Chapter 2:Atoms, Elements, Isotopes,and IonsA Review of ChemistryAtoms• Atoms are composed of Protons, NeutronsProtons, NeutronsProtons, NeutronsProtons, Neutronsand ElectronsElectronsElectronsElectrons–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 negligiblerelative to protons and neutrons.Atoms and Elements•The chemical properties of an elementThe chemical properties of an elementThe chemical properties of an elementThe chemical properties of an elementdepend on the number of protons (i.e.depend on the number of protons (i.e.depend on the number of protons (i.e.depend on the number of protons (i.e.the net electric chargethe net electric chargethe net electric chargethe net electric charge) of the nucleus.) of the nucleus.) of the nucleus.) of the nucleus.•The number of protons in the nucleus isThe number of protons in the nucleus isThe number of protons in the nucleus isThe number of protons in the nucleus isknown as the known as the known as the known as the atomic numberatomic numberatomic numberatomic number of the of the of the of theelement.element.element.element.•Atomic numbers for natural elementAtomic numbers for natural elementAtomic numbers for natural elementAtomic numbers for natural elementrange from 1 (hydrogen) to 92 forrange from 1 (hydrogen) to 92 forrange from 1 (hydrogen) to 92 forrange from 1 (hydrogen) to 92 foruranium.uranium.uranium.uranium.Chemistry•The chemical reactions an elementis capable of is determined by theelectron configuration•Atoms with complete outer shellsdon’t enter chemical reactions(Inert).•The number of lost electrons (netelectric charge) is the valence.Ions and Valence•Atoms with 1,2, 3, or 4 outer electrons maylose them and form positive ions (cations).•Atoms with 6 or 7 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 ratiosto maintain charge balance•H2•H2O•SiO2•CaCO3Isotopes•The number of protons plusneutrons in the nucleus is knownas the mass numbermass numbermass numbermass number of the atom.•Atoms of a given element (atomicnumber) may have differingnumbers of neutrons.•Atoms of the same element withdifferent mass numbers areknown as isotopes.Mass Number• The mass numbers or isotopes of an elementare denoted as preceding superscripts.• For example the stable isotopes of theelement oxygen are denoted 18O, 17O and 16O.• Oxygen has an atomic number of 8 (eightprotons).• The nucleus of 16O thus contains eight protonsand eight neutrons.• How many neutrons are in the nucleus of 18O?(ans.: 10).Atomic Weight•A given element may haveseveral stable isotopes.•The average mass number of anelement is the atomic weightatomic weightatomic weightatomic weight....•This is not an integer.•The atomic weight of H is 1.008Nucleosynthesis• The elements H, He, and minor amounts ofLi were formed in the original Big Bang.• All heavier elements were formed from theprimordial H and He by nuclear fusionfusionfusionfusionreactions 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 asupernova explosion.• Our solar system formed from the remnants ofa supernova.Condensation and Accretion•Each supernova will have a uniquedistribution of stable isotopes.•The solar gas collapsed into a diskthat heated and then cooled tocondense solid particles of mineralscalled chondrules.•The chondrules accreted to formplanetesimals and the planetesimalsaccreted to form planets.Accretion andDifferentiation•The accretion process was rapid andthe early Earth melted.•The elements partitioned according totheir fluid-phase chemical affinities.–Siderophile - Metallic–Chalcophile - Covalent–Lithophile - Ionic–Atmophile - VanderWaals
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