Chapter 2:Atoms, Elements, Isotopes, and IonsA Review of ChemistryAtoms• Atoms are composed of Protons, Neutronsand Electrons–A proton has an electric charge of +1 and a restmass of 1.67 x 10-24 gm.–A neutron has a charge of 0 and a rest mass of1.67 x 10-24 gm. (about the same as a proton).–An electron has a charge of -1 and a rest mass of9.11 x 10-28 gm. (much, much less than a proton).• The electron mass is negligible relative toprotons and neutrons.Atoms and Elements• The chemical properties of an element dependon the number of protons (i.e. the net electriccharge) of the nucleus.• The number of protons in the nucleus is knownas the atomic number of the element.• Atomic numbers for natural element range from1 (hydrogen) to 92 for uranium.Chemistry• The chemical reactions an element is capableof is determined by the electron configuration• Atoms with complete outer shells don’t enterchemical reactions (Inert).• The number of lost electrons (net electriccharge) is the valence.Ions and Valence• Atoms with 1,2, 3, or 4 outer electrons maylose them and for positive ions (cations).• Atoms with 7 or 8 outer electrons may gainelectrons to form negative ions (anions).• The number of lost electrons is the valence• The elements are arranged by chemistryinto the Periodic Table.Ions and ValenceCharge Denoted by Superscript• Cations•H+1•Na+1•Mg+2•Al+3•Si+4• Anions•F-1•O-2•S-2Chemical Compounds• Elements occur in integer ratios to maintaincharge balance•H2•H2O•SiO2•CaCO3Isotopes• The number of protons plus neutrons in thenucleus is known as the mass number of theatom.• Atoms of a given element (atomic number)may have differing numbers of neutrons.• Atoms of the same element with different massnumbers are known as isotopes.Mass Number• The mass numbers or isotopes of an element aredenoted as preceding superscripts.• For example the stable isotopes of the elementoxygen are denoted 18O, 17O and 16O.• Oxygen has an atomic number of 8 (eight protons).• The nucleus of 16O thus contains eight protons andeight neutrons.• How many neutrons are in the nucleus of 18O?(ans.: 10).Atomic Weight• A given element may have several stableisotopes.• The average mass number of an element isthe atomic weight.• This is not an integer.Nucleosynthesis• The elements H, He, and minor amounts ofLi were formed in the original Big Bang.• All heavier elements were formed form theprimordial H and He by nuclear fusionreactions in stars.• The fusion reaction proceeds in steps instars massive enough to undergo the fullsequence.Nucleosynthesis• Large stars undergo successive fusionreactions until Fe is formed by direct fusion.• Heavier elements are formed by neutroncapture.• The final fusion stage results in a supernovaexplosion.• Our solar system formed from the remnantsof a supernova.Condensation and Accretion• Each supernova will have a unique distributionof stable isotopes.• The solar gas collapsed into a disk that heatedand then cooled to condense solid particles ofminerals called chondrules.• The chondrules accreted to form planetesimalsand the planetesimals accreted to form planets.Accretion andDifferentiation• The accretion process was rapid and theearly Earth melted.• The elements partitioned according to theirfluid-phase chemical affinities.– Siderophile - Metallic– Chalcophile - Covalent– Lithophile - Ionic– Atmophile - VanderWaals
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