1. Discuss in pairs
What is an isotope?

2. Learning Objectives Grade C/D Recall what an isotope is.
Define relative atomic mass and relative formula mass.
Grade A/B
Calculate RAM of an element using information on the relative abundances of its isotopes.
Calculate the RFM of a compound and the percentage by mass of an element in a compound.

3. These are the mass numbers of atoms of these elements
What is the mass of...
1 atom of carbon?
1 atom of lithium?
2 atoms of mercury?
5 atoms of neon?
These are the mass numbers of atoms of these elements

4. ATOMIC MASS Atoms are so small that we can’t use units like grams.
Instead, the mass of all atoms is compared with the mass of an atom of a carbon-12 isotope (the carbon-12-scale).
These atoms weigh exactly 12 units.
If we compare an atom of the most common isotope of magnesium with carbon-12, it has exactly double the mass (magnesium-24). This is the relative isotopic mass.
An atom of the most common isotope of hydrogen weighs one-twelfth of carbon-12 (1 unit)

5. Relative Atomic Mass (Ar)
The Ar of an element is the weighted average mass of the isotopes of the element.
It is always measured on a scale on which a carbon-12 atom has a mass of 12 units.
As it is the average, it is often not a whole number.

6. How were the Ar‘s of each element worked out?
If you take a sample of an element then you are likely to have a mixture of atoms of different isotopes of the element.
E.g. In a sample of chlorine, some atoms have a mass of 35 and others have a mass of 37. However, this doesn’t mean that the average Ar is 36.
A typical sample contain 75% chlorine-35 and 25% chlorine-37
- For 100 atoms of Cl, the mass is:
(75 x 35) + (25 x 37) = 3550
atom of Cl has an average mass of 35.5

7. Example
A magnesium sample contains 78.6% Mg-24, 10.1% Mg-25 and 11.3% Mg-26.
Assume there are 100 atoms in the sample, calculate the Ar of magnesium to 1 decimal place.
Total mass = (78.6 x 24) + (10.1 x 25) + (11.3 x 26)
Ar = /100 = 24.3

8. You could also be asked to use a graph like this one to calculate the Ar of an element

9. Relative Formula Mass
The relative masses of all the atoms in a compound added together. eg Copper Sulphate, CuSO4 contains: 1 x Cu atom (mass 64) 1 x S atom (mass 32) 4 x O atoms (mass 4 x 16) Total = = 160
Remember:
No units!!!

10. Find the RFM of the following compounds
NH3
(NH4)2SO4
NH4Cl
Mg(NO3)2
H2SO4
Ca(NO3)2
HNO3
Mg(OH)2
CaCO3
Ca(OH)2 17
53.5 98 63
100
Brackets:
132
148
164 58 74

11. % of elements in a compound
Malachite (copper ore) has the formula CuCO3
What is the % by mass of copper in this ore?
Total mass = (16 x 3) = 124
64 of the 124 is copper
(64/124) x 100 = 51.6%

12. Solve the following:
What is the % by mass of copper in copper oxide (CuO)?
What is the % by mass of water in CaSO4.2H2O?
What is the % by mass of sulphur in sulphuric acid (H2SO4)?

13. Learning Objectives Grade C/D Recall what an isotope is.
Define relative atomic mass and relative formula mass.
Grade A/B
Calculate RAM of an element using information on the relative abundances of its isotopes.
Calculate the RFM of a compound and the percentage by mass of an element in a compound.