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Chapter 4 Structure of the Atom 4.3 How Atoms Differ.

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Presentation on theme: "Chapter 4 Structure of the Atom 4.3 How Atoms Differ."— Presentation transcript:

1 Chapter 4 Structure of the Atom 4.3 How Atoms Differ

2 Section 4.3 How Atoms Differ Explain the role of atomic number in determining the identity of an atom. Define an isotope; be able to identify and give an example of one Write an isotope in any one of the 3 standard notations for them. Explain how the atomic mass unit (amu) is defined Describe in semi-quantitative terms the relative masses of the neutron, the proton and the electron. The number of protons and the mass number define the type of atom. Objectives

3 Section 4.3 How Atoms Differ 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. The number of protons and the mass number define the type of atom. Objectives (cont)

4 Section 4.3 How Atoms Differ Key Concepts The 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 electrons mass number = atomic number + number of neutrons Atoms 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 atom The atomic mass of an element is a weighted average of the masses of all of its naturally occurring isotopes.

5 Atomic Number (AN) AN = # of protons For any neutral element # of protons = # of electrons Different ANs  different elements Lithium, Li AN = 3 Carbon, C AN = 6

6 Practice Atomic Number Practice Problems, page – 15 Chapter Assessment, page , 59, 66, 75 Appendix Suppl. problems, pp (a-f), 2&3(a-d)

7 Isotopes / Mass Number Isotopes have same number of protons but a differing number of neutrons Same # protons  Same element Mass number = sum of number of protons and neutrons in the nucleus Mass number = atomic number + number of neutrons Mass number not the same as atomic mass

8 Isotopes / Mass Number To distinguish a particular isotope from another, use element name followed by dash followed by mass number Potassium-39 Hydrogen-3 (aka tritium) Can also use chemical symbol K-39 H-3

9 Potassium, K (Figure 4.17) K AN = 19 (19 protons, 19 electrons) 19e - 19p + 20n 0 19p + 20n 0 19p + 21n 0 19p + 22n 0 K-39 K-40 K-41

10 Alternate Notation for Isotopes Mass Number Atomic Number Rather than using silver-107, silver-109 for isotopes, often preferable to use Note that numbers are to left of symbol

11 Example Problem 4.2, page 118 One of neon’s isotopes found to have atomic number 10 & mass number 22 Find # protons, electrons, neutrons Name isotope and give its symbol AN = # protons = 10 Neutral atom, # electrons = protons=10 Mass number = # protons + # neutrons # neutrons = 22 – 10 = 12 Isotope = neon-22 Ne symbol

12 Practice Atomic & mass number and isotope symbols Practice Problems, page , 17 Chapter Assessment, page – 62, 64, 67-69, Appendix Suppl. Problems, page 978 4, 5, 6(a-f)

13 Mass of Individual Atoms So far, only discussed mass number Atomic mass has definition in terms of a chosen atomic standard Carbon-12 atom assigned a mass of exactly 12 atomic mass units (amu) => One amu = 1/12 mass of 12 6 C All masses of atoms or atomic particles expressed in terms of amu

14 Masses of Subatomic Particles Protons and neutrons do not have mass of exactly 1 amu Proton (p +1 ) and neutron (n 0 ) masses slightly different Electron mass ~ 1/1840 (p +1 or n 0 ) ParticleMass (amu) Electron Proton Neutron

15 Mass Spectrum & Mass Spectrometer Q. How do you “weigh” these atoms to get their masses? A.Mass spectroscopy Charge (ionize) atom or molecule Accelerate in electric field Laws of physics predict path of ion in a known magnetic field Specific path and place where it strikes a detector depends on ion’s mass See page 125

16 Mass Spectrometer Magnetic Field Detector Heating coil to vaporize sample + ions Electric field accelerates ions Slits Least massive ions Most massive ions

17 Mass Spectrometer Launch video from misc Mass Spectroscopy (Royal Soc Chem) (7m 58s) Essentials: from start to 1:54 & from 3:25 to 4:48 Mass Spectroscopy (Royal Soc Chem)

18 Mass Spectrometer Ionization Acceleration Deflection Detection Vaporized Sample Electromagnet To vacuum pump

19 Mass Spectrum – Mercury Isotopes

20 Mass Spectrum - Mercury Natural abundance Hg-196, 0.146% Hg-198, 10.02% Hg-199, 16.84% Hg-200, 23.13% Hg-201, 13.22% Hg-202, 29.80% Hg-204, 6.85% Mass Number Relative Abundance

21 Atomic Mass - Elements Atomic mass of element is weighted average of the isotopes of that element AM(element) = AVG wt = Mass(1) x Abundance(1) + Mass(2) x Abundance(2) + … where Mass(i) = atomic mass of isotope(i)

22 Atomic Mass - Elements Atomic 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 integer b) Even if isotope masses had integer values, process of doing weighted average over isotopes generally gives result which is not an integer

23 35 17 Cl Atomic mass = amu % abundance = % Contribution to weighted avg = amu Cl Atomic mass = amu % abundance = % Contribution to weighted avg = amu AVG wt = = amu This is value listed in period table for Cl Chlorine Example – Fig 4.18, page 119

24 Example Problem 4.3, page 121 Unknown element X 6 X amu 7.59% abundance 7 X amu 92.41% abundance Calculate contributions to weighted avg amu  = amu amu  = amu Sum to find mass; Atomic mass = = amu Matches atomic mass of lithium (Li)

25 Practice At mass of from isotope abundance Practice Problems, page Section Assessment, page Chapter Assessment, page (data source?), 76 – 78 Appendix Suppl. Problems, page 978 7, 8


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