CHEM 1112: Nuclear Chemistry

Nuclear Composition

• nuclei contain two types of nucleons:

  • protons (+ charge)

  • neutrons (neutral charge)

Proton, Neutron, and Electron charge/mass

Atomic # vs. Mass #

• Atomic #:

  • Z

  • number of protons in nucleus

• Mass #:

  • A

  • number of protons + neutrons in nucleus

Principal Isotropes of H

• protium

  • most abundant

• deuterium

• tritium

Nuclide

• a particular nucleus

• characterized by # of protons (Z) and neutrons (N) that they possess

  • chlorine-37 is a nuclide

Isotopes

• atoms of the same element whose nuclei contain different numbers of neutrons

  • chlorine-37 is an isotope of chlorine

• some isotopes are unstable, and tend to rearrange or disintegrate

Mass Deficit

• ∆m

• a nucleus has a lower mass than the sum of its parts

  • this is because so much energy is released upon the formation of a stable nucleus, that the nucleus actually loses mass

• can be calculated by comparing the “sum of the parts” to the observed mass

Nuclear Binding Energy

• a positive mass defect (∆m) times the speed of light (s) to find a change in energy (∆E)

• energy corresponding to the mass defecit

  • positive sign represents the energy required to disassemble the nucleus

• nuclides with mass # around 56 are the most stable

Relative Nuclear Stability

• to compare which nucleus of two separate nuclides is more stable, we must compare binding energy per nucleon

• ∆E/nucleon

  • higher binding energy per nucleon → more stable

Unstable Nuclei

• too many or too few neutrons (for the # of protons) which causes the nucleus to be unstable

  • unstable nuclei may become more stable by giving off this excess matter and/or energy, or by capturing a nearby electron

• nuclei that are too large (too many protons in the small area of the nucleus) will be unstable

  • such nuclei may become more stable by giving off the excess matter or even splitting to form two new nuclei

• all isotopes of elements with atomic numbers greater than 83 are unstable

• Unstable nuclides decay to form more stable nuclides

Belt of Stability

• unstable nuclides in green

• stable nuclides in blue

  • generally, most stable isotopes have n > Z

Nuclear Stability

• balance between attractive forces and repulsive forces

  • attractive forces only over very very short distances

• elements with large atomic # (Z) need more neutrons

  • this increases the value of net nuclear binding force

  • this increases the distances between protons

  • this decreases repulsion

• more stable nuclides have roughly equal numbers of neutrons and protons

Nuclear Decay

• decomposition of unstable nuclides into more stable ones, often accompanied by a change in mass

  • radioactive decay

• decay MUST conserve mass # and charge !!!!!

Radioactivity

• spontaneous nuclear disintegration of atoms with emission of particles or electromagnetic radiation.

• radiation that is emitted from a radioactive isotope may include:

  • alpha particles

  • beta particles

  • positrons

  • gamma rays

Alpha Particle

• ɑ

• high energy helium nuclei consisting of two protons and two neutrons

  • mass # is 4

  • charge is 2+

Beta Particle

• β

• high energy negatively charged particle (electron)

  • mass # is 0

  • charge is -1

• will not exist long, usually will promptly collide with an electron

  • both will be annihilated with the release of energy (usually 2 gamma photons)

Positron

• composed of + charged particles (positrons)

  • mass # is 0

  • charge is +1

• will not exist long, will promptly collide with an electron

  • both will be annihilated with the release of energy (usually 2 gamma photons)

Gamma Photons

• 𝛾

• very high energy electromagnetic radiation

  • mass # is 0

  • charge is 0

• a photon

• very penetrating, and can pass through a human body

Chemical Change

• in chemical change, no material is created or destroyed

• end up with the same number/types of atoms

• net charge is conserved

• energy may be absorbed or produced, but any mass change is not measurable

Radioactive Decay

• total mass number is conserved

• total charge is conserved

• types of atoms will usually change

Nuclear vs. Chemical Reaction

ɑ-emission

• Z decreased by two units and A decreases by 4 units

β-emission

• Z increases by one unit and A does not change

Positron Emission

• Z does not change, and A is decreased by 1 unit

Electron Capture

• Z does not change, and A is decreased by 1 unit

• an inner shell electron of the atom is a reactant

Types of Nuclear Decay

• Too big ?

  • ɑ-emission

• Protons too large ?

