Solar System Composition & Nuclear Stability

Question 1

How do the relative abundance of elements change with increasing mass number?

Answer 1

In general, element relative abundance decreases with increasing mass number.

Question 2

What is the mass defect?

Answer 2

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

Question 3

What is the binding energy?

Answer 3

the energy required to separate an atom/nucleus into its constituents

Question 4

What is branched decay?

Question 5

What is beta decay?

Question 6

why can we approximate the proportions of the elements in the solar system by looking at the relative element abundances in the sun?

Answer 6

because about 99.9% of the Solar System mass are stored in the Sun.

Question 7

how are the suns elemental abundances measured?

Answer 7

by measuring the intensity of certain wavelengths of light emitted by the elements from the Sun’s photosphere

Question 8

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.

Answer 8

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.

Question 9

what element are the abundances of elements plotted relative to?

Answer 9

Si = 10^6 atoms

Question 10

apart from the dominance of H and He, what are the five features of the relative element abundance of the solar system?

Answer 10

1. generally, abundance of elements decreases with increasing atomic number
2. prominent peak in Fe and neighbouring elements - the Fe peak elements. Iron is about 1000 times more abundant than someone of its neighbours.
3. Li, Be and B abundances anomalously low
4. saw-toothed pattern across whole curve - elements with even atomic numbers higher abundance than elements with odd atomic numbers (odd-even nuclear stability)
5. all periodic table elements are present, except those which have no stable or long-lived radioactive isotopes (Technetium, promethium and the trans-uranic actinides)

Question 11

what are the iron peak elements?

Answer 11

the elements (inc. Fe) around iron on relative abundance have an abundance more elevated than expected

Question 12

how can the observed mass defect of an atom be used to quantify the nuclear stability of an isotope?

Answer 12

using E = mc^2
Due to the equivalence of mass and energy, this decrease in mass implies that energy is released in the process

Question 13

which elements have the highest binding energy?

Answer 13

Fe and Ni

Question 14

do elements with a high binding energy have high abundances or low abundances?

Answer 14

high binding energy = high abundances

Question 15

which elements have anomalously low binding energy?

Answer 15

Li and Be

Question 16

what are the x and y axis in the chart of nuclides?

Answer 16

x = neutron number, y = proton number

Question 17

on the chart of nuclides, what are the isotopes?

Answer 17

horizontal trends for nuclei with the same number of protons but different number of neutrons

Question 18

on the chart of nuclides, what are the isobars?

Answer 18

diagonal trends (same mass number, beta decay)

Question 19

on the chart of nuclides, what are the isotones?

Answer 19

vertical trends for nuclei with the same number of neutrons

Question 20

what is the valley of stability for nuclides?

Answer 20

a narrow trend within which the number of protons and neutrons an atom needs to be stable is defined.

Question 21

what are the two gradients for the valley of stability on the chart of nuclides, for light elements and for heavy elements

Answer 21

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

Question 22

what do elements outside of the valley of stability do?

Answer 22

decay, either through positron or beta/negatron decay

Question 23

which elements decay through Positron decay?

Answer 23

elements above the valley of stability (more protons than neutrons)

Question 24

which elements decay through Beta/negatron decay?

Answer 24

elements below the valley of stability (more neutrons than protons)

Question 25

what happens in Beta or negatron decay?

Answer 25

neutron rich nuclides. negatron releases a electron and an anti neutrino and decay energy

Question 26

what happens in positron decay?

Answer 26

proton rich nuclides. releases a positron, neutrino and decay energy

Question 27

what happens in electron capture?

Answer 27

proton rich nuclides react with an electron to form a neutron.

Question 28

what happens in branched decay?

Answer 28

decay of a nuclide by two or more pathways. e.g. decay of 40K to both 40Ca and 40Ar

Question 29

what happens in alpha decay?

Answer 29

mainly heavy nuclides with Z > 58 (Cerium), releases an alpha particle (4He)

Question 30

what happens in spontaneous fission?

Answer 30

heaviest nuclides 232Th, 235U, 238U. the spontaneous breakup of a nuclide into two or more fairly heavy daughter nuclides.

Question 31

If isobaric nuclides have the same mass number, why do they have different stabilities?

Answer 31

due to odd-even systematics - they have small differences in their exact atomic weights and in the energy released in the decay process.

Question 32

rank odd-odd, even-even, and even-odd isobars from most to least stable

Answer 32

most stable
even-even
even-odd
odd-odd
least stable

Question 33

why are even-even isobars lighter than odd-odd and even-odd isobars?

Answer 33

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.

Question 34

how many stable isobars do even-even nuclides have?

Answer 34

two or more

Question 35

how many stable isotopes do odd mass number nuclides have?

Answer 35

only one

Question 36

how are differences in nuclide stability translated into differences in nuclide and element abundances?

Answer 36

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

Question 37

what are magic numbers in nuclei?

Answer 37

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

Question 38

why are 4He, 16O, 40Ca, 48Ca, 138Ba, and 208Pb so stable compared to their neighbours?

Answer 38

they have magic numbers of protons and/or neutrons.