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Homework 4 - due Wednesday 14 Feb Homework 5 posted-due Wednesday 21 Feb

Learn about the modern definition of an element, the role of protons and neutrons, and how to represent isotopes. Practice writing isotopes and deducing atomic parts and identities. Compute average atomic masses using fractional and percentage abundances.

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Homework 4 - due Wednesday 14 Feb Homework 5 posted-due Wednesday 21 Feb

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  1. Homework 4 - due Wednesday 14 Feb Homework 5 posted-due Wednesday 21 Feb

  2. Modern View of Elements (p. 47) • . All atoms of a given element have the same proton count. • Atoms of different elements have different proton counts. • Modern twist: a given element can have different neutron counts in the nucleus (isotopes) and still be the same chemically=> same element can differ in weight !

  3. How the entries on the Modern Periodic Table reflect the `Modern’ definition of an element proton count Average mass of protons and neutrons

  4. Representing Elements with Atomic symbols p = atomic number (Z) defines element p + n= mass number=M (several choices for an element) p = e in neutral atom Isotope = element with specific count of n M Z p How to represent a specific isotope (p. 48)

  5. U-Do-IT for Neon Isotopes Isotope symbol ? Proton count (p+) neutron count (no) p+ + no 20Ne 10 10 10 20 21Ne 10 10 1121 10 1222 22Ne 10 Alternative symbol (drop p…) 21 Ne

  6. In-Class Practice Writing more Isotopes (use Periodic tables here) 1 p + 0 n 1 p + 2 n 5 p + 5 n 17 p + 18 n 29 p + 34 n

  7. Yet another alternative way to write isotopes: 20Ne=20Ne 10 =Ne-20 By definition, the proton count for Ne is 10 so it doesn’t have to be shown explicitly.

  8. The isotope species: 13C has:6 • 13 protons and 6 neutrons • 13 neutrons and 6 protons • 7 protons and 6 neutrons • 6 protons and 7 neutrons

  9. Fe-56 has how many neutrons ? • 56 • 30 • 26 • 36

  10. A Neon isotope with 9 neutrons is written as: • 1910Ne • 109Ne • 910Ne • 1019Ne

  11. Deducing atomic parts and identities: A quick board primer

  12. ATOMIC BOOKKEEPING Atomic # mass# symbol #p+ #no#e-atom charge 12 24 Mg 12 15 15 P 16 8 8 -2 Pertinent section of Periodic table

  13. ATOMIC BOOKKEEPING (cont.) Brain toss variant…. Atomic # mass# symbol #p+ #no#e-atom charge 10 10 Ne 10 N 7 7 8 26 26 26 56 35 17 -2 Cl Let’s go down a column left to right…. 1 mole buck/right answer with explanation

  14. Isotopic C (12C) Chemist’s C 

  15. Why the chemist’s C lists 12.01 and not 12 Imagine `fishing’ out 100 atoms of Carbon from a sample of graphite (pure carbon). What would you catch ? # p # n mass # caught out of 100 C atoms 6 6 Both kinds isotopes of C act exactly the same, chemically so chemists just average the masses 12 99 6 7 13 1 99*12 + 1*13 100 = 12.01 Average mass of each C=

  16. Averages Written As Sums Of Fractional Contributions 99*12 + 1*13 100 Average mass = of each C = 12.01 =99 *12 + 1 * 13 100 100 fm = fraction of C atoms with mass M =f12*12 + f13*13

  17. Averages written as sums of % contributions Average mass of each C =99 *12 + 1 * 13 100 100 =(99 % *12 + 1% * 13) 100 Pm = % of C atoms with mass M = (P12 *12 + P13 *13) 100

  18. Take home lesson : Average mass is computable from fractional abundances fkand mkso: AV. MASS =f1*m1+f2*m2 +….. Or, from % abundances Pk and mk so: AV. MASS =P1*m1+P2*m2 +….. 100

  19. `U-Do-it’ Example: Boron Isotope P= % abundance 11B 81% 10B 19% Average B mass ??? AV. MASS =P1*m1+P2*m2 +….. 100 =81*11 + 19*10 100 =10.81

  20. estimate average atomic masses for elements below Isotope ~mass % abundance ~ average mass 1H 1.008 99.985 2H 2 .014 0.015 1.008 28Si 27.98 92.2 29Si 28.98 4.8 30Si 29.97 3.0 28.081 35Cl 34.97 75.8 37Cl 36.97 24.2 35.454

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