Chapter 2 Atoms Elements Isotopes and Ions A Review of Chemistry Atoms 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 electron mass is negligible relative to protons and neutrons Atoms and Elements The chemical properties of an element depend on the number of protons i e charge of the nucleus the net electric charge 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 Chemistry The chemical reactions an element is capable of is determined by the electron configuration Atoms with complete outer shells don t enter chemical reactions Inert The number of lost electrons net electric charge is the valence Ions and Valence Atoms with 1 2 3 or 4 outer electrons may lose them and form positive ions cations Atoms with 6 or 7 outer electrons may gain electrons to form negative ions anions The number of lost electrons is the valence The elements are arranged by chemistry into the Periodic Table Ions and Valence Charge Denoted by Superscript Cations H 1 Na 1 Mg 2 Al 3 Si 4 Anions F 1 O 2 S 2 Chemical Compounds Elements occur in integer ratios to maintain charge balance H2 H2O SiO2 CaCO3 Isotopes 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 Mass Number 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 Atomic Weight A given element may have several stable isotopes The average mass number of an element is the atomic weight weight This is not an integer The atomic weight of H is 1 008 Nucleosynthesis The elements H He and minor amounts of Li were formed in the original Big Bang All heavier elements were formed from 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 Nucleosynthesis Large stars undergo successive fusion reactions until Fe is formed by direct fusion Heavier elements are formed by neutron capture The final fusion stage results in a supernova explosion Our solar system formed from the remnants of a supernova Condensation and Accretion Each supernova will have a unique distribution of stable isotopes The solar gas collapsed into a disk that heated and then cooled to condense solid particles of minerals called chondrules The chondrules accreted to form planetesimals and the planetesimals accreted to form planets Accretion and Differentiation The accretion process was rapid and the early Earth melted The elements partitioned according to their fluid phase chemical affinities Siderophile Metallic Chalcophile Covalent Lithophile Ionic Atmophile VanderWaals Inert
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