Isotopes (DP IB Chemistry)
Revision Note
Isotopes
What are isotopes?
Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
These are atoms of the same elements but with different mass numbers
The way to represent an isotope is to write the chemical symbol (or the word) followed by a dash and then the mass number
E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively
These isotopes could also be written as 12C or C-12, and 14C or C-14 respectively
Isotopes of hydrogen
Using the chemical symbols of hydrogen to determine the number of subatomic particles in each isotope
Calculating Relative Atomic Mass
What is relative atomic mass?
The relative atomic mass (Ar) of an element is the ratio of the average mass of the atoms of an element to the unified atomic mass unit
The definition of relative atomic mass is:
'the average mass of one atom of an element compared to one twelfth of the mass of an atom of carbon-12'
How to calculate relative atomic mass
The mass of an element is given as relative atomic mass (Ar) by using the average mass of all of the isotopes
The relative atomic mass of an element can be calculated by using the percentage abundance values
The percentage abundance of an isotope is either given or can be read off the mass spectrum
For example, if you have two isotopes A and B:
Find the mass of 100 atoms by multiplying the percentage abundance by the mass of each isotope:
total mass of 100 atoms = (% abundanceA x massA) + (% abundanceB x massB)
Then divide by 100, to find the average / relative atomic mass:
relative atomic mass =
Worked Example
A sample of oxygen contains the following isotopes:
Isotope | Percentage abundance |
---|---|
16O | 99.76 |
17O | 0.04 |
18O | 0.20 |
What is the relative atomic mass of oxygen to 2 dp?
A 16.00
B 17.18
C 16.09
D 17.00
Answer:
The correct answer is A
Total mass of 100 atoms = (99.76 x 16) + ( 0.04 x 17) + (0.20 x 18) = 1600.44
Mass of 1 atom = = 16.0044
So, the relative atomic mass, rounded to 2 decimal places, is 16.00
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