1. Atomic Symbols = = mass # atomic # protons + neutrons protons
Unit 2 Notes 2
protons + neutrons =
mass #
atomic # =
protons

2. Elements are identified by atomic number (protons).
Neutrons
Electrons 19 20 S 17
39Ca 14 6 39 K 19 19 33 16 16 16 16 20 20 19 20 20 6 6 8 C 6

3. H H H Isotopes 1 2 1 3 1 element: same or different mass:
_________:
same # of protons & electrons,
BUT different # of neutrons 1 H 2 1 H 3 1 H
protium
deuterium
tritium

4. Quick (but challenging) Quiz!
1. The number of protons in the nucleus is…
the mass number.
the atomic number.
the same as the number of neutrons.
the total of protons and neutrons.

5. Quick (but challenging) Quiz.
2. The mass number is…
the number of protons.
the number of neutrons.
the total of protons and electrons.
the total of protons and neutrons.

6. Quick (but challenging) Quiz.
3. Isotopes of an element have…
the same mass number.
different atomic numbers.
the same number of protons but different mass numbers.
the same number of protons but different numbers of neutrons.

7. Quick (but challenging) Quiz.
4. How many neutrons are in an atom of potassium–40 ? (40K)
(Atomic Number = 19)
19 neutrons
21 neutrons
40 neutrons
59 neutrons

8. Average Atomic Mass If there are 2 naturally occurring isotopes of
Neon including
Ne-20 and Ne-22.
then the average mass of Neon atoms should be ____ amu. 21
Why does the PT lists it as amu?
Calculated as a weighted average of atoms in a sample of naturally occurring isotopes.

9. 63Cu is 69% abundant 65Cu is 31% abundant Average Atomic Mass
A weighted average atomic mass is calculated from both the
masses of each isotope and the
relative abundance (%) of each isotope.
63Cu is 69% abundant
65Cu is 31% abundant
To which will the
average mass be closer,
63 or 65?

10. = 63.55 Calculating Average Atomic Mass
copper-63
has a mass of amu
and natural abundance of 69.15%
copper-65
has a mass of amu
and has a natural abundance of 30.85%
Calculating Average Atomic Mass
Avg. Mass = (Mass1)(%) + (Mass2)(%) …
= (62.93)(0.6915) + (64.93)(0.3085)
= 63.55

11. neon neon-22
Which isotope is in greater relative abundance in natural samples of neon?
because…
the weighted average atomic mass of neon from the PT is amu.
90% - 20, <1% - 21, 9% - 22

12. = 32.2 Avg. = (M1)(%) + (M2)(%) = (32)(0.900) + (34)(0.100)
Quick Quiz!
1. If an unusually selected sample of sulfur contained 90.0% sulfur-32 and % sulfur-34, what would be its average atomic mass?
32.2 amu
32.9 amu
33.5 amu
34.2 amu
Avg. = (M1)(%) + (M2)(%)
= (32)(0.900) + (34)(0.100)
= 32.2