1. Periodic table data and Isotopes
Learning Objective
recall the meanings of the terms Atomic Number, Mass number and Relative atomic mass.
understand the term ‘Isotope’
calculate the relative atomic mass using the mass numbers and abundance of isotopes.

2. Atomic Number (Proton Number)
This is the number of Protons found in the nucleus of an atom. All atoms of a particular element have the same number of protons in their nuclei.
Relative Atomic Mass (Ar)
 The relative atomic mass of an element, symbol Ar, is defined as the relative mass of its atoms compared to the mass of the carbon-12 isotope. 
Mass Number
This is the relative mass of atoms. It is calculated from adding the number of protons (Ar = 1) to the number of neutrons (Ar = 1). Electrons are regarded as having a mass of zero.

3. Relative atomic mass = 7
Protons = 3
Electrons = 3
Neutrons = = 4
Atomic number = 3

4. Look at your periodic table
Look at your periodic table. The atomic masses are all whole numbers except for….
The relative Atomic Mass of Chlorine is recorded as 35.5.
35.5 – 17 = 18.5
You can’t have half a neutron. So how can this be explained?
35.5 Cl
Chlorine 17

5. All chlorine atoms have 17 Protons
Some chlorine atoms have 18 Neutrons giving them a mass number of 35
Some chlorine atoms have 20 Neutrons giving them a mass number of 37
These atoms are called ISOTOPES of chlorine
Isotopes are atoms that have the same number of Protons but different numbers of Neutrons in their nuclei.

6. 75% of the Chlorine in existence is Cl35
This means that the atomic mass shown in the periodic table is an average of the masses of both isotopes taking into account their relative proportions.
(75 x 35) + (25 x 37) = 35.5
100

7. Nearly all the elements in the Periodic table have isotopes
Nearly all the elements in the Periodic table have isotopes. But for most of them the vast majority of the atoms are one particular isotope and so the average mass is always close to a whole number H3 H2 H1
Hydrogen has three isotopes. However 99.98% of all hydrogen is H1 so the average Ar is so close to 1.0 as to make no difference.

8. The periodic table states that the Relative Atomic Mass (Ar) of Boron is 11
Boron has two naturally occurring isotopes.
80% of all Boron is B11 (6 neutrons)
20% of all Boron is B10 (5 Neutrons)
Calculate the average relative atomic mass of Boron.
(80 x 11) + (20 x 10) = 100
10.8

9. The periodic table states that the Relative Atomic Mass (Ar) of Copper is 64
Copper has two naturally occurring isotopes.
69% of all Copper is Cu63 (34 neutrons)
31% of all Copper is Cu65 (36 Neutrons)
Calculate the average relative atomic mass of Boron.
(69 x 63) + (31 x 65) = 100
63.6

10. The periodic table states that the Relative Atomic Mass (Ar) of Magnesium is 24
Magnesium has three naturally occurring isotopes.
79% of all Magnesium is Mg24 (12 neutrons)
11% of all Magnesium is Mg25 (13 Neutrons)
10% of all Magnesium is Mg26 (14 neutrons)
Calculate the average relative atomic mass of Magnesium.
(79 x 24) + (11 x 25) + (10 x 26) =
24.3

11. Periodic table data and Isotopes
Learning Objectives
To be able to
recall the meanings of the terms Atomic Number, Mass number and Relative atomic mass.
understand the term ‘Isotope’
calculate the relative atomic mass using the mass numbers and abundance of isotopes.