (dont need to know)

• unstable isotopes that break down over time
• uses
• cobalt 60
• radiation treatment for cancer
• carbon 14
• used to date objects up to 60,000
• years old
• iodine 125 and iodine 131
• ingested and used for
• medical imaging

Dangerous, but worth the risk
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The Mass Spectrometer
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• has many applications, but one of the simplest is
  to determine the natural abundances of the
  isotopes of a particular element
• the relative atomic mass can be calculated from
  the data from the mass spectrometer

Mass spectrometer video (226) http//www.youtube.
com/watch?v_L4U6ImYSj0
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Calculate the relative atomic mass of magnesium
with the provided data.

• magnesium results from the mass spectrometer
• 80 24Mg
• 10 25Mg
• 10 26Mg

• just a simple weighted mean
• .80(24) .10(25) .10(26) 24.3 amu