Solar System Composition & Nuclear Stability Flashcards
How do the relative abundance of elements change with increasing mass number?
In general, element relative abundance decreases with increasing mass number.
What is the mass defect?
The difference between the measured mass of an atom (Ma) and the combined mass of its constituent particles (Mc) (protons, neutrons, electrons). DeltaM = Mc -Ma
What is the binding energy?
the energy required to separate an atom/nucleus into its constituents
What is branched decay?
What is beta decay?
why can we approximate the proportions of the elements in the solar system by looking at the relative element abundances in the sun?
because about 99.9% of the Solar System mass are stored in the Sun.
how are the suns elemental abundances measured?
by measuring the intensity of certain wavelengths of light emitted by the elements from the Sun’s photosphere
what type of meteorites have similar relative element abundances to the Sun? what benefit does this give us in terms of measuring these relative element abundances.
CI carbonaceous chondrites. since the Sun and CI carbonaceous chondrites are in strong agreement, the ability to precisely measure element abundances of CI CC allows us to put a more accurate constraint on the sun, and therefore the solar systems composition, as the measurements of the sun are at lower precision.
what element are the abundances of elements plotted relative to?
Si = 10^6 atoms
apart from the dominance of H and He, what are the five features of the relative element abundance of the solar system?
- generally, abundance of elements decreases with increasing atomic number
- prominent peak in Fe and neighbouring elements - the Fe peak elements. Iron is about 1000 times more abundant than someone of its neighbours.
- Li, Be and B abundances anomalously low
- saw-toothed pattern across whole curve - elements with even atomic numbers higher abundance than elements with odd atomic numbers (odd-even nuclear stability)
- all periodic table elements are present, except those which have no stable or long-lived radioactive isotopes (Technetium, promethium and the trans-uranic actinides)
what are the iron peak elements?
the elements (inc. Fe) around iron on relative abundance have an abundance more elevated than expected
how can the observed mass defect of an atom be used to quantify the nuclear stability of an isotope?
using E = mc^2
Due to the equivalence of mass and energy, this decrease in mass implies that energy is released in the process
which elements have the highest binding energy?
Fe and Ni
do elements with a high binding energy have high abundances or low abundances?
high binding energy = high abundances
which elements have anomalously low binding energy?
Li and Be
what are the x and y axis in the chart of nuclides?
x = neutron number, y = proton number
on the chart of nuclides, what are the isotopes?
horizontal trends for nuclei with the same number of protons but different number of neutrons
on the chart of nuclides, what are the isobars?
diagonal trends (same mass number, beta decay)
on the chart of nuclides, what are the isotones?
vertical trends for nuclei with the same number of neutrons
what is the valley of stability for nuclides?
a narrow trend within which the number of protons and neutrons an atom needs to be stable is defined.
what are the two gradients for the valley of stability on the chart of nuclides, for light elements and for heavy elements
for light elements: N ~ 1xZ
for heavy elements: N ~ 1.5xZ
i.e. heavier elements need increasingly more neutrons compared to protons to be stable
what do elements outside of the valley of stability do?
decay, either through positron or beta/negatron decay
which elements decay through Positron decay?
elements above the valley of stability (more protons than neutrons)
which elements decay through Beta/negatron decay?
elements below the valley of stability (more neutrons than protons)
what happens in Beta or negatron decay?
neutron rich nuclides. negatron releases a electron and an anti neutrino and decay energy
what happens in positron decay?
proton rich nuclides. releases a positron, neutrino and decay energy
what happens in electron capture?
proton rich nuclides react with an electron to form a neutron.
what happens in branched decay?
decay of a nuclide by two or more pathways. e.g. decay of 40K to both 40Ca and 40Ar
what happens in alpha decay?
mainly heavy nuclides with Z > 58 (Cerium), releases an alpha particle (4He)
what happens in spontaneous fission?
heaviest nuclides 232Th, 235U, 238U. the spontaneous breakup of a nuclide into two or more fairly heavy daughter nuclides.
If isobaric nuclides have the same mass number, why do they have different stabilities?
due to odd-even systematics - they have small differences in their exact atomic weights and in the energy released in the decay process.
rank odd-odd, even-even, and even-odd isobars from most to least stable
most stable
even-even
even-odd
odd-odd
least stable
why are even-even isobars lighter than odd-odd and even-odd isobars?
the smaller the mass, the more strongly the atom is bound together. even-even isobars, have a high mass defect and thus high binding energy. it will take a lot to separate them.
odd-odd isobars are not as tightly bound, they have a low mass defect and therefore a low binding energy.
how many stable isobars do even-even nuclides have?
two or more
how many stable isotopes do odd mass number nuclides have?
only one
how are differences in nuclide stability translated into differences in nuclide and element abundances?
nuclear reactions typically take place to turn energetically less stable nuclei into more stable nuclei. therefore the most stable nuclei will have the highest abundances and vice versa. e.g. E-E isotopes of tin Sn have the highest natural abundance
what are magic numbers in nuclei?
nuclei with a magic number of protons and/or neutrons have energetically favourable complete shells within the atomic nucleus, making them particularly stable.
2,8,10,20,28,50,82,126
why are 4He, 16O, 40Ca, 48Ca, 138Ba, and 208Pb so stable compared to their neighbours?
they have magic numbers of protons and/or neutrons.
