Nuclides planetory redox
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Biogeochemistry Wk 1
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45 Terms
What determines the chemical reactivity of an atom?
The arrangement of electrons
What is the ‘valley of stability’?
A region where stable nuclides lie; unstable ones are high energy and lie either side.
What are magic numbers in nuclear stability?
Nuclei with the Z and/or N of 2,8,20,28,50 and 126 are anamously stable are are magic.
What is the difference even and odd Z numbered elements?
Even Z typically have more stable isotopes than elements with odd Z
Odd Z tend to have nuclides with odd masses and even Z tend to have even masses
What happens during beta decay?
A neutron emits an electron and anti-neutrino → increasing the atmoic number by 1
What happens during positron emission?
A proton becomes a neutron, emitting a positron and a neutrino
What happens during 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 a nucleus transitions from an excited to a stable state.
Name 3 heavy isotopes with long-half lives.
232Th, 235U, 238U
What is alpha decay?
Emission of a 4He nucleus which reduces the mass by 4 and atomic number by 2
What is spontaneous fission?
A nucleus splits into two fragments without external stimulation
Why is spontaneous fission significant?
It produces neutron-rich fragments that undergo multiple beta decays
How are heavier elements formed?
Through stellar nucleosynthesis - fusion and neutron addition in stars
What is galactic chemical evolution?
The process by which elemental and isotropc compositions change over time as matter cycles through stars
Why are younger stars more metal rich?
As they contain more lements beyond H and He due to prior stellar generations
What light nuclei formed minutes after the Big Bang?
2H, 3He, 4He, 7Li
Why are nuclei with mass numbers 5 and 8 rare?
As they are highly unstable and radioactive
What is the sun’s composition?
~73% hydrogen, ~25% helium, ~2% metals
Why are Li, Be and B scarce?
They are destroyed by fusion rather than created
Why is Fe a local maximum in abundance?
It represents the peak of nuclear stability
What is the mass defect?
The difference between the mass of reactants and products, converted into binding energy
What prevents nuclei from fucing easily?
Electrostatic repulsion between positively charged nuclei
Why is fusion of H to He fusion efficient?
Low repulsion and high energy field
What happens when H is exhausted in small stars?
Core contracts → red giant forms → nebula formation
What happens when fuel is exhausted in large stars?
Core collapse → super red giant → supernova → black hole/neutron star
What process creates heavier nuclides?
Neutron addition (s-process and r-process)
What is photodissociation?
Neutron loss bia high-energy gamma irradiation
Where does the s-process occur?
In low-mass stars during hydrostatic burning
What follows neutron addition in the s-process?
Beta decay to reach stability
Where does the r-process occur?
In high-energy environments like supernoave or neutron stars
What is the r-process?
Flux of neutron addition and once the flux stopped undergoes beta decay to become stable and form neutron rich nuclides
What is the p-process?
Gamma-induced neutron loss - reverse of neutron gain
Where does the p-process occur?
In high-temperature environments beyond the Fe peak
Why is N2 abundnat in the atmosphere?
It’s chemically inert and poorly soluble so it remains afetr other gases are removed
What governs volcanic gas speciation?
Mantle redox state and oxygen fugacity
What species dominate under low fO2?
Methane (CH4)
What species dominate under high fO2?
Carbon dioxide (CO2)
What is a redox buffer?
A mineral reaction that stabilises oxygen fugacity by counteracting changes in O2
Name 3 redox buffers
IW (Iron-Wüstite),
WM (Wüstite-Magnetite),
FMQ (Fayalite-Magnetite-Quartz)
What does the IW buffer involve?
Fe + ½O₂ ⇌ FeO — sets fO₂ when both phases are present.
How does mantle composition affect IW buffering?
Activities of Fe and FeO are lower than pure phases, shifting fO₂ ~2 log units below pure IW.
What drives mantle oxidation?
Iron disproprtionation in the lower mantle
What is the thermal water trap?
A mechanism that retains water in the lower atmosphere by freezing it before it reaches UV- rich layers. Without it water would be lost to space
Why does methane escape more easily?
It rises higher, undergoes photodissociation, and releases hydrogen radicals that excape Earth’s gravity.