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Nuclear Decay

Key Concepts

  • alpha particles:
        identity: helium nucleus, 42He
        charge: 2+

  • beta particles:
        identity: high energy electrons, 0-1e
        (emitted when a neutron decays to form a proton and an electron)
        charge: -1

  • gamma rays:
        identity: high energy electromagnetic radiation (accompany alpha or beta decay)
        charge: none

  • Mass Numbers (A) on the left hand side of the nuclear decay equation must
        equal the sum of the mass numbers on the right hand side of the equation

  • Atomic Numbers (Z) on the left hand side of the nuclear decay equation must
        equal the sum of the atomic numbers on the right hand side of the equation

Animated Tutorial

Examples

alpha decay

238     234     4  
  U   decays to produce     Th   and     He
92 90 2
Mass Number (A) 238     234     4   238 = 234 + 4
    U     →       Th     +       He  
Atomic Number (Z) 92 90 2 92 = 90 + 2

When uranium-238 decays it releases 2 protons and 2 neutrons as an alpha particle.

The daughter nucleus therefore has an atomic number (Z) = uranium's atomic number - 2

Daughter's atomic number (Z) = 92 - 2 = 90

From the periodic table we see that the element with atomic number 90 is thorium, Th.

The mass number (A) of this thorium atom = uranium's mass number - 4 = 238 - 4 = 234

The daughter nucleus (thorium-234) has 2 less protons and 2 less neutrons than the parent nucleus (uranium-238).

beta decay

14     14     0  
  C   decays to produce     N   and     e
6 7 -1
Mass Number (A) 14     14     0   14 = 14 + 0
    C     →       N     +       e  
Atomic Number (Z) 6 7 -1 6 = 7 + -1

When carbon-14 decays, a beta particle is released from the nucleus.
A beta particle has a mass number of 0 and an atomic number of -1.

The daughter nucleus therefore has an atomic number = carbon's atomic number - -1

Daughter's atomic number = 6 - -1 = 6 + 1 = 7

From the periodic table we see that the element with atomic number 7 is nitrogen, N.

The mass number of this nitrogen atom = carbon's mass number - 0 = 14 - 0 = 14

The daughter nucleus (nitrogen-14) has 1 more proton and 1 less neutron than the parent nucleus (carbon-14)


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