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Flashcards in SNS - General Chemistry - Nuclear Phenomena Deck (15):
1

Isotopes

Different forms of an atom with the same number of protons but a different number of neutrons. Therefore isotopes will have the same atomic number but a different atomic mass

2

Nuclear Reactions

Radioactive Decay

[At] = [Ao] e-kt

Where [At] is concentration of A at time t, [Ao] is initial concentration of A, k is a rate constant and t is elapsed time

Occurs as a result of an unstable nucleus that spontaneously emits energy and/or small subatomic particles Radioactive isotopes emit subatomic particles including electrons (beta particle), neutrons, helium nucleii (alpha particle) and positrons As a result, nuclear mass and/or nuclear charge changes forming a radioisotope

3

Nuclear Reactions Radioactive Decay Alpha

Charge - +2 Radiation - helium nucleus (2 protons, two neutrons) Penetrating power - 1

4

Nuclear Reactions

Radioactive Decay

Beta

ß-

Charge - -1 Radiation - Electrons Penetrating power - 100

Occurs when a neutron decays into a proton and a ß- (anti-neutrino). Results in no change to atomic mass, A and increase in atomic number, Z by one

5

Nuclear Reactions Radioactive Decay Gamma

Charge - 0 Radiation - High energy photons Penetrating power - 1000

6

Nuclear Reactions

Radioactive Decay

Beta

ß+

Proton splits into a positron and a neutron. Results in no change to A and decrease in Z by one

Reactions involving positrons may involve elecron capture, a process whereby an electron combines with a proton to form a neutron

7

Nuclear Reactions Radioactive Decay Gamma Rays

Emitted when decay occurs leaving the nucleus with excess energy. The loss of this excess energy is called gama radiation. No change in charge or mass occurs

8

Conversion amu to kg and g

1 amu = 1.66 x 10-24 g = 1.66 x 10-27 kg

 

9


Mass Defect

Every nucleus (except 11H) has a smaller mass than the combined mass of its constituent protons and neutrons. The amount by which the two differ is called the mass defect

Occurs due to the conversion of matter to energy - binding energy which holds the nucleons together in the nucleus

10


Rest energy of 1 amu

932 MeV

11


Calculate the mass defect and binding energy for 4He

proton = 1.00728 amu

neutron = 1.00867 amu

Atomic mass 4He = 4.00260 amu

Mass defect = 4.00260 - ((1.00728 x 2) + (1.00867 x 2)) = 0.02930

E = mc2      c2 = 932 MeV / amu

Binding Energy = 0.02930 x 932 = 27.3 MeV

12


Supposing a parent isotope AZX emits a ß- and turns into an excited state of the isotope A'Z'Y* which then γ decays into A''Z''Y which undergoes alpha decay to A'''Z'''W. If W = 60Fe, what is AZX  

60Fe : A''' = 60, Z''' = 26

Y : A'' = 64, Z'' = 28

Y* : A' = 64, Z' = 28

X : A = 64, Z = 29, X = 64Cu

13


Electron Capture

Rarely, certain unstable radionuclides are capable of capuring an inner (K or L shell) electron which combines with a proton to form a neutron. May be thought of as inverse ß- decay

14


Decay constant

λ = 0.693 / t1/2

15


If at time t=0 there is a 2 mole sample of radioactive isotopes of decay constant 2 (hour)-1 how many nuclei remain after 45 minutes?

n = n0 e-λt

n0 = (2 x 6 x 1023) = 1.2 x 1024, λ = 2 hr, t = 0.75 hr

n = 2.6 x 1023 particles

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