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
21Ne 10
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. 10 =Ne-20 Yet another alternative way to write isotopes: 20Ne=20Ne
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 ?30 26 36

10. A Neon isotope with 9 neutrons is written as:

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

12. Pertinent section of Periodic table
ATOMIC BOOKKEEPING
Atomic # mass# symbol #p #no #e atom charge . 12 31 15 17 O 9 10 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 20 10 7 14 -1 Fe 30 17 18 19 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 * * 13
fm = fraction of C atoms with mass M
=f12* f13*13

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

18. AV. MASS =f1*m1+f2*m2 +….. AV. MASS =P1*m1+P2*m2 +….. 100
Take home lesson
: Average mass is computable from fractional abundances fk and mk so:
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 %
10B %
Average B mass ???
AV. MASS =P1*m1+P2*m2 +…..
100
=81* *10
100
=10.81

20. estimate average atomic masses for elements below
Isotope ~mass % abundance
~ average mass 1H 2H
1.008
28Si
29Si
30Si
28.081
35Cl
37Cl
35.454