  • β-emission

• Protons too small ?

  • positron emission

  • electron capture

Decay Sequences

• series of radioactive decays that an unstable nucleus undergoes in order to become stable

• may either be ɑ-decay or β-decay

• all decays obey the 1st order rate law

  • Rate= k[A]

  • t_1/2 = 0.693/k

  • ln[A]_t / [A]_o = -kt

Unstable Nuclei

• stable isotopes remain stable, whereas unstable isotopes continuously disintegrate

  • thus, every element should be composed of ONLY stable isotopes

• unstable isotopes present either have half-lives longer than the age of the earth, or are products of the decay of these nuclides

Nuclear Transmutation

• transformation of one element into another

  • occurs during nuclear reactions

  • is not restricted to natural decay processes

  • can also occur through induced nuclear reactions

Induced Nuclear Reaction

• occurs when a nuclear projectile collides and reacts with another nucleus

• classified according to the nature of the nuclear projectile

  • most common: neutron-capture

Neutron Capture

• always exothermic

• product is a metastable, highly excited nuclide that usually decays by either proton or gamma emission

• neutrons could come from the sun, or produced in a nuclear reactor

  • the earths atmosphere is constantly bombarded with solar neutrons

  • can be captured by the most abundant element in atmosphere ¹⁴N to form unstable ^15mN, which will fragment into ¹²C + ³H

Fission

• when heavy nuclides fragment

Fusion

• when light nuclides combine

Fission Chain Reaction

• the fission of a large nucleus produces two or three neutrons, each of which is capable of causing fission of another nucleus by the reactions shown attached.

  • if this process continues, a nuclear chain reaction occurs

Subcritical Mass

• fissile material is too small and allows too many neutrons to escape the material, so a chain reaction does not occur

Critical Mass

• a large enough number of neutrons in the fissile material induce fission to create a chain reaction

Advantages of Nuclear Fission (energy production)

• generates a tremendous amount of electricity from a small amount of fuel

• generates no air pollution, or greenhouse gases

Problems with Nuclear Fission (energy production)

• potential for nuclear accidents as the fission reaction can overheat

• the disposal of products brings issues, as they may be radioactive and have long half-lives

Advantages with Nuclear Fusion (energy production)

• generates a tremendous amount of electricity from a small amount of fuel

• generates no air pollution, or greenhouse gases

• provides about 10x more energy per gram of fuel than fission

• does not produce radioactive waste products

Problems with Nuclear Fusion (energy production)

• very high temperatures required for fusion to occur, and no existing material can withstand those temperatures for long

• need to contain using magnetic fields

Tokamak Fusion Reactor

• uses a powerful magnetic field to confine plasma

  • able to reach temperatures around 100-150 million ^oC

• difficult to attain conditions to allow the reaction to be self-sustaining and controlled

  • record is 22 minutes of maintained plasma reaction, reached a few months ago

Applications of Radioactivity

• radiocarbon dating

• applications to human health

  • diagnosis of disease

  • treatment of disease

Radiocarbon Dating

• in the upper atmosphere ¹⁴C both forms and degrades at a constant rate, thus there is a relatively constant ratio of ¹⁴C:¹²C in the atmosphere

  • from this, scientists can work backwards to determine the length of time since an organism had ‘died’

• can date organisms up to 50,000 years old with 5% accuracy

Uranium/Lead Dating

• can estimate age of older objects that were never alive, using other techniques

  • U-238 decays to Pb-206 with a half-life of 4.5×10⁹ years

Positron Emission Tomography

• PET scan

• detects gamma rays emitted during positron annihilation

• used commonly to detect areas of high metabolic activity

  • active areas of brain

  • sites of tumours

Warburg Effect

• the rate of glycolysis is elevated in almost all tumours

• useful for potential treatment strategies

• useful for diagnosis of cancer

Glycolysis

• set of reactions that converts glucose to pyruvate or lactate

Boron Neutron Capture Theory

• novel approach where tumour cells are allowed to pick up compounds rich in boron-10.

  • the tissues are then bombarded with neutrons

  • boron-10 will capture the neutrons, forming radioactive boron-11

• targets:

  • brain tumours

  • head and neck cancer

  • melanoma

  • liver cancer

  • lung cancer

  • mesothelial tumour

  • breast cancer

• able to target tumour cells since target cells are actively producing proteins that contain tyrosine