The Atom and Radioactivity

1
The Atom and Radioactivity

• The Birth of Modern Physics

2
Basic Structure of an Atom

• An atom consists of negatively charged electrons
  surrounding a positively charged nucleus
• The nucleus consists of both positively charged
  protons and neutral particles called neutrons

3
Properties of Atoms

• An atoms always has the same number of electrons
  and protons
• The number of protons in the nucleus of a
  particular atoms is its atomic number
• Example Carbon atoms have six protons so the
  atomic number of carbon is six
• The atomic number determines the chemical
  properties of an atom

4
The Nucleus

• The nucleus consists of a dense ball of neutrons
  and protons
• The total number of neutrons and protons is the
  atomic mass
• Protons and neutrons are roughly equal in mass,
  but they are almost 2000 times more massive than
  the electron

5
Discovery of the Electron

• In 1897, J.J. Thompson discovered the electron
• The electron is a negatively charged particle
  that is thousands of times lighter than the
  lightest atom

Thompsons experiment to prove that electrons are
light, negatively charged particles
6
Isotopes

• Isotopes have the same atomic number but a
  different atomic mass
• That is, the number of protons is the same, the
  number of neutrons is different
• Example Hydrogen has three isotopes called
  hydrogen, deuterium and tritium (each has only
  one proton)

7
Notation

• Sometimes, the atomic mass of an isotope is
  indicated after the name of the element, e.g.
• uranium-238
• carbon-14
• Sometimes, the atomic mass is written as a
  superscript preceding the symbol for the element,
  e.g.
• 238U or 14C

8
Radioactivity

• In the late 19th century, Becquerel and Marie and
  Pierre Curie discovered that certain elements
  emit radiation
• Definition Radioactivity is the property of
  elements that emit radiation
• Definition Radiation is the stream of particles
  that are actually emitted from a radioactive
  substance

9
Types of Radiation

• Unstable nuclei emit three common types of
  radiation
• ? rays heavy positive charge
• ? rays light negative charge
• ? rays neutral particles

?
10
What Causes Radiation?

• Certain isotopes have unstable nuclei
• In order to become stable again, they must emit a
  particle (decay)
• Example Tritium is unstable and emits a beta
  particle (electron)

11
Alpha Radiation

• Some nuclei decay by emitting an alpha particle
• An alpha particle consists of two protons and two
  neutrons (i.e. a helium nucleus)
• Example Americium-241 (241 atomic mass)
  becomes Neptunium-237 after emitting an alpha
  particle

12
Discovery of the Nucleus

• Rutherford, Geiger and Marsden observed that most
  alpha particles passed straight through a gold
  foil (1911)
• Occasionally, a particle was deflected as it
  struck the nucleus

13
Beta Radiation

• Some nuclei decay be emitting a beta particle
• A beta particle is just an electron!
• When tritium decays, a neutron becomes a proton
  and an electron is emitted

14
Gamma Radiation

• Some nuclei decay by emitting a gamma particle
• A gamma particle is just pure energy (a photon)
• Helium-3 has too much energy, so it emits a
  gamma-ray photon

15
Comments on Radioactive Decay

• When an atom decays by alpha or beta radiation,
  it transmutates or changes into a different
  element
• Example When uranium decays by emitting an alpha
  particle, it becomes a thorium atom
• Many radioactive elements decay into other
  radioactive elements which themselves decay
• Example Uranium will eventually decay into lead
  through several intermediate decays (i.e. uranium
  to thorium to protactinium ... to lead)

16
Half-life

• The time it takes half of the atoms in a
  radioactive sample to decay is the half-life
• Example Uranium-238 has a half life of 4.5
  billion years
• After 4.5 billion years, only half of the
  original quantity of uranium will remain
• After 9 billion years, only one fourth will
  remain