1. Understanding Chemical Reactions
Lesson 1

2. Lesson Objectives Ensure familiarity with concept of atomic structure.
Understand what isotopes are.
Understand the properties of isotopes
Understand the concept of relative atomic mass (RAM)
Be able to calculate the RAM from information about the isotopes of an element.

3. Isotopes of hydrogen
There are 3 isotopes of hydrogen

4. Hydrogen-1
Contains 1 proton and 1 electron – as all isotopes of H must contain.
No neutrons
The most common type of H atom.

5. Hydrogen-2, deuterium
Contains 1 proton and 1 neutron in the nucleus.

6. Hydrogen-3, tritium
Contains 1 proton and 2 neutrons in the nucleus.

7. Isotopes
All isotopes have identical chemical properties but different physical properties.

8. Isotopes Chlorine exists as two isotopes, chlorine-35 and chlorine-37.
Chlorine has the atomic number 17.
Work out the number of neutrons in chlorine-35 and chlorine-37.

9. Isotopes
Chlorine- 35
17 protons
No. of neutrons
= 35 – 17
= 18

10. Isotopes
Chlorine- 37
17 protons
No. of neutrons
= 37 – 17
= 20

11. Relative atomic mass
Atoms are too small to be weighed individually, even with the best balance!
We can, however, compare the masses of different atoms.

12. Relative atomic mass
We give the lightest atom (a hydrogen-1 atom) a mass of 1.
All other atoms are compared so many times heavier than this.
We call this relative atomic mass (RAM).

13. Relative atomic mass E.g. a helium atom has a RAM of 4.
This means a helium atom is four times heavier than a hydrogen atom.

15. Relative atomic mass
A magnesium atom has a RAM of 24. How many times heavier is a magnesium atom than a helium atom?
SIX times heavier! (He has a RAM of 4, 24/4 = 6)

16. Relative atomic mass
Some elements have RAM that are not whole numbers.
This is because the elements exist as a number of isotopes – each of which has atoms of different mass.
Tin has 10 different isotopes!

17. The RAM of chlorine The RAM of Cl is 35.5, how do we explain this?
Cl exists as two isotopes, Cl-35 & Cl-37.
In any sample of Cl, 75% of the atoms are Cl- 35 and 25% are Cl-37.

18. The RAM of chlorine
The average mass, taking into consideration the different number of atoms of the two isotopes, is:
(75% x 35) + (25% x 37)
= 35.5

19. The relative atomic mass of hydrogen is 1.008 rather than exactly 1.
Why?