Biogeochemistry - Nuclides planetary redox

What determines the chemical reactivity of an atom?

The arrangement of electron shells

What determines nuclide stability?

Nuclear structure governs stability; electron structure does not

What characterises stable nuclides at low atomic numbers?

They have nearly equal numbers of protons and neutrons

What compositions are favoured at higher atmoic numbers?

More neutron-rich nuclides

What is the valley of stability?

The region where stable nuclides lie, where unstable nuclides are on the higher-energy slopes on either side.

Which elements have more stable isotopes?

Elements with even proton numbers are more stable isotopes than odd

What are magic numbers?

Nuclei with proton or neutron number = 2,8,20,28,50, 126
they are anomously stable nuclei

What is a double magic number?

Where both Z and N are magic numbers

What happens in beta decay (β⁻) ?

A neutron becomes a proton, emitting an electron and an anti-neutrino

How does beta decay change the atmoic number?

Z increases by 1

What happens in positron emission (β⁺)

A proton becomes a neutron, emitting a positron and a neutrino

What happens in electron capture?

The nucleus captures an electron, converting a proton into a neutron.

Why is ²⁶Al important?

It has a long half-life and is used in early solar system chronology and exposure dating.

What are gamma rays?

hIgh-energy photons emitted when an excited nucleus returns to its ground state.

Which isotopes are considered heavy and long-lived?

²³²Th, ²³⁵U, ²³⁸U

What is emitted in alpha decay?

A 4He nucleus (mass 4 and atmoic number 2)

What does ²³⁸U ultimately decay into?

Stable ²⁰⁶Pb via multiple α and β decays.

What is spontaneous fission?

A nucleus splits into two uneven fragments without needing neutron stimulation

Why are fission products radioactive?

They lie on the neutron rich side of teh valley of stability and undergo multiple beta decays

How are all elements which are heavier than Li formed?

Via stellar nucleosynthesis

What is galactic chemical evolution?

Progressive enrichment of the galaxy in heavy elements as stars process matter over ~13Ga - Younger stars are more metal rich

What nuclei formed minutes after the Big Bang?

²H, ³He, ⁴He, and ⁷Li by protons and neutrons combining

Why are masses higher than 5 and 8 rare?

As they are highly radioactive and unstable

What is the Sun’s composition?

~73% H, 25% He, ~2% of metallicity

Why are even—Z elements more abundant?

Nuclei prefer even Z numbers as they are more stable making them more abundant

Why are Li, Be and B low in abundance?

They are destroyed by fusion in stars rather than created

What is binding energy?

Energy released when nucleons combine into a nucleus.

Why does fusion release energy up to Fe?

The binding energy per nuclean increaess up to Fe

Why does fusion stop up to Fe?

Beyond Fe as fusion requires much more energy

What prevents nuclei from fusing easily?

Electrostatic repulsion between positively charged nuclei

Where can fusion occur naturally?

In stars - extremely high temperatures

What is hydrogen burning?

Fusion of ¹H → ⁴He through a series of chain reactions.

Why do stars spend most of their lives burning H?

As these light nuclides have the lowest repulsion and yield the greatest amount of energy

What is the triple-alpha process?

⁴He + ⁴He → ⁸Be;

⁸Be + ⁴He → ¹²C;

¹²C + ⁴He → ¹⁶O.

What happens when small stars exhaust H?

Core contracts → red giant → planetary nebula → white dwarf

What burning stages occur in massive stars?

H → He → C → Ne → O → Si → Fe/Ni core

What ends a massives stars life?

Core catastrophically collapses → supernova → neutronstar / black hole

What processes create heavy nuclides?

s-process, r-process, p-process

Where does s-process occur?

Low-mass stars

What happens after neutron capture?

Sequences of neutron addition and beta decay to move nuclides back towards stability

Where doe r-process occur?

require much more energy → in supernovae and neutron star nergers

What characterises the r-process?

Extremely high neutron flux; nuclei become very neutron-rich

What is the p-process (photodissociation)?

Gamma-induced neutron loss (rverse of neutron capture)

Where does the p-processes occur?

High temperature enviornments (supernova)

What are the dominant volcanic gases emitted by the mantle?

Water and carbon dioxide

In what form is sulfur typically emitted in volcanic gases?

Sulfur is largely oxides as SO2

Why is N_2 the most abundant atmospheric gas?

It is chemically inert and poorly soluble, so it accumulates

What species dominate at low fO2?

CH4

What species dominates at higher fO2?

CO2

What is a redox buffer?

A mineral reaction that fixes/corrects oxygen fugacity by consuming or releasing O2 to maintain equilibrium.

Name the three key redox buffers?

IW (Iron-Wüstite), WM (Wüstite-Magnetite), and FMQ (Fayalite-Magnetite-Quartz)

What is the IW buffer reaction?

Fe + ½O₂ = FeO.

What does the presence of Fe +FeO do?

Fixes fO₂ at the IW value until one phase is exhausted.

Why is the FMQ more realistic for mantle comparison?

It uses mantle minerals (product is SiO)

Why are buffers useful in temrs of temp?

Their relative positions remain temperature-independant

What is the thermal water trap?

A cold region that freezes out water, preventing it from reaching the upper atmosphere

Why is the thermal trap essential?

Without it, all water would be lost to space

Why can CH4 reach the upper atmosphere?

It has a freezing point so cannot be trapped

What happens to CH4 in the upper atmosphere?

UV photodissociation → H radicals → H escape to space.

What U minerals dominate before 2.4 Ga?

Detrital UO2 (uranite)

What U deposits dominate after 2.4 Ga?

Hydrothermal U deposits

Why does O2 eventually accumulate?

Once Ch4 is destroyed too quickly to reach upper atmosphere, H escape slows and O2 builds up