Radioactivity

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Radioactivity

• BrainPop
• Atomic Model

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• A charge can be either positive or negative.
• Like charges repel
• Two positive charges try to push each other apart
• unlike charges attract
• Ex. A positive charge draws closer to a negative
  charge
• art.

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• Protons are positive charges
• Electrons are negative charges
• A proton and an electron will be attracted to
  each other
• Neutrons dont have any charge so they wont be
  attracted or repelled by anything else

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• The nucleus of an atom contains protons and
  neutrons
• But protons repel each other!
• The closer together they are, the greater the
  force that wants to push them apart

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• Protons and neutrons are also called nucleons
• A proton is a nucleon
• A neutron is a nucleon

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Check Your Understanding

• An atomic nucleus has 20 protons and 25 neutrons.
  How many nucleons does it have?
• Protons Neutrons
  Nucleons
• 20 25
  45
• A nucleus has 10 protons and 12 neutrons. How
  many nucleons does it have?

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• Protons Neutrons Nucleons
• 40 50
• 51 91
• 100 257
• 83 139
• BrainPop Atoms

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Strong Nuclear Force

• The atom does not fall apart when 2 or more
  positive charges are extremely close together, so
  there has to be another force acting,
• Physicists call this force the Strong nuclear
  force.

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The Atomic Nucleus

• Nucleon The principal building block of an
  atomic nucleus
• The nucleus of an atom is composed of nucleons
• Ex protons and neutrons
• The mass of protons and neutrons are nearly the
  same

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• The number of protons determines the properties
  of an atom (i.e. what type of atom it is)
• Carbon always has 6 protons, hydrogen always has
  1 proton, oxygen always has 8 protons
• You change the of protons, you change the type
  of atom
• Neutrons have no affect on an atoms properties

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The Strong Force

• Nucleons are bound together by the strong force
• It is the strongest force in the Universe
• Aka Strong Nuclear Force
• It is effective over extremely short distances
• Protons want to repel each other
• Neutrons help to keep the protons from repelling
  out of the nucleus
• Neutrons also carry the Strong force

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• Light elements usually only need equal number of
  protons as neutrons
• Ex Carbons most common form has 6 protons and
  6 neutrons
• Heavier elements need more neutrons than protons
  to hold the nucleus together.
• Ex Leads most common form has 82 protons and
  126 neutrons
• For elements with MORE than 83 protons, they
  remain unstable no matter how many neutrons are
  added.

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Check Your Understanding

• What is the weakest force in the universe?
• Gravity
• What is the strongest force in the universe?
• The strong force (or strong nuclear force)
• If the strong nuclear force is so strong, why do
  we feel gravity more?
• B/c gravity has a much longer range than the
  strong force. The strong force only extends as
  far as an atoms nucleus.
• BrainPop Radioactivity

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Radioactive Decay

• Atoms can be stable or unstable
• If they are unstable, they try to change their
  number of protons, neutrons, or electrons to make
  themselves more stable

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• Radioactive term applied to an atom with a
  nucleus that is unstable and that can
  spontaneously emit a particle and become the
  nucleus of another type of atom
• All elements with more than 83 protons will
  decayso they are radioactive

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3 Types of Radioactive Decay

• Three rays can be emitted during radioactive
  decay
• Alpha (a)
• Beta (ß)
• Gamma (?)
• Alpha Particle
• Always positive
• 2 protons and 2 neutrons
• A helium nucleus

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• Beta particle
• Always negative
• An electron
• Gamma rays
• Always neutral (i.e. no charge)
• No mass
• Very high energy EM waves

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Radiation Penetrating Power

• There is a great difference between the
  penetrating power of the 3 types of radioactive
  rays
• Alpha rays are the easiest to stop
• They can be stopped by a heavy sheet of paper
• Alpha particles move the slowest
• Beta rays go thru paper but are stopped by
  sheets of Aluminum foil
• Electrons move faster than alpha particles
• Gamma rays require lead to block them
• Move at the speed of light

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Check Your Understanding

• Pretend you are given radioactive cookies-one
  alpha, one beta, and the other gamma. If you
  MUST eat one, hold one, and pocket one, which
  would you do what to in order to minimize your
  exposure to radiation?

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• Hold the alpha your skin will protect you
• Pocket the beta your clothing will protect you.
• Eat the gamma it will penetrate your body anyway.

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Radioactive Isotopes

• An atom has more than one form, based on the of
  neutrons it has
• These are called isotopes
• Some isotopes are more stable than others
• Unstable isotopes are also called radioactive
  isotopes
• Ex Carbon 14 is a radioactive isotope because
  it can decay over time
• BrainPop Isotopes

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Radioactive Half-Life

• Half-life The time required for half of the
  atoms of a radioactive isotope of an element to
  decay.
• Radioactive isotopes decay at different rates
• The amount of a substance halves every half-life
• Ex Uranium 238 has a half-life of 4.5 billion
  years, Carbon 14 has a half-life of 5730 years

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Check Your Understanding

• If there is 100 g of carbon 14 at time zero, how
  much carbon 14 will there be after 5730 years
  (one half life)?
• 50 g. There has been one half-life, so only half
  the amount is left.
• How much will be left in another 5730 years
  (11460 years total)?
• 25 g. The amount is cut in half again, so ¼ of
  the original amount remains.

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Check Your Understanding

• If a sample of a radioactive isotope has a
  half-life of one year, how much of the original
  sample will be left at the end of two years?
• One quarter (or half of a half). Half decayed
  during the 1st year, and half of the half
  remaining decayed during the 2nd year.

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Dating

• Scientists often use radioactive half-life
  information to date older materials
• Carbon 14 is used to date organic materials that
  have been dead for centuries
• Ex dinosaur bones, wooden artifacts
• Uranium 238 and 235 are used to date much older
  objects that were never living
• Ex dating the age of Earth

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Check Your Understanding

• Why cant Uranium 238 be used to date younger
  objects like skyscrapers?
• Not enough Uranium has undergone radioactive
  decay to measure a change. The half-life of
  U-238 is 4.5 billion years!
• BrainPop Carbon Dating