1. AQA Additional Science
Chemistry 2
Lesson 3.1 Mass number

2. Aims Class work 6 September 2007 Mass Number LO’s
What is the mass number?
What’s an isotope?
Write it down!!
Page 50
© C Meyer

3. Chemistry Unit 2 Sections
Structures Structures
and bonding  and properties  How much?
3 of 7
© C Meyer

4. Aims All Know the relative masses of sub atomic particles
Know what an isotope is
Most
Work out the mass number of an atom
Some
Explain how isotopes are different

5. What have I learnt? T/F Quiz – Back Atoms are charged particles
Electrons are found in energy levels
Electrons are found in shells
Electrons have a negative charge
Protons are in the nucleus of the atom
Neutrons are found in shells around the nucleus
The atomic number is the number of protons

6. What have I learnt? What does the structure of the atom look like?
There are two properties of sub-atomic particles that are especially important:
Electrical charge (looked at before)
Mass (Need to look at now)
Charge
Particle -1
Electrons
Neutrons +1
Protons
Overall neutral
Particle
Charge
Relative Mass
Protons +1 1
Neutrons
Electrons -1
0.0005
Mass of proton = mass of neutron (relative mass = 1)
Ignore mass of electron m5

7. 15 Mins

8. What have I learnt? Atomic/Proton Number
The atom of any particular element always contains the same number of protons. E.g.
Hydrogen atoms always contain 1 proton
Carbon atoms always contain 6 protons
Magnesium atoms always contain 12 protons
The number of protons in an atom is known as its atomic/proton number.
It is the smaller of two numbers shown in most periodic tables 12 C 6

9. Activity
How Many Protons?
Here is some data about elements taken from the Periodic Table.
What is the atomic number of the elements? 23 Na 11 56 Fe 26
119 Sn 50 19 F 9 27 Al 13 11 26 50 9 13

10. What have I learnt? How Many Protons?
Note elements have a definite and fixed number of protons.
Changing number of protons in atom changes that atom into a different element.

11. What is the mass number? Mass Number – What is it?
The mass of each atom comes almost entirely from the number of protons and neutrons that are present. (Remember that electrons have a relatively tiny mass).
MASS NUMBER = Number of protons + number of neutrons
Atom
Protons
Neutrons
Mass Number
Hydrogen 1
Lithium 3 4 7
Aluminium 13 14 27 m2

12. Activity
What is the mass number?
In back - Calculate the mass number of the atoms shown.
Atom
Protons
Neutrons
Mass Number
Helium 2
Copper 29 35
Cobalt 27 32
Iodine 53 74
Germanium 41 4 64 59
127 73
MASS NUMBER = Number of protons + number of neutrons

13. What is the mass number? Mass number = protons + neutrons
It is the bigger of two numbers shown in periodic table 12 C 6
Atomic/proton number = protons.
It is the smaller of two numbers shown in periodic table m1

14. Activity
What is the mass number?
In back - Calculate the number of neutrons in these atoms.
Number of Neutrons = mass number - atomic number
Atom
Mass Number
Atomic Number
Number of Neutrons
Helium 4 2
Fluorine 19 9
Strontium 88 38
Zirconium 91 40
Uranium
238 92 2 10 50 51
146

15. What have I learnt? In Front - Copy and fill in the blank columns 5 5
Activity
What have I learnt?
In Front - Copy and fill in the blank columns
Atom
Protons
Neutrons
Electrons
Atomic number
Mass Number
Boron 5 6
Potassium 19 20
Chromium 24 28
Mercury 80
121
Argon 18 22 5 5 11 19 19 39 24 24 52 80 80
201 18 18 40
Note – atomic number is defined as the number of protons rather than electrons because atoms can lose (or gain) electrons but do not normally lose protons

16. 30 Mins

17. What’s an isotope?
Natural samples of elements are often a mixture of isotopes. About 1% of natural carbon is carbon-13. C 13 6 1% C 12 6
99%
What’s the
difference? 6 6 7
Protons
Electrons
Neutrons m1

18. ATOMS are just like the Ford Fiesta
What is relative mass?
ATOMS are just like the Ford Fiesta
ISOTOPE m2

19. There are different versions of the FIESTA
SPEEDY m3

20. We also have different versions of the same element’s atoms WE CALL THEM
ISOTOPES m1

21. The only difference between the atoms in an isotope is that they contain different numbers of
NEUTRONS

22. Remember: The number of
What’s an isotope?
It is not strictly true to say that elements consist of one type of atom.
Atoms in an element always have the same
number of protons but may have different
numbers of neutrons.
Atoms like this are called isotopes.
Remember: The number of
protons defines
the element

23. H H H Isotopes: Hydrogen
Hydrogen exists as 3 isotopes although Hydrogen-1 makes up the vast majority of the naturally occurring element.
All can make water H 1 H 2 1 H 3 1
Protons
Electrons
Neutrons
Hydrogen
Protons
Electrons
Neutrons
(Deuterium)
Protons
Electrons
Neutrons
(Tritium) m4

24. Cl Cl Isotopes: Chlorine
About 75% of natural chlorine is 35Cl the rest is 37Cl. Cl 35 17
75% Cl 37 17
25%
How many
neutrons? 17
Protons
Electrons
Neutrons
Protons
Electrons
Neutrons 17 18 20

25. Activity
Isotopes of Oxygen 16 O 8
Almost all of natural oxygen is 16O but about 0.2% is 18O.
Produce a Table showing the particles in each isotope.
Oxygen-16
Oxygen-18
Protons
Electrons
Neutrons 8
Protons
Electrons
Neutrons 8 8 8 8 10

26. What’s an isotope?
Isotopes are virtually identical in their chemical reactions as have same number of protons and electrons.
The uncharged neutrons only affect physical properties such as melting point and density.

27. What have I learnt?
Draw the electronic structure of carbon 12 and carbon 14
Say how many protons, neutron
Draw up a table of similar properties and differences
Same
Different

28. C What have I learnt? 12 6 99% 13 1% Same Different
Protons
Electrons
Neutrons 6 7
Same
Different
Same number electrons
Different number neutrons
Same number protons
Different mass number
Same chemical properties
Different physical props
Same proton number

29. Homework
Section 2 test

30. What have I learnt?

31. The End