SNS - General Chemistry - Nuclear Phenomena

Question 1

Isotopes

Answer 1

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

Question 2

Nuclear Reactions

Radioactive Decay

Answer 2

[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

Question 3

Nuclear Reactions Radioactive Decay Alpha

Answer 3

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

Question 4

Nuclear Reactions

Radioactive Decay

Beta

ß-

Answer 4

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

Question 5

Nuclear Reactions Radioactive Decay Gamma

Answer 5

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

Question 6

Nuclear Reactions

Radioactive Decay

Beta

ß+

Answer 6

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

Question 7

Nuclear Reactions Radioactive Decay Gamma Rays

Answer 7

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

Question 8

Conversion amu to kg and g

Answer 8

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

Question 9

Mass Defect

Answer 9

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

Question 10

Rest energy of 1 amu

Answer 10

932 MeV

Question 11

Calculate the mass defect and binding energy for ⁴He

proton = 1.00728 amu

neutron = 1.00867 amu

Atomic mass ⁴He = 4.00260 amu

Answer 11

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

Question 12

Supposing a parent isotope ^A_ZX 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 ^A_ZX

Answer 12

⁶⁰Fe : A’’’ = 60, Z’’’ = 26

Y : A’’ = 64, Z’’ = 28

Y* : A’ = 64, Z’ = 28

X : A = 64, Z = 29, X = ⁶⁴Cu

Question 13

Electron Capture

Answer 13

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

Question 14

Decay constant

Answer 14

λ = 0.693 / t_1/2

Question 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?

Answer 15

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