Nuclear Chemistry - Exam 1

Radiation

Any energy that is propagating in the form of a wave or particle

Photons

Packets of waves

Ionizing radiation

When a particle/wave/photon has enough energy to complete the knockoff of an electron or proton from an atom/molecule

• electron is most likely

X-rays, gamma rays, alpha particles, beta particles

Types of ionizing radation

>10 eV

Energy needed to ionize

Radioactivity

When energy is being emitted from the nucleus of an atom

1 u=1.6654×10^-24 g

Unified mass unit (u/amu)

Strongforce

Holds the nucleus together by attracting nucleons to other nucleons

10^-15 m

Range of the strongforce

Nucleons

Things in the nucleus

Atomic number, number of protons

Z

Mass number, number of protons and neutrons

A

Number of neutrons

N

Isotopes

Nuclides with the same number of protons, different number of neutrons

Isotones

Nuclides with the same number of neutrons, different number of protons

Isobars

Nuclides with the same A, different number of protons and neutrons

Hadrons

Composite particles made up of 3 quarks. Ex: protons and neutrons

Up (+2e/3) and down (-1e/3), charm (+2e/3) and strange (-1e/3), top (+2e/3) and bottom (-1e/3)

The six quarks and their charges

Strongforce and gluons, another type of elementary particle

How are quarks held together?

Up up down (uud)

Quarks that make up a proton

Up down down (udd)

Quarks that make up a neutron

Nuclear decay (fissure)

Unstable nuclides spit out a particle/photon while turning into something more stable

• alpha, beta, and gamma decay

Nuclear reaction (fusion)

A particle and/or a photon interacts with one or more nuclei to form another nuclide, plus the emission of a particle or photon

How to balance a nuclear equation

1. Sum of Z must be equal on both sides

2. Sum of A must be equal on both sides

3. Deduce symbol of unknown element from Z

⁴He nucleus = 2 protons and 2 neutrons

Alpha particle

Alpha decay

Nuclear decay where an unstable nucleus emits an alpha particle

A-4, Z-2

Net effect of alpha decay

Beta decay

Loss of an electron from an unstable nucleus

An electron

Beta particle

Beta decay, positron decay, electron capture

Three types of beta decay

A stays the same, Z+1

Net effect of beta decay

Positron decay

Loss of a positron

Positive electron

Positron

Antimatter

Particle which mirrors normal matter with opposite property (usually charge)

Annihlation

Mutual destruction and energy release

A stays the same, Z-1

Net effect of positron decay

Electron capture

Orbital electron is pulled into the nucleus and converts a proton into a neutron

A stays the same, Z-1

Net effect of electron capture

Gamma decay

Nuclear rearrangement from an excited nuclear state to a lower energy nuclear state; energy released in the form of a gamma ray

A high energy photon

Gamma ray

A and Z stay the same, *P to D

Net effect of gamma decay

Isomer

A nuclide that is only different in its energy state; * denotes a higher isomeric state

Metastable

Isomeric state which exists for more than a few seconds; denoted by an m next to A

Neutron bombardment

Nuclear reaction between a parent nuclide and a neutron to form a daughter nuclide and a gamma ray

A+1, Z stays the same, gamma ray is emitted

Net effect of neutron bombardment

Radioactive nuclide

Unstable nuclide which spontaneously undergoes nuclear decay

Bad N/Z ratio, odd number of protons and neutrons, nucleus is too large

Three main types of nuclide instability

Neutron rich (above the belt on segre chart), will convert neutrons to protons via beta decay

When N/Z is too high or A>atomic weight

Neutron poor, will convert protons to neutrons via positron decay or maybe electron capture

When N/Z is too low or A<atomic weight

Some form of beta decay

If Z is odd and/or if N is odd

Z>82, alpha decay

What size are nuclei no longer stable, and what kind of decay will they undergo?

Magic numbers

Exceptionally stable nuclides contain these numbers of protons and/or neutrons

Doubly magic

The number of protons and neutrons are both magic

alpha, beta, gamma/X-rays

Ionizing radiation from most ionizing to least ionizing

Less penetrating

More ionizing

More penetrating

Less ionizing

Cell damage/death, incorrect replication

Ionization inside the body

Hormesis

Theory that exposure to low dose radiation stimulates cell repair mechanisms

As low as reasonably achieveable

What does ALARA stand for?

Cosmic rays and radioactive materials present on Earth

Sources of natural radiation

Cosmogenic radiation, primordial nuclides, fission products

Types of radioactive materials present on Earth

Cosmogenic radiation

Lighter elements in the upper atmospheres are attacked by cosmic rays, producing unstable isotopes

Primordial nuclides

Unstable nuclides with a very long half life

• eventually all decay to lead

4.6×10⁹ years

Minimum half life for primordial nuclides

Fission products

Radionuclides produced during decay chain processes

Nuclear weapons, power plants, disposal of nuclear waste, nuclear medicine

Sources of anthropogenic radiation

<0.1%

What percentage of the average annual radiation dose in the US is anthropogenic?

6.2 mSv per annum

Average annual dose of radiation in the US

Average atomic mass

Weighted average of all naturally occuring isotopes of an element

Isotropic

Particles/photons emitted equally in all directions

Detectors

Records a small fraction of total activity

dps and cps are proportional, cps<dps

Relationships between dps and cps

Percent efficiency

Percentage of total decay that the detector sees

Specific activity

Activity per unit mass of substance

Radiometric dating

Determining how old an object is by how many radionuclides are left

A₀ as the activity for the new object, A_t for the old object

How to use carbon dating

14 dpm/g

Specific activity of ¹⁴C before 1950

Can’t use for objects post-1950s, good to about 50,000 y (10 half lives), about 2 sig figs of accuracy

Limitations of carbon dating

Uranium-lead dating

Useful for rocks 1 mil-4.5 bil yrs old; precision of around 0.1-1%

General method for dating

Amount of parent nuclide at t=0 is extrapolated by adding current amount of parent nuclide to the amount of daughter present

Isochron dating

Uses several samples from the same rock; normalized with a stable isotope

Branched decay

More than one possible decay pathway for an isotope

Branch ratio (BR)

Relative percentages for each form of decay

t_1/2=ln2/ln(A₂/A₁)*t

How to calculate half-life with branch decay for a short half-life

t_1/2=ln2*N/A

How to calculate half-life with branch decay for a long half-life

True (total) half-life

All decays of an isotope

Partial half-life

Only one branch of decay of an isotope

partial t_1/2/100 x BR

Converting partial t_1/2 to true t_1/2

true t_1/2/BR x 100

Converting true t_1/2 to partial t_1/2

Secular equilibrium

When t_1/2 of the parent is much bigger than t_1/2 of daughter

• rate of formation of daughter = rate of decay of daughter (occurs ~7 t_1/2 of daughter)

A_D=A_P(1-e^λ_Dt)

Secular equilibrium equation

Mass ratio easily found, can find t_{1/2 P} easily, mass of parent can be calculated from activity of any daughter and vice versa

Applications of secular equilibrium

Transient equilibrium

When t_1/2 of the parent is similar to or slightly larger than t_1/2 of the daughter

• parent has exponential decay, daughter’s activity starts at 0, grows and peaks, then relative activity of daughter and parent stabilize

Equations for secular equilibrium

Equations for transient equilibrium