Isotopes are atoms of the same element that have different numbers of neutrons. Atoms of the same element can have different mass numbers depending on their number of neutrons. The atomic mass listed on the periodic table is an average that takes into account the relative abundances of each isotope of that element. Atomic mass calculations involve multiplying the mass and abundance percentage of each isotope and adding them together.

1. Isotopes

2. Isotopes
Na
23
11
sodium-23
Mass number
Atomic number

3. Isotopes
Isotopes: Atoms with the same number of protons,
but different numbers of neutrons.
Isotopes are atoms of the same element (same
atomic number) with different mass numbers
Isotopes of chlorine
35Cl 37Cl
17 17
chlorine 35 chlorine 37

4. Isotopes
Example: Carbon isotopes
12 13 14
C C C
6 6 6
98.9% 1.1% >0.1%

5. Isotopes
Atomic mass
-Listed on the periodic table
-Gives the mass of “average” atom of each element
compared to 12C
-Average atomic mass based on all the isotopes and
their abundance %
-Atomic mass is not a whole number…it’s a weighted
average
Na
22.99

6. Isotopes
Atomic mass calculations:
Gallium is a metallic element found in small lasers
used in compact disc players. In a sample of
gallium, there is 60.2% of gallium-69 (68.9 u)
atoms and 39.8% of gallium-71 (70.9 u) atoms.
What is the atomic mass of gallium?
Mass1 x Abundance1 + Mass2 x Abundance2 = Average atomic mass
69.7u = Average atomic mass
68.9u x 60.2% + 70.9u x 39.8% = Average atomic mass

7. Isotopes
Atomic mass calculations analogy:
A chemistry student’s grade is weighted. Tests
are worth 50.%, labs are 25%, and homework is
worth 25%. A student's test average is 85.0%, lab
average is 77.0%, and homework is 91.0%.
What is the student’s average?

8. Isotopes
Atomic mass calculations:
A sample of boron consists of 10B (mass 10.0 u) and
11B (mass 11.0 u). If the average atomic mass of B
is 10.8 u, what is the % abundance of each boron
isotope?

9. Isotopes
A sample of boron consists of 10B (mass 10.0 u) and 11B (mass 11.0 u). If the average
atomic mass of B is 10.8 u, what is the % abundance of each boron isotope?
Assign X and Y values:
X = % 10B Y = % 11B
Determine Y in terms of X
X + Y = 1 therefore Y = 1 - X
Solve for X:
X (10.0) + (1 - X )(11.0) = 10.8
10.0X + 11.0 – 11.0X = 10.8
10.0X – 11.0X = 10.8 –11.0
- 1.0X = - 0.2
X = 0.2
X = 20%
Y = 80%
.: the % abundances of 10B and 11B are 20% and 80%, respectively