Chapter 21 - Nuclear Chemistry

Radioactivity

Emission of subatomic particles or high-energy electromagnetic radiation by the nuclei of certain atoms

Phosphorescence

Long-lived emission of light by atoms or molecules that sometimes occurs after they absorb light

Uranic rays

Isotope

Atoms of the same element with different numbers of neutrons

Mass number formula

Mass number = number of protons + neutrons

Isotopic notation (nuclide)

^A_ZX (where A = mass number, Z = atomic number, and X = chemical symbol)

Proton symbol

Regular symbol: p⁺; nuclear symbol: ¹₁H, ¹₁p

Neutron symbol

Regular: n⁰; nuclear: ¹₀n

Electron symbol

Regular: e^-; nuclear: ⁰_-1e

Alpha symbol

Regular: α; nuclear: ⁴₂α, ⁴₂He

Beta symbol

Regular: β, β^-; nuclear: ⁰_-1β, ⁰_-1e

Positron symbol

Regular: β, β⁺; nuclear: ⁰₊₁β, ⁰₊₁e

Alpha rays

Have a charge of +2 and mass of 4 amu (what we now know to be helium nucleus)

Beta rays

Have a charge of -1 c.u. and negligible mass (electron-like)

Gamma rays (γ)

Form of light energy (not a particle like α and β)

Positron

Charge of +1 and negligible mass (antiparticle of electron), some unstable nuclei emit positrons

Electron capture

When an inner orbital/low energy electron is pulled into the nucleus (also action of some unstable nuclei), no particle emission, but atom changes (same result as positron emission)

When a proton combines with the electron to make a neutron:

Mass number stays the same, atomic number decreases by 1

Nuclear equations

Atomic numbers and mass numbers are conserved

Alpha decay

Occurs when an unstable nucleus emits a particle composed of two protons and two neutrons, most ionizing but least penetrating

Loss of an alpha particle means:

Atomic number decreases by 2 and mass number decreases by 4

Beta decay

A neutron becomes a proton when an unstable nucleus emits an electron, about 10 times more penetrating than alpha decay, but only about half the ionizing ability

Loss of a beta particle means:

Atomic number increases by 1, mass number remains the same

Gamma emission

No loss of particles from the nucleus, no change in composition of the nucleus (same atomic # and mass #), least ionizing but most penetrating, generally occurs after the nucleus undergoes some type of decay and the remaining particles rearrange

Ionizing power

The ability of radiation to ionize other molecules and atoms

Penetrating power

The ability to penetrate through matter

Positron emission

A proton becomes a neutron when a positron is emitted from its nucleus

When an atom loses a positron:

Mass number remains the same and atomic number decreases by 1

Strong force

A very strong attractive force holding the particles in the nucleus together (lol???), acts over only very short distances

N/Z ratio

The ratio of neutrons:protons

If N/Z ratio is too high:

Neutrons are converted to protons through beta decay

If N/Z ratio is too low:

Protons are converted to neutrons via positron emission or electron capture (or alpha decay, though not as efficiently)

Valley of stability

For Z = 1-20, stable N/Z ~ 1; for Z = 20-40, stable N/Z approaches 1.25; for Z = 40-80, stable N/Z approaches 1.50; for Z > 83, there are no stable nuclei

Magic numbers

Decay series

Atoms with Z > 83 are radioactive, one radioactive nuclide changes into another radioactive nuclide, all radioactive nuclides are produced one after the other until a stable nuclide is reached

Ways of detecting radioactivity

Thermoluminescent dosimeters, Geiger-Muller counter, scintillation counter

Constant half-life

Length of time for a radionuclide required to lose half its radioactivity

Kinetics of radioactive decay

Rate of change in amount of radioactivity is constant and different for each radioactive isotope, constant half-life follows first-order kinetics, not affected by temperature

Rate

kN (N = # of radioactive nuclei)

Half-life formula

t_1/2 = 0.630/k

The shorter the half life:

the more nuclei decay every second; therefore we say the sample is hotter

Radioactive decay formula

ln(N_t/N₀) = -kt (N_t = # of radioactive nuclei at time, t; N₀ = initial # of radioactive nuclei) (can also be written as N_t = N₀e^-kt)

C-14 half-life

5730 years

Radiocarbon dating

While still living, ¹⁴C/¹²C is constant because the organism replenishes its supply of carbon. Once the organism dies, the ¹⁴C/¹²C ratio decreases. The half-life of ¹⁴C is 5715 years.

Radiometric dating (slides 55 and 56)

Nuclear fission

A large nucleus splits into two smaller nuclei via reaction with neutron

Fusion

Small/light nuclei can be accelerated to smash together to make a larger/heavier nucleus, energy source of stars, basis for hydrogen bombs, requires high energy input to initiate the reaction (need to overcome repulsion of positive nuclei)

Does fission or fusion release more energy per gram?

Fusion releases 10 times more energy per gram as fission with less problematic products

fissionable material (slide 59)

Fission chain reaction

Occurs when a reactant in the process is also a product, in the fission process it is the neutrons

Critical mass

The minimum amount of fissionable isotope needed to sustain the chain reaction

Nuclear power plant

Uses about 50 kg of fuel to generate enough electricity for 1 million people, no air pollution

Coal-burning power plant

Uses about 2 million kg of fuel to generate enough electricity for 1 million people, produces CO₂ (greenhouse gas), NO₂, and So_x (acid rain)

Problems with nuclear power

Core meltdown (water loss from core; heat melts core), waste disposal (highly radioactive waste)

Energy from fission equation

E = mc², each mole of U-235 that fissions produces about 1.7 × 10³ J of energy, a very exothermic chemical reaction produces 10⁶ J per mole

Nuclear transmutation

When atoms of one element are transformed into atoms of a different element by nuclear reaction, usually involve accelerated high-energy particles smashing into target nuclei

Artificial transmutation

Bombardment of one nucleus with another/neutrons causing new atoms to be made (can be done in a particle accelerator)

Three types of radiation effects

Acute radiation damage, increased cancer risk, and genetic effects

Acute effects of radiation

High levels of radiation over a short period of time kill a large number of cells, causes a weakened immune system and lower ability to absorb nutrients from food

Exposure

Number of decay events to which a person is exposed

Dose

Amount of energy actually absorbed by body tissue

Units of radiation exposure

1) Curie (Ci) = 3.7 × 10¹⁰ events per second no matter the kind of radiation

2) Gray (Gy) measures amount of energy absorbed by body tissue, 1 Gy = 1 J/kg body tissue

3) Rad also measures amount of energy absorbed by body tissue, 1 rad = 0.01 Gy

Relative biological effectiveness (RBE)

A correction factor used to account for a number of factors that affect the result of the exposure, result is the dose in rems (roentgen equivalent man), dose in rads x RBE = dose in rems

Roentgen

Amount of radiation that produces 2.58 × 10^-4 C of charge per kg of air