04_01_88 - THE ELEMENTS C R Hammond One of the most...

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Unformatted text preview: THE ELEMENTS C. R. Hammond One of the most striking facts about the elements is their un- equal distribution and occurrence in nature. Present knowledge of the chemical composition of the universe, obtained from the study of the spectra of stars and nebulae, indicates that hydrogen is by far the most abundant element and may account for more than 90% of the atoms or about 75% of the mass of the universe. Helium atoms make up most of the remainder. All of the other elements together contribute only slightly to the total mass. The chemical composition of the universe is undergoing contin- uous change. Hydrogen is being converted into helium, and helium is being changed into heavier elements. As time goes on, the ratio of heavier elements increases relative to hydrogen. Presumably, the process is not reversible. Burbidge, Burbidge, Fowler, and Hoyle, and more recently, Peebles, Penzias, and others have studied the synthesis of elements in stars. To explain all of the features of the nuclear abundance curve — obtained by studies of the composition of the earth, me- teorites, stars, etc. — it is necessary to postulate that the elements were originally formed by at least eight different processes: (1) hy- drogen burning, (2) helium burning, (3) χ process, (4) e process, (5) s process, (6) r process, (7) p process, and (8) the X process. The X process is thought to account for the existence of light nuclei such as D, Li, Be, and B. Common metals such as Fe, Cr, Ni, Cu, Ti, Zn, etc. were likely produced early in the history of our galaxy. It is also probable that most of the heavy elements on Earth and elsewhere in the universe were originally formed in supernovae, or in the hot interior of stars. Studies of the solar spectrum have led to the identification of 67 elements in the sun’s atmosphere; however, all elements cannot be identified with the same degree of certainty. Other elements may be present in the sun, although they have not yet been detected spectroscopically. The element helium was discovered on the sun before it was found on Earth. Some elements such as scandium are relatively more plentiful in the sun and stars than here on Earth. Minerals in lunar rocks brought back from the moon on the Apollo missions consist predominantly of plagioclase {(Ca,Na)(Al,Si)O 4 O 8 } and pyroxene {(Ca,Mg,Fe) 2 Si 2 O 6 } — two minerals common in terrestrial volcanic rock. No new elements have been found on the moon that cannot be accounted for on Earth; however, three minerals, armalcolite {(Fe,Mg)Ti 2 O 5 }, pyrox- ferroite {CaFe 6 (SiO 3 ) 7 }, and tranquillityite {Fe 8 (Zr,Y)Ti 3 Si 3 O 2 }, are new. The oldest known terrestrial rocks are about 4 billion years old. One rock, known as the “Genesis Rock,” brought back from the Apollo 15 Mission, is about 4.15 billion years old. This is only about one-half billion years younger than the supposed age of the moon and solar system. Lunar rocks appear to be relatively en- riched in refractory elements such as chromium, titanium, zirco-...
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This note was uploaded on 02/04/2011 for the course CHEM 101 taught by Professor Dr.n during the Spring '10 term at McMaster University.

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04_01_88 - THE ELEMENTS C R Hammond One of the most...

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