University of Colorado GEOLOGY 1010 Class Note 2

Atoms and Elements, Isotopes and Ions

Atoms are composed of protons, neutrons, and electrons. A proton has an electric charge of +1 and a rest mass of 1.67 x 10-24 gm. A neutron has a charge of 0 and a rest mass of 1.67 x 10-24 gm. (about the same as a proton). An electron has a charge of -1 and a rest mass of 9.11 x 10-28 gm. (much, much less than a proton). The important point here is that the electron mass is negligible relative to protons and neutrons.

The heavy particles (protons and neutrons) are bound into the nucleus, whereas the electrons form complex orbitals about the nucleus.

The chemical properties of an element depend on the number of protons (i.e. the net electric charge) of the nucleus. The number of protons in the nucleus is known as the atomic number of the element. Atomic numbers for natural element range from 1 (hydrogen) to 92 for uranium.

The number of protons plus neutrons in the nucleus is known as the mass number of the atom. Atoms of a given element (atomic number) may have differing numbers of neutrons. Atoms of the same element with different mass numbers are known as isotopes. The mass numbers or isotopes of an element are denoted as preceding superscripts. For example the stable isotopes of the element oxygen are denoted 18O, 17O and 16O. Oxygen has an atomic number of 8 (eight protons). The nucleus of 16O thus contains eight protons and eight neutrons. How many neutrons are in the nucleus of 18O? (ans.: 10). Because elements may have several stable isotopes, the average mass number of an element is the atomic weight and is commonly not an integer.

Atoms may not change their atomic numbers or mass numbers except by very energetic nuclear reactions. However atoms may gain or lose electrons in ordinary chemical reactions. If an atom has the same number of electrons as protons, it is a neutral atom. If it has a net charge, (more or less electrons than protons) it is an ion. If it has more electrons than protons it has a net negative charge and is known as an anion. If it has fewer electrons than protons it has a net positive charge and is known as a cation. The ionic state may be denoted as a following superscript (e.g. O2-, Fe2+). The common ionic states of a atom are known as its valences.

Because electrons arrange themselves in discreet orbitals about the nucleus and the orbitals repeat in shells, the chemical properties of the elements tend to repeat as the atomic number increases. This periodicity of properties gives rise to the periodic table of the elements.

The elements H, He, and minor amounts of Li were formed in the original Big Bang. All heavier elements were formed form the primordial H and He by nuclear fusion reactions in stars. The fusion reaction proceeds in steps in stars massive enough to undergo the full sequence. (Our sun is not massive enough to form elements more massive than He by direct fusion and will die when all the H is consumed.) First H is consumed to form He. When the H is consumed, the star collapses until He is "ignited" to form Be and C. There are many free neutrons in these reactors and nuclei will capture enough neutrons to stabilize themselves. Most of the heavy elements are formed by neutron capture rather than by direct fusion. In the last stage Fe is formed by direct fusion of Si and other light elements. This reactions is rapid and results in an explosion. Our solar system condensed from the remnants of one of these supernova explosions.

Chemical bonds may be either ionic, metallic, covalent or vander Waals (mirror charge), and the bond type preferred by the various elements will determine their geochemical affinity. Ionically bonded elements are termed lithophile and combine with the most abundant element, O, and are enriched in the silicate and oxide minerals (rocks). Metallically bonded elements are termed siderophile and combine with native iron and are enriched in the core. Covalently bonded elements are termed chalcophile and combine with sulfur and are enriched in ore minerals. The atmophile elements form only very weak vander Waals bonds and did not condense in the inner solar system. They are depleted in Earth and enriched in the outer planets.

Here is a full Periodic Table of the Elements. Geological Periodic Table

GEOL 1010 Syllabus

GEOL 1010 Class Note 3

GEOL 1010 Class Note 1

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