1. Isotopes are atoms of the same element that have different numbers of neutrons. This document discusses isotopes and how to calculate relative atomic mass. 2. It provides examples of different isotopes like chlorine-35, chlorine-37, and hydrogen isotopes. The number of protons and electrons are the same between isotopes but the number of neutrons can differ. 3. To calculate relative atomic mass, you must know the natural abundances of each isotope and take a weighted average of their masses based on these abundances. For chlorine, 75% is chlorine-35 and 25% is chlorine-37, giving a relative atomic mass of 35.5.

1. Island School Chemistry 3e – 3g
READ, COMPLETE AND UPLOAD TO YOUR BLOG
Isotopes
Aim: Understand the terms isotopes and relative atomic mass (Ar)
Calculate the relative atomic mass of an element from the relative abundances of its isotopes
Info and Questions:
Atoms of the same element all contain the same number of protons (and therefore
electrons), but the number of neutrons may be different. Atoms of the same element
which have a different number of neutrons are called ISOTOPES.
e.g. Chlorine has two kinds of atoms with different mass numbers, i.e. chlorine has two
isotopes:
35
Cl and 37
Cl
17 17
Q1. Calculate the number of protons, neutrons, electrons in each.
P:17 P: 17
E:18 E: 17
N:17 N : 20
Since the chemical reactions of an atom depend on its electrons, all chlorine atoms
react in the same way. The number of neutrons in the nucleus does not affect
chemical reactions but it does affect some physical properties such as boiling point or
speed of diffusion.
e.g. Hydrogen has three isotopes, 1
H, 2
H and 3
H. They are often referred to as
hydrogen-1, hydrogen-2 and hydrogen-3. Hydrogen-2 is also called deuterium and
hydrogen-3 is also called tritium.
Q2. Calculate the number of protons, electrons, neutrons in each hydrogen isotope.
Relative Atomic Mass
Any sample of chlorine is a mixture of two isotopes – 75% is chlorine–35 and 25% is
chlorine-37
So if you have 100 chlorine atoms, 75 will be chlorine-35 and 25 will be chlorine-37.
We can work out the average mass (or relative atomic mass) of chlorine:
Average mass = total mass = (75 x 35) + (25 x 37) = 35.5
Number of atoms 100
The relative atomic mass is the average mass and takes into account the different
proportions of each isotope in the natural mixture.
(The mass is based on a scale where carbon-12 is given a mass of exactly 12)Key words: Relative atomic mass, isotopes, neutrons, protons, atoms.

2. Island School Chemistry 3e – 3g
1. Use the words in the boxes to complete the statements:
A Protons and neutrons are found in the nucleus
B Protons and neutrons are called nucleons
C The number of protons in an atom is the atomic number
D Atoms of the same element containing different numbers of neutrons are called istopes
E The number of protons plus neutrons in an atom gives the .
F Carbon-12 and carbon-14 are two . of carbon
Two particles are present in equal number in carbon-12 and carbon-14 atoms.
G One is positively charged. It is a .Proton
H The other is negatively charged. It is an .Electron
I One particle present in different numbers in carbon-12 and carbon-14 atoms is the .
J Protons and the neutrons in a carbon-12 atom are found in the .
2. The table gives information about three types of oxygen atom. Finish the table.
number of protons number of neutrons number of electrons
oxygen-1 6 8 8 8
oxygen-1 7 A=8 B=9 C=8
oxygen-1 8 D=8 E=10 F=8
3. Which two particles, W, X, Y, Z represent different isotopes of the same element?.
particles
Number of one particle
protons neutrons electrons
W 1 1 1
X 1 2 1
Y 9 10 9
Z 9 10 10
• atomic number
• isotopes
• mass number
• nucleons
• nucleus
• proton
• isotopes
• electron
• neutron
• nucleus
Atoms and isotopes
Aim: to review the names and arrangement of particles in the atom

3. Island School Chemistry 3e – 3g
Isotopes & Relative Atomic Mass Questions
Aim:
Calculate the relative atomic mass of an element from the relative abundances of its isotopes
Questions:
1. Silicon (atomic no. 14) has 3 naturally occurring isotopes, with masses 28, 29,30.
a) Write the symbols for the 3 isotopes including both the mass and atomic numbers.
M: 28, M:29, M:30
A: 14 A:14 A:14
b) For each isotope work out the number of protons, electron and neutrons.
14, 15, 16
c) Natural silicon contains 93% silicon-28; 5% silicon-29 and 2% silicon-30
Calculate is the relative atomic mass of silicon (show working)
93/100 = 28, 5/100 = 29, 2/100 = 30 = 28.09
2. (a) In a sample of copper, 69.1% of the atoms have a mass number of 63 and the remainder
have a mass number of 65. Use this information to calculate the relative atomic mass of
copper. Give your answer to 3 significant figures. (5)
(b) Explain why copper atoms with different numbers of neutrons have identical
chemical properties. (1)