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