1. Workshop: Isotopes and Average Atomic Mass
Workshop: Isotopes and Average Atomic Mass

2. A quick review of the atom
A quick review of the atom
If you change the number of…
It will affect the…
And the result will be a…
Protons
Electrons
Neutrons
completely different atom
Identity of the atom
Charge
Ion
Mass
Isotope

3. What Are Isotopes?
They are atoms of the same element that have different numbers of neutrons.

4. There are 3 different “versions” of a hydrogen atom
An Example
Hydrogen has 3 isotopes
There are 3 different “versions” of a hydrogen atom
Note how the number of
protons never change!!!

5. There are 3 different “versions” of a hydrogen atom
An Example
Hydrogen has 3 isotopes
There are 3 different “versions” of a hydrogen atom
Having more neutrons
increases the mass

6. There are 3 different “versions” of a hydrogen atom
An Example
Hydrogen has 3 isotopes
There are 3 different “versions” of a hydrogen atom
Mass number = 1
Mass number = 3
Mass number = 2

7. Why does The atomic mass on the periodic table have so many decimal Places? 6 C 13 Al
26.982 10 Ne
20.180

8. Atomic mass 1 H
This atomic mass is the one number that best represents the mass of all three versions of hydrogen.

9. No atom of hydrogen anywhere in the universe actually has this mass.
Atomic mass 1 H
No atom of hydrogen anywhere in the universe actually has this mass.

10. Atomic mass 1 H
This number is a “weighted” average of the masses of every isotope of hydrogen

11. Calculating a “weighted” average
Calculating a “weighted” average
1. First, you have to know the mass of each isotope
2. Second, you have to know the percent abundance* of each isotope
*see next slide

12. Percent abundance?
Percent abundance is how often that particular isotope occurs in a sample of the element
A sample of magnesium collected anywhere in the universe will contain three isotopes of magnesium.
Sample of Magnesium

13. Percent abundance?
These are the percent abundances of each isotope of magnesium
78.90% of this magnesium will be the isotope named “magnesium 24”
10.00% of this magnesium will be the isotope named “magnesium 25”
Sample of Magnesium
11.10% of this magnesium will be the isotope named “magnesium 26”

14. Percent abundance?
This data will be provided to you in problems or you will be given a table where you can look it up.
78.90% of this magnesium will be the isotope named “magnesium 24”
10.00% of this magnesium will be the isotope named “magnesium 25”
Sample of Magnesium
11.10% of this magnesium will be the isotope named “magnesium 26”

15. Calculating a “weighted” average
Calculating a “weighted” average
The formula (Mass of isotope #1)(percent abundance) 100 (Mass of isotope #2)(percent abundance) (Mass of isotope #3)(percent abundance)

16. Calculate the atomic mass of magnesium From the given data
# protons
# neutrons
Mass (amu’s)
% abundance
Isotope 1
(magnesium-24) 12
78.90%
Isotope 2
(magnesium-25) 13
10.00%
Isotope 3
(magnesium-26) 14
11.10%

17. Calculating the Atomic mass of magnesium
Using the formula for calculating a weighted average
(.7890)
(.1000)
(.1110)
amu’s
Since its a “weighted”
average this number
has more influence since
this mass occurs 79% of
the time
It’s NOT just a straight
average where you add
the masses and divide
by 3
Note how the %
abundances were
converted to decimals
(just divide each by 100)

18. Try another example…

19. Calculate the Atomic Mass of Chromium
Calculate the Atomic Mass of Chromium
Isotope
Mass
% abundance #1
4.35 #2
83.79 #3
9.50 #4
2.36

20. Calculate the atomic mass of chromium
Calculate the atomic mass of chromium
(0.0435) (0.8379) (0.0950) (0.0236) amu’s

21. Terms associated with isotopes
Vocabulary review
Terms associated with isotopes

22. Atomic number = the number of protons
Vocabulary review
Atomic number = the number of protons
It’s the same as the element’s position on the periodic table
For example, carbon is the 6th element on the table, so its atomic number is 6 which means it has 6 protons (and 6 electrons if it’s a neutral atom)

23. Vocabulary review
Atomic mass = the “weighted” average mass of all the different isotopes of an atom
This is the “weird” number on the periodic table that usually has a bunch of places past the decimal point

24. Mass number = the sum of protons + neutrons
Vocabulary review
Mass number = the sum of protons + neutrons
This one kind of makes sense because the electrons essentially don’t have any mass
This HAS to be a whole number because you can’t have part of a proton or neutron
Mass number is NOT given on the periodic table

25. Let’s Practice Those Terms…
What is the mass number of a carbon atom that has 6 neutrons? 7 neutrons?
How many neutrons does a nitrogen atom have if its mass number is 14?
How many neutrons does a chlorine atom have if its mass number is 36?
What is the atomic number of neon?
What is the atomic mass of neon?

26. Answers
12 and 13 7 19 10
amu’s

27. AXZ Notation for Isotopes
AXZ Notation for Isotopes
A shorthand way to write isotopes

28. A Short Way to Write Isotopes
A Short Way to Write Isotopes
“A” is the symbol
for Mass Number
(#protons + #neutrons)
AXZ
“Z” is the
symbol for
Atomic Number
(# of protons)
“X” will be the
symbol of the element

29. Jay - Z Yo! If you’re doin’ chem problems I feel bad for you son….
I got 99 problems, and here’s the first one….HIT ME!!!!

30. Write aXz notation for an isotope that has 17 protons and 18 neutrons.
Problem #1
Write aXz notation for an isotope that has 17 protons and 18 neutrons.

31. Write aXz notation for an isotope that has 17 protons and 18 neutrons.
Problem #1
Write aXz notation for an isotope that has 17 protons and 18 neutrons.
35Cl17

32. Write aXz notation for an isotope that has 19 protons and 21 neutrons.
Problem #2
Write aXz notation for an isotope that has 19 protons and 21 neutrons.

33. Write aXz notation for an isotope that has 19 protons and 21 neutrons.
Problem #2
Write aXz notation for an isotope that has 19 protons and 21 neutrons.
40K19

34. Problem #3
Write aXz notation for an isotope that has 15 protons, 18 electrons, and 16 neutrons.

35. Problem #3
Write aXz notation for an isotope that has 15 protons, 18 electrons, and 16 neutrons.
31 P3- 15
This isotope is
also an ion!!!

36. How many protons, electrons, and neutrons are in this isotope?
Problem #4
How many protons, electrons, and neutrons are in this isotope?
41 Ca2+ 20

37. Problem #4
How many protons, electrons, and neutrons are in this isotope?
41 Ca2+ 20
20 protons, 18 electrons, 21 neutrons

38. How many protons, electrons, and neutrons?
More examples
How many protons, electrons, and neutrons?
56Fe2+26
27Al3+13
79Se2-34

39. How many protons, electrons, and neutrons?
More examples
How many protons, electrons, and neutrons?
56Fe , 24, 30
27Al , 10, 14
79Se , 36, 45

40. More practice Write aXz notation for:
Isotope with 21 protons, 18 electrons, and 24 neutrons
Isotope with 53 protons, 74 neutrons, and 54 electrons

41. 45Sc3+21 127I-53 More practice Write aXz notation for:
Isotope with 21 protons, 18 electrons, and 24 neutrons
45Sc3+21
Isotope with 53 protons, 74 neutrons, and 54 electrons
127I-53

42. Different Forms of AXZ 35Cl17 is the same as 3517Cl
Might also see “Cl-35” or “chlorine-35”