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Title: Radioactivity

  • BrainPop
  • Atomic Model

  • 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
  • art.

  • 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

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

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

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

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

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

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

  • 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

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

  • 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

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

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

  • 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

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

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

Radiation Penetrating Power
  • There is a great difference between the
    penetrating power of the 3 types of radioactive
  • 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

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?

  • 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.

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
  • Ex Carbon 14 is a radioactive isotope because
    it can decay over time
  • BrainPop Isotopes

Radioactive Half-Life
  • Half-life The time required for half of the
    atoms of a radioactive isotope of an element to
  • 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

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.

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.

  • 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

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