Atomic models

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1. Atomic models
2. Subatomic particles
3. Atomic structure
4. Change in atomic structure

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Atomic Models

• Models are used to help us visualize something
  that cannot be seen

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Democritus

• This Greek philosopher is credited with coining
  the term atomos
• This was the term for the smallest possible
  particle of matter

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J.J. Thomson

• Deflected a beam of particles with magnets
• Determined that particles must have a negative
  charge

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J.J. Thomson (1897)

• Plum Pudding model
• Electrons are embedded in a positive ball of
  matter
• This creates a neutral atom

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Rutherford (1911)

• Developed the gold foil experiment

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Rutherford

• A stream of positive alpha particles were
  directed at foil
• Thomsons model predicted that particles should
  pass through neutral material

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Rutherford

• Expectations
• Actual results
• Rutherfords model
• http//www.mhhe.com/physsci/chemistry/essentialche
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• http//micro.magnet.fsu.edu/electromag/java/ruther
  ford/

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Rutherford

• Rare deflections indicated two things
• A positive mass existed in the atom
• The positive center is very small

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Rutherford

• He determined that
• There is a positively charged nucleus in the
  center of all atoms
• The nucleus is VERY small
• Most of an atom is empty space

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Bohr

• Used color of light emitted from hydrogen atoms
  to determine the energies of its electron

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Bohr

• Energy indicates location of electron in the atom
• Low energy electrons are closer to nucleus
• High energy electrons are further from nucleus

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Bohr (1913)

• Called the orbital model or planetary model

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Schrodinger (1926)

• Used calculus to determine probability of finding
  an electron within a certain region
• The points in the diagram are all the possible
  locations of one electron

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Schrodinger

• Two names have been given to this model
• Quantum mechanical model
• Electron cloud model

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Recap Atomic Models
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Whats in an Atom?

1. Atomic models
2. Subatomic particles
3. Atomic structure
4. Change in atomic structure

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Proton (p)

• Has a positive charge
• Defines the atom as a specific element
• Has a mass of 1 amu (atomic mass unit)
• Is located in the nucleus

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Electron (e-)

• Has a negative charge
• It is located OUTSIDE of the nucleus in the
  electron cloud
• It has mass but it is so small relative to the
  mass of the proton and neutron that it is
  considered to be zero mass

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Neutron (no)

• Has no charge
• Has a mass of 1 amu (atomic mass unit)
• Is located in the nucleus

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Recap - Subatomic Particles
Symbol Charge Mass Location
Proton
Electron
Neutron
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Recap - Subatomic Particles
Symbol Charge Mass Location
Proton p 1 amu nucleus
Electron e- - 0 electron cloud
Neutron no 0 1 amu nucleus
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Whats in an Atom?

1. Atomic models
2. Subatomic particles
3. Atomic structure
4. Change in atomic structure

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Atomic Number

• The atomic number of an element is always the
  number of protons found in the atom
• The number of protons defines the element!

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Atomic Number

• Found at bottom, left of symbol

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Nuclear Charge

• Nuclear charge the positive charge of the
  nucleus caused by protons
• ie sodium (Na) has a nuclear charge of 11
  because it has 11 protons

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Atoms are NEUTRAL!

• ALWAYS!
• This means that the number of positive and
  negative charges are equal
• This means that the number of protons and
  electrons are always equal in ANY ATOM

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Atomic Structure

• Electrons have relatively NO MASS
• All the mass of an atom is in the nucleus
• Each proton has a mass of 1 amu
• Each neutron has a mass of 1 amu
• Summing protons neutrons mass number

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Mass Spec

• The heavier the atom, the more charge it
  requires..

..to maintain its path to the detector
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Isotopes

• Isotopes are
• Atoms of the same element (atomic number is the
  same)
• Atoms that have a different number of neutrons
  (mass number is different)

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Mass Number

• Mass number p no
• It is found in the top, left corner

The number of neutrons can be calculated by
subtracting the atomic from the mass
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Isotopic Symbols

• Indicates the unique mass for one specific atom
• Also shown by writing hydrogen-3

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Calculate the number of Neutrons

1. 2311Na
2. 94Be
3. 11950Sn
4. Carbon-13

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Hydrogen Isotopes
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Writing Isotopic Symbols

• Write both forms of an isotopic symbol for an
  atom having
• 5 p and 4 no
• 1 p and 1 no
• 8 no and 6 p

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Average Atomic Mass

• The weighted average of the isotope masses is
  found on the periodic table

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Average Atomic Mass

• Calculating a weighted average
• Multiply the percent abundance x isotope mass
• Sum all results

10.81 amu
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47.92 amu
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Recap atomic structure

• Note number of e- number of p
• Note number of no does not have to equal either
  the number of protons or electrons

What is the atomic mass of this atom?
39 amu
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Recap Atomic Structure
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Whats in an Atom?

1. Atomic models
2. Subatomic particles
3. Atomic structure
4. Change in atomic structure

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Change in number of Neutrons

• Neutrons have a mass of 1 amu
• Mass is added or removed
• mass changes
• This causes the formation of an isotope
• Ex 126C 2no ? 146C

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Change in number of Electrons

• Atom protons electrons
• This particle is neutral
• Ion protons ? electrons
• This particle is charged

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Change in number of Electrons

• Electrons have a negative charge
• Adding electrons to an atom results in
• electrons gt protons
• neg. charge gt pos. charge
• Negative ion ANION

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Change in number of Electrons

• Electrons have a negative charge
• Removing electrons from an atom results in
• protons gt electrons
• pos. charge gt neg. charge
• Positive ion CATION

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Changing the Number of Protons

• Changes
• the
• Element!

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Changing the Number of Protons

• This is not a chemical change
• This is a nuclear change

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4 Forces in the Universe
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Electrostatic Energy

• Opposites attract
• Likes repel

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Electrostatic Energy
Protons in the nucleus should repel each other
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The Strong Force

• When protons become close enough, they experience
  the strong force that holds them together

This is the energy that is released from a
nuclear plant or bomb
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Fission

• The splitting of a large nucleus into 2 midsized
  nuclei
• Produces neutrons that can continue the process
• Critical mass the minimum amount of radioactive
  material required to sustain fission

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Fusion

• The joining of two nuclei
• Occurs in stars
• Requires extreme heat to start process

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Alpha Decay

• The loss of an alpha particle occurs when
  electrostatic forces overcome the strong force

42He alpha!
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Alpha Decay Problems

• When uranium-238 undergoes alpha decay, it
  produces an alpha particle and a new particle
• 238U92 ? 4He2 ?
• 238U92 ? 4He2 234Th90
• Try the next 2 problems
• 239Pu94 ? 4He2 ?
• 241Am95 ? 4He2 ?

235 U 92 and 237 Np 93
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Beta Decay

• Beta decay creates another proton from a neutron
• This improves the proton/neutron ratio

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Beta Decay Problems

• 238U92 ? 0e-1 ?
• 238U92 ? 0e-1 238Np93
• Try the next 2 problems
• 239Pu94 ? 0e-1 ?
• 241Am95 ? 0e-1 ?

239Am95 and 241Cm96
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Half Life

• Half life the time it takes for HALF of a
  sample to decay

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Half Life

• Half life problems always contain 4 values
• Initial quantity
• Final quantity
• Half life
• Time elapsed
• Always begin a problem with the 2 like values

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Half Life
A 100 mg sample of cesium-134 has a half-life of
15 yrs. How much of the sample will remain after
45 years?
Half-life Time elapsed Initial quantity Final quantity
15 yrs 45 yrs 100 mg
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Recap Changing Particles

1. Change in the number of neutrons creates
   isotopes.
2. Change in the number of electrons creates ions.
3. Change in the number of protons creates a new
   element.

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The World of Particles

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Nuclear Particles

• Gluons
• Pions
• Hadrons
• Quarks
• Leptons
• Besons
• Meson