Chapters 21

1
Chapters 21

• Early Quantum Theory and Models of the Atom

2
The Atom

• Protons Found in the Nucleus
• Carry a Positive Charge
• Have a Mass of 1 x 10-27 kg
• Neutrons Found in the Nucleus
• Carry no charge
• Have a mass that is approximately the same as a
  Proton
• Electrons travel around the nucleus in specific
  orbits related to their energy
• Carry a negative charge
• Have a mass of 9 x 10-31 kg

3
Light Quanta

• Light has a dual nature it is made up of
  packets of energy called quanta or photons which
  are carried on a wave.
• When an atom absorbs energy electrons move from
  their normal or ground state to a higher energy
  or excited state. To go back to their ground
  state atoms give off this excess energy as light.

4
Reminder The Wave Equation

• By Definition
• V f?
• Where
• v wave velocity (meters/second)
• f wave frequency (hertz)
• ? wavelength in meters.

5
The Energy of a Photon

• The energy of any photon of light given off in
  this way is determined by the equation
• E hf
• Where E energy in joules
• h 6.626 x 10-34 Js (Plancks Constant)
• f frequency of the light
• Because the amount of energy is quite small it is
  often expressed in Electron Volts symbolized by
  eV
• 1 eV 1.6 x 10-19 Joules

Practice A p. 755 1 - 4
6
The Photoelectric Effect

• The photoelectric effect is the ejection of
  elections from a material when light falls upon
  it.
• The material is sensitive to the frequency of the
  light not its intensity.

7
Chapters 22

• Nuclear Physics Radiation, Radioactivity its
  Applications

8
Nuclear Energy

• The Nucleus of an atom contains
• Protons Positively Charged
• Neutrons no charge
• Atomic Mass Number denoted by the letter A,
  this number represents the total number of
  protons neutrons in the nucleus, telling you
  what isotope of the element you have.
• Atomic Number denoted by the letter Z, this
  number represents the number of protons in the
  nucleus, telling you what element you have.

9
Nuclear Energy

• Atomic Symbol for a given isotope of an element
  is generally given as noted to the right.
• A prime example is an alpha particle or helium
  nucleus

10
Nuclear Reactions

• Two Types of Nuclear reactions produce vast
  amounts of energy according to Einsteins famous
  equation
• E mc2
• Fission the splitting of an atom into smaller
  parts
• Fusion- the joining of two small nuclei to
  produce one larger nucleus

11
Nuclear Reactions

• Mass defect is the amount of mass that is
  converted to energy during fission or fusion.
• Calculation of Mass defect is the difference
  between the actual mass of the atom and the known
  mass of each of its parts
• The amount of energy that this mass is converted
  into is called the binding energy

12
Sample Problem

• Calculate the mass defect and energy released in
  the creation of Carbon-13.
• Solution
• Expected Mass
• Protons 6 (1.007276 u) 6.043656 u
• Neutrons 7(1.008665 u) 7.060655 u
• 
  13.104311 u
• Known Mass -13.003355 u
• Mass Defect .100956 u
• Energy Released (931.5 MeV/u)(0.100956 u)
  94.04 MeV

Practice A P. 796 1 3
13
Radioactivity

• Three types of Radioactivity
• Alpha a is the nucleus of a helium atom
• Can be stopped by a sheet of paper, is harmful
  only if ingested
• Beta ß emission of an electron or positron
• Can be stopped by a sheet of lead, is harmful to
  all living tissue
• Gamma ? emission of a high energy photon
• Cannot be completely stopped. Very harmful to
  all living tissue.

14
Nuclear Reactions

• Alpha Decay
• Beta Decay
• Gamma Emission

15
Half Life

• The half life of a radioactive material is the
  amount of time required for ½ of the sample to
  decay into another element or isotope.
• Half lives are calculated according to the
  equation
• a a0(½)x

16
Half Life

• a amount of material left at any time
• a0 amount of material that you begin with
• x the number of half lives that have passed
  since you have begun counting
• This type of decay is said to be exponential
  since it can be described graphically as a
  hyperbola

17
Sample Problem

• Carbon-14, a radioactive isotope of carbon, has a
  half life of 5730 years. If a 20 gram sample of
  carbon-14 is allowed to decay for 10,000 years,
  how much remains at the end of this period?

18
Solution

• a a0(½)x
• a0 20 grams
• x 10,000 yrs/5730 yrs/half life
• 1.75
• So a 20 grams(½)1.75
• 5.95 grams

Practice C P. 805 Problems 4 5
19
Applications of Nuclear Processes

• Energy can be released in a nuclear reaction by
  one of two processes
• Fission the splitting of a nucleus into smaller
  nuclei
• Fusion the joining of two smaller nucleui into
  a larger nuclei

20
Fission

• Usually caused by neutron bombardment of the
  nucleus, causing the nucleus to split
• Mass is converted into energy
• All current nuclear reactor technology uses
  fission
• Fission is controlled by using a moderator, a
  material which absorbs neutrons to keep the chain
  reaction under control

21
Fusion

• Fusion reactions take lighter nuclei, often an
  isotope of hydrogen called deuterium and fuse
  them together to make a heavier nuclei, often
  helium
• This must occur at high energy and is very
  difficult to produce under laboratory conditions
• Currently no workable fusion reactor has been
  produced on earth
• The sun and stars all produce energy due to
  nuclear fusion