Nuclear Reactions

•Changes in matter originating from the nucleus of an atom.

Radioactivity

•Spontaneous emission of particles and/radiation

–

+

Lead block

Radioactive substance

• Three types of ray are produced by the decay, or breakdown, of radioactive substances

•Alpha rays ( α ) – consist of positively charged particles called alpha particles and therefore are deflected by the positively charged plate.

•Beta rays ( β ) – or beta particles – are electrons and deflected by negatively charged plate

•Gamma rays ( γ ) – have no charged and are not affected by an external electric field or magnetic field

Radioactive•When a nuclei change spontaneously,

emitting energy

Nucleons •Particles in the nucleus

Radionuclide•Radioactive nucleus

Radioisotopes•Atoms containing radionuclide

Nuclear Equations

•Radionuclides are unstable and spontaneously emit particles and/or electromagnetic radiation

Example: Uranium-238▫Emits alpha particles (helium-4 particles)

•Radioactive decay

Balancing Nuclear Equations

1. Conserve mass number (A). The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants.

1n0U235

92 + Cs13855 Rb96

371n0

+ + 2

235 + 1 = 138 + 96 + 2x1

2. Conserve atomic number (Z) or nuclear charge. The sum of nuclear charges in the products must equal the sum of

nuclear charges in the reactants.

1n0U235

92 + Cs13855 Rb96

371n0

+ + 2

92 + 0 = 55 + 37 + 2x023.1

Three Types of RadiationΑlpha Radiation•The loss of alpha particles (helium-4)

Beta Radiation•Loss of an electron from the nucleus•High-speed electrons beta particles

Gamma Radiation•Loss of high-speed photons from the

nucleus

Types of Radioactive decay

Positron emission•Positron same mass as an electron but

opposite in sign

Electron capture•The nucleus captures an electron from the

electron cloud surrounding the nucleus.

Strong Nuclear Force

•Cohesive forces in the nucleus that hold the nucleons together.

Interactions in the nucleus•Proton-proton•Proton – neutron•Neutron – neutron

n/p too large

beta decay

X

n/p too small

positron decay or electron capture

Y

23.2

Belt of Stability• Portion of the graph

that contains all stable nuclei

• Nuclei with 84 or more protons are radioactive

• Nuclei above the belt beta emission

• Nuclei below the belt positron emission or electron capture

Predicting the mode of decay

1. High n/p ratio (too many neutrons; lie above band of stability) --- undergoes beta decay

2. Low n/p ratio (neutron poor; lie below band of stability) --- positron decay or electron capture

3. Heavy nuclides ( Z > 83) --- alpha decay

Rate of Radioactive DecayHalf-life•Indicates the rate at which a radionuclide

decays•Time it takes for one half of a given

quantity of a radioisotope to decay

Exercises:

Nuclear Transmutations•Nuclear reactions that are induced in a way that nucleus is struck by a neutron or by another nucleus (nuclear bombardment) a.Positive ion bombardment

- alpha particle is the most commonly used positive ion

- uses acceleratorsExamples:14

7 N + 42 He → 17

8 O + 11 H

94 Be + 4

2 He → 126 C + 1

0n

b.Neutron bombardment- neutron is bombarded to a nucleus to form a new

nucleus- most commonly used in the transmutation to form

synthetic isotopes because neutron are neutral, they are not repelled by nucleus

Example: 58

26 Fe + 10 n → 59

26 Fe → 5927 Co + 0

-1 e

Transuranium elements-Element with atomic numbers above 92-Produced using artificial transmutations, either by:

a. alpha bombardmentb. neutron bombardmentc. bombardment from other nucleiExamples:

a.

b.

4He2Pu239

94 + Cm24296

1n0

+

1n0U238

92 + U23992

0 e-1Pu239

94 + Np23993 + 0

-1 e

14N7U238

92 + Es24799

1n0

+ 5c.

Nuclear Energy

Recall:Nucleus is composed of proton and neutronThen, is the mass of an atom equal to the total mass of all the proton plus the total mass of all the neutron?

Example for a He atom:Total mass of the subatomic particles= mass of 2 p+ + mass of 2n0

= 2 ( 1.00728 amu ) + 2 (1.00867 amu)= 4.03190 amuAnd atomic weight of He-4 is 4.00150

Why does the mass differ if the atomic mass = number of protons + number of neutrons?

Mass defect

-mass difference due to the release of energy-this mass can be calculated using Einstein’s equation

E =mc2

2 11 H + 2 2

0 H → 42 He + energy

Therefore:-energy is released upon the formation of a nucleus from the constituent protons and neutrons-the nucleus is lower in energy than the component parts.-The energy released is a measure of the stability of the nucleus. Taking the reverse of the equation:

42 He + energy → 2 1

1 H + 2 20 n

Therefore,-energy is released to break up the nucleus into its component parts. This is called the nuclear binding energy.-the higher the binding energy, the stable the nuclei.-isotopes with high binding energy and most stable are those in the mass range 50-60.

Nuclear binding energy per nucleon vs Mass number

nuclear binding energynucleon nuclear stability

23.2

-The plot shows the use of nuclear reactions as source of energy.-energy is released in a process which goes from a higher energy state (less stable, low binding energy) to a low energy state (more stable, high binding energy).-Using the plot, there are two ways in which energy can be released in nuclear reactions:

a. Fission – splitting of a heavy nucleus into smaller nuclei

b. Fusion – combining of two light nuclei to form a heavier, more stable nucleus.

Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons.

BE + 19F 91p + 101n9 1 0

BE = 9 x (p mass) + 10 x (n mass) – 19F mass

E = mc2

BE (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984

BE = 0.1587 amu 1 amu = 1.49 x 10-10 J

BE = 2.37 x 10-11J

binding energy per nucleon = binding energy

number of nucleons

= 2.37 x 10-11 J

19 nucleons= 1.25 x 10-12 J

23.2

23.3

Radiocarbon Dating14N + 1n 14C + 1H

7 160

14C 14N + 0 + 6 7 -1 t½ = 5730 years

Uranium-238 Dating

238U 206Pb + 8 4 + 6 092 -182 2

t½ = 4.51 x 109 years

23.3

Nuclear Transmutation

Cyclotron Particle Accelerator

14N + 4 17O + 1p7 2 8 1

27Al + 4 30P + 1n13 2 15 0

14N + 1p 11C + 47 1 6 2

23.4

Nuclear Transmutation

23.4

Nuclear Fission

23.5

235U + 1n 90Sr + 143Xe + 31n + Energy92 54380 0

Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2

Energy = 3.3 x 10-11J per 235U

= 2.0 x 1013 J per mole 235U

Combustion of 1 ton of coal = 5 x 107 J

Nuclear Fission

23.5

235U + 1n 90Sr + 143Xe + 31n + Energy92 54380 0

Representative fission reaction

Nuclear Fission

23.5

Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions.

The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass.

Non-critical

Critical

Nuclear Fission

23.5

Schematic diagram of a nuclear fission reactor

Annual Waste Production

23.5

35,000 tons SO2

4.5 x 106 tons CO2

1,000 MW coal-firedpower plant

3.5 x 106

ft3 ash

1,000 MW nuclearpower plant

70 ft3 vitrified waste

Nuclear Fission

23.5

Nuclear Fission

Hazards of the radioactivities in spent fuel compared to uranium ore

From “Science, Society and America’s Nuclear Waste,” DOE/RW-0361 TG

23.6

Nuclear Fusion

2H + 2H 3H + 1H1 1 1 1

Fusion Reaction Energy Released

2H + 3H 4He + 1n1 1 2 0

6Li + 2H 2 4He3 1 2

6.3 x 10-13 J

2.8 x 10-12 J

3.6 x 10-12 J

Tokamak magnetic plasma confinement

23.7

Radioisotopes in Medicine• 1 out of every 3 hospital patients will undergo a nuclear medicine

procedure

• 24Na, t½ = 14.8 hr, emitter, blood-flow tracer

• 131I, t½ = 14.8 hr, emitter, thyroid gland activity

• 123I, t½ = 13.3 hr, ray emitter, brain imaging

• 18F, t½ = 1.8 hr, emitter, positron emission tomography

• 99mTc, t½ = 6 hr, ray emitter, imaging agent

Brain images with 123I-labeled compound

Geiger-Müller Counter

23.7

23.8

Biological Effects of RadiationRadiation absorbed dose (rad)

1 rad = 1 x 10-5 J/g of material

Roentgen equivalent for man (rem)

1 rem = 1 rad x Q Quality Factor

-ray = 1

= 1

= 20

Chemistry In Action: Food Irradiation

Dosage Effect

Up to 100 kiloradInhibits sprouting of potatoes, onions, garlics. Inactivates trichinae in pork. Kills or prevents insects from reproducing in grains, fruits, and vegetables.

100 – 1000 kilorads Delays spoilage of meat poultry and fish. Reduces salmonella. Extends shelf life of some fruit.

1000 to 10,000 kiloradsSterilizes meat, poultry and fish. Kills insects and microorganisms in spices and seasoning.