SCH 3U
Unit 1: Matter
Isotopes
1.
Name: ____________________
Date: ____________________
Isotopes and Isotope Notation
We have already reviewed and talked about the concepts of atomic number and mass number. The atomic number
of an element is represented by the letter Z. The mass number is represented by the letter A. In short:
Where the letters are defined as:
A
Z
X = the symbol for the name of element
X NE
N = the number of neutrons (usually not shown, but can be derived
from A and Z where N = A - Z)
Z = the number of protons (atomic number)
A = the number of protons and neutrons
A = (Z + N) = the atomic mass (approx.)
A = mass number
E = charge on the ion (when produced)
When dealing with isotopes, you may see the notation above as in
131
53
I or you may see the isotope written out
with its mass number as iodine-131.
2.
Defining Isotopes and Radioisotopes
a) Isotopes
Atoms with the same number of protons (so the same atomic number) but with different numbers of neutrons (so
different mass numbers) are called isotopes. For example,
14
6
C , 136 C , 126 C all have the atomic number 6
(so all have 6 protons) but they have different mass numbers (different numbers of neutrons: 8, 7, 6 respectively).
They can also be written as carbon-14, carbon-13 and carbon-12.
b) Radioisotopes
Smaller atoms tend to have the same number of protons and neutrons. As atoms get larger, the number of neutrons
exceeds the number of protons by increasing amounts. This is because neutrons tend to have a stabilizing effect on
the repulsive forces between the protons (protons are attracted to neutrons). When the forces between protons
and neutrons are not balanced, the nucleus “decays” or breaks down to form a more stable nucleus. Atoms with
“unbalanced” nuclei are called radioisotopes because they often produce some form of radiation as their nuclei
decay.
Example:
60
27
Co  e1 +
60
28
Ni
Here, a neutron within the unstable Cobalt-60 atom decays to form a proton
and an electron. The electron is ejected as a beta particle (
nickel-60 with atomic number 28 instead of 27.
e1 ) and the extra proton means the cobalt-60 is now
3.
Average Atomic Mass and Isotopic Abundance
 Most elements found in nature are really a mixture of two or more isotopes. The atomic mass listed in the periodic
table for a given element is really a weighted average atomic mass of all of the isotopes of that element. Often when
1
doing these calculations, we use the "unified atomic mass unit, u" as the unit. Theoretically, 1 u = 12 the mass of one
carbon atom or 1 u = 1.66
 The isotopic abundance (pg. 18) is defined as the amount of a given isotope of an element that exists in nature. This
value is usually given as a percentage of the total amount of the element in a typical sample. These values are used to
help to calculate the average weighted atomic masses displayed on the periodic table for each element.
4.
Calculating Average Atomic Mass on the Periodic Table Using Isotopic Abundance
Example:
79
81
Bromine in nature is 50.69% 35 Br and 49.31% 35 Br . What is the average atomic mass of a sample of bromine
atoms?
SOLUTION:
Av. atomic mass = (atm mass of 79Br x fraction of 79Br present) + (atm. mass 81Br x fraction of 81Br present)
= (79 u x
50.69
49.31
100 ) + (81 u x 100 )
= 40.0451 u + 39.941 u
= 79.9862 u
The average atomic mass for bromine is about 80. u. (NOTE: 2 sig figs)
[Need more information? Read through “Sample Problem” pg. 165]
Practice
1a) A neutral atom of a particular element has Z = 32 and A = 73. Identify the element and state how many protons, electrons
and neutrons it has.
b) The nucleus of an atom of bromine contains 35