unit 7: nuclear chemistry colin johnson and george fourkas
TRANSCRIPT
Unit 7: Nuclear Chemistry
Colin Johnson and George Fourkas
Nuclear DecayDecay Particles
• Alpha• Beta• Positron Emission• Electron Capture
• The element loses two protons and two
neutrons, decreasing the mass number by four and atomic number by two for each alpha decay. Alpha decay is the most common in elements with an atomic number greater than 84.
• An alpha particle is also known as a He-4 particle
ALPHA DECAY
Beta Decay• One neutron changes into one proton
and one negative beta particle. The atomic number (Z) increases by one due to the new proton. The mass number (X) is unchanged because a neutron is gone. A negative Beta particle is also formed to keep the particle neutral.
• Beta decay follows the form:
Electron Capture
• The nucleus captures an electron from the electron cloud.
• The electron is consumed rather than formed, so it is included on the reactant side of the equation.
POSITRON EMISSION
• One proton changes into one neutron and one positron particle. The atomic number decreases by one due to the loss of a proton. Since it changed into a neutron, the mass number is unchanged.
Zone of Stability
• The stable nuclei are in the shaded area (belt of stability). Radioactive nuclei are outside and must go through some type of decay to regain stability.
• N/P – if N is too high, usually beta decay
occurs.– If N is too low, usually positron
emission or electron capture occurs.
– For nuclei with atomic numbers over 84, alpha decay occurs.
http://lhs.lps.org/staff/sputnam/chem_notes/UnitII_Radioactivity.htm
Sample Decays
U He + Th
Pb Bi + β
Mn Cr + e
W + e Ta
23892
42
21182
21183
23490
0-1
5025
5024
01
17974
0-1
17973
Alpha Decay
Beta Decay
Positron Emission
Electron Capture
Nuclear Energy
• Binding Energy: The energy required to separate a nucleus into its individual nucleons
• E=Δmc2
– E the binding energy– m the mass defect– c2 the speed of light (2.997 924 58 x 108 m/s)
Nuclear Energy
• Mass defect: the mass lost when a nuclear change occurs.– the mass of the products is less than the
mass of the reactants
– Example: Calculate the binding energy per nucleon for C-12
• C= 11.996708 amu • Proton= 1.00783 amu• Neutron= 1.00867 amu
126
Half Life (t1/2)
• Radioactive elements decay according to their half life.
• The time required for the concentration of a reactant substance to decrease to half its initial value; the time required for half of a sample of a particular radioisotope to decay.
Rate of Decay
• The decay rate is the speed at which a substance disintegrates.
• The following equation represents the relationship between the number of nuclei remaining, N, the number of nuclei initially present, NO, the rate of decay, k, and the amount of time, t.
N=Noe-kt
K=ln2/t1/2
N=N0e-kt
Activity =
-dN
dt= kN
ST
0
-dN
N ST
0= d
t
k
ln Nt-ln N0=-k(t-0)
Nt=N0e-kt