1. A r and M r

2. Let’s try counting the number of rice particles in a bucket of rice. Is it easy?

3. Relative atomic mass, A r Atoms are very tiny particles . Atoms have very small masses . For example, mass of hydrogen atom = 1.4 X 10 -24 g mass of lead atom = 2.9 x 10 -22 g Mass of carbon – 12 atom = 1.7 X 10 -23 g It is not practical to use actual masses of atoms in calculations.  Chemists often compare masses of different atoms with each other.

6. Relative Atomic Mass The relative atomic mass of any atom is the number of times the mass of one atom of an element is greater than 1/12 of the mass of one carbon-12 atom . Relative atomic mass = Mass of 1/12 of an atom of carbon-12 Mass of one atom of the element

7. Relative Atomic Mass The symbol for relative atomic mass is A r . Relative atomic mass is a ratio and has no unit . The relative atomic mass of each element is given in the Periodic Table.

10. Relative Atomic Mass of Some Elements Some A r values are not whole numbers. 35.5 Chlorine 64 Copper 23 Sodium 1 Hydrogen Relative atomic mass, A r Element

12. Why are some A r values not whole numbers? This is because such elements occur as mixtures of isotopes . For example, chlorine exists in two isotopic forms: chlorine-35 and chlorine-37.

13. How do we derive the A r of chlorine? Hence, relative atomic mass of chlorine = (0.75 × 35) + (0.25 × 37) = 26.25 + 9.25 = 35.5 A sample of chlorine is made up of 75% of chlorine-35 atoms and 25% of chlorine-37 atoms.

17. How do we calculate the relative molecular mass of chlorine gas (Cl 2 )? A r of Cl = 35.5 M r of Cl 2 = 35.5 X 2 = 71

18. How do we calculate the relative molecular mass of ammonia (NH 3 )? A r of N = 14 A r of H = 1 M r of NH 3 = 14 + 1  3 = 17

19. Relative Formula Mass, M r Many substances such as water are covalent and exist as molecules. However, substances like sodium chloride, NaCl, are ionic and do not exist as molecules. The ‘relative molecular mass’ of an ionic compound is more accurately known as relative formula mass, M r .

21. Relative Formula Mass Many substances such as water are covalent and exist as molecules. However, substances like sodium chloride are ionic and do not exist as molecules. The ‘relative molecular mass’ of an ionic compound is more accurately known as relative formula mass .

23. Calculating M r of MgSO 4 A r of Mg = 24 A r of S = 32 M r of MgSO 4 = 24 + 32 + (16  4) = 120 A r of O = 16

24. (1 X 64) + (1 X 32) + (9 x 16) + (10 x 1) = 250 CuSO 4. 5H 2 O Copper (II) sulphate crystals (1 X 40) + (2 X 14) + (6 X 16) = 164 Ca(NO 3 ) 2 Calcium nitrate (1 X 26) + (1 X 32) + (4 X 16) = 120 MgSO 4 Magnesium sulphate Relative formula mass, M r Chemical Formula Compound