Presentation on theme: "Chapter 4 Structure of the Atom"— Presentation transcript:
1Chapter 4 Structure of the Atom 4.3 How Atoms Differ
2Section 4.3 How Atoms Differ The number of protons and the mass number define the type of atom.ObjectivesExplain the role of atomic number in determining the identity of an atom.Define an isotope; be able to identify and give an example of oneWrite an isotope in any one of the 3 standard notations for them.Explain how the atomic mass unit (amu) is definedDescribe in semi-quantitative terms the relative masses of the neutron, the proton and the electron.
3Section 4.3 How Atoms Differ The number of protons and the mass number define the type of atom.Objectives (cont)Explain why atomic masses are not whole numbers.Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.Calculate the atomic mass of an element given the isotope masses and abundances (and variations of this problem).Explain what a mass spectrum is, identify the instrument used to obtain it and describe the basic ideas behind its operation.
4Section 4.3 How Atoms Differ Key ConceptsThe atomic number of an atom is given by its number of protons. The mass number of an atom is the sum of its neutrons and protons.atomic number = number of protons = number of electronsmass number = atomic number + number of neutronsAtoms of the same element with different numbers of neutrons are called isotopes.The atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atomThe atomic mass of an element is a weighted average of the masses of all of its naturally occurring isotopes.
5Atomic Number (AN) AN = # of protons For any neutral element # of protons = # of electronsDifferent ANs different elementsLithium, Li AN = 3Carbon, C AN = 6
7Isotopes / Mass NumberIsotopes have same number of protons but a differing number of neutronsSame # protons Same elementMass number = sum of number of protons and neutrons in the nucleusMass number = atomic number + number of neutronsMass number not the same as atomic mass
8Isotopes / Mass NumberTo distinguish a particular isotope from another, use element name followed by dash followed by mass numberPotassium-39Hydrogen-3 (aka tritium)Can also use chemical symbolK-39H-3
9K AN = 19 (19 protons, 19 electrons) Potassium, K (Figure 4.17)K AN = 19 (19 protons, 19 electrons)19e-19p+20n021n022n0K K K-41
10Alternate Notation for Isotopes Rather than using silver-107, silver-109 for isotopes, often preferable to useMass NumberAtomic NumberNote that numbers are to left of symbol
11Ne Example Problem 4.2, page 118 One of neon’s isotopes found to have atomic number 10 & mass number 22Find # protons, electrons, neutronsName isotope and give its symbolAN = # protons = 10Neutral atom, # electrons = protons=10Mass number = # protons + # neutrons# neutrons = 22 – 10 = 12Isotope = neon-22Ne2210symbol
12Practice Atomic & mass number and isotope symbols Practice Problems, page 11816, 17Chapter Assessment, page 128-960 – 62, 64, 67-69, 72-74Appendix Suppl. Problems, page 9784, 5, 6(a-f)
13Mass of Individual Atoms So far, only discussed mass numberAtomic mass has definition in terms of a chosen atomic standardCarbon-12 atom assigned a mass of exactly 12 atomic mass units (amu)=> One amu = 1/12 mass of 126CAll masses of atoms or atomic particles expressed in terms of amu
14Masses of Subatomic Particles Protons and neutrons do not have mass of exactly 1 amuProton (p+1) and neutron (n0) masses slightly differentElectron mass ~ 1/1840 (p+1 or n0)ParticleMass (amu)ElectronProtonNeutron
15Mass Spectrum & Mass Spectrometer Q. How do you “weigh” these atoms to get their masses?Mass spectroscopyCharge (ionize) atom or moleculeAccelerate in electric fieldLaws of physics predict path of ion in a known magnetic fieldSpecific path and place where it strikes a detector depends on ion’s massSee page 125
16Mass Spectrometer Detector Electric field accelerates ions Least massive ions+ ionsMost massive ionsSlitsMagnetic FieldHeating coil to vaporize sample
17Mass Spectrometer Launch video from misc Mass Spectroscopy (Royal Soc Chem) (7m 58s)Essentials: from start to 1:54 & from 3:25 to 4:48Mass Spectroscopy (Royal Soc Chem)
18Mass Spectrometer Acceleration Ionization Electromagnet To vacuum pumpVaporized SampleDeflectionDetection
21Atomic Mass - ElementsAtomic mass of element is weighted average of the isotopes of that elementAM(element) =AVGwt = Mass(1) x Abundance(1) +Mass(2) x Abundance(2) +…where Mass(i) = atomic mass of isotope(i)
22Atomic Mass - ElementsAtomic masses don’t have integer values because:a) Protons and neutrons have masses close to but not exactly 1 amu, so mass of a given isotope not integerb) Even if isotope masses had integer values, process of doing weighted average over isotopes generally gives result which is not an integer
23Chlorine Example – Fig 4.18, page 119 3517Cl Atomic mass = amu% abundance = %Contribution to weighted avg = amu3717Cl Atomic mass = amu% abundance = %Contribution to weighted avg = amuAVGwt = = amuThis is value listed in period table for Cl
24Example Problem 4.3, page 121 Unknown element X 6X amu % abundance7X amu % abundanceCalculate contributions to weighted avg6.015 amu = amu7.016 amu = amuSum to find mass; Atomic mass == amuMatches atomic mass of lithium (Li)
25Practice At mass of from isotope abundance Practice Problems, page 121 Section Assessment, page 121Chapter Assessment, page 12971 (data source?), 76 – 78Appendix Suppl. Problems, page 9787, 8