LACKING LIGHT ELEMENTS Shown here are the relative abundances of the chemical elements from hydrogen (Z=1) to germanium (Z=32). These abundances have been shown relative to that of carbon (Z=6), with the scale shifted so that the abundances are all >1 and hydrogen is at 10^12. Several patterns emerge hydrogen and helium are significantly more abundant than the other elements in the universe, the relative abundance generally declines with atomic number, and an alternation between higher abundance of even atomic numbers and lower abundance of odd atomic numbers. Perhaps most striking however is the very low abundance of the light elements between helium (Z=2) and carbon. The elements lithium (Z=3), beryllium (Z=4) and boron (Z=5), in fact boron is 10^7 times less abundant than carbon. This is the result of relative stabilities of the elements and how they are produced. With the big bang forming protons (or hydrogen ions), neutrons, and electrons, in the space of minutes, no pun intended, the majority of helium had been produced, as well as very small amounts of deuterium (a proton and a neutron for the ion and with an electron for the atom), and lithium-7 (3 protons and 4 neutrons). However the rapid expansion and cooling meant that until these early nuclides recoalesced in the stars the heavier elements were not produced in significant quantities. Our star is formed of the dust of previous stars and so contains heavier elements, despite only being in the main hydrogen to helium fusion stage. However in the later stages of large stars helium, is able to fuse into successively heavier elements until iron (Z=26) which has the highest binding energy per nucleon (protons and neutrons). Some slightly heavier elements are able to form in equilibrium with iron, but the majority of remaining heavier nuclides form by successive neutron capture and beta-decay. Beta-decay happens because the number of neutrons increases beyond the stable proton-neutron balance and neutrons decay into protons and electrons. However the relative binding energy per nucleon, shown, in the second image, for the first 24 stable nuclides after hydrogen which being a single proton doesn’t have another nucleon to bind with, illustrates the reason why lithium, beryllium, and boron are severely lacking. The very high binding energy per nucleon of helium-4 means that fusion to the next 5 stable nuclides lithium-6, lithium-7, beryllium-9, boron-10, and boron-11 would actually take in energy! These nuclides are thought to produced when high energy cosmic rays smash together large and small nuclides producing light nuclides such as Li, Be, and Bo. This process is known as spallation and is evidenced in the first graphics cosmic ray composition which has much greater proportion of these light elements than the solar(universe proxy). Note: the term ‘binding-energy’ is a little confusing and convention is it is the energy required to break the nuclide into its respective nucleons. A= atomic mass (neutrons + protons), Z = atomic number (protons). References: Trimble, V. (1975). The origin and abundance of the chemical elements. Reviews of Modern Physics 47, 977-976. Friedlander, G., Kennedy, J. W., Marcias, E. S., and Miller, J. M. (1981). Nuclear and radiochemistry. John Wiley and Sons.
Posted on: Tue, 26 Aug 2014 13:01:00 +0000
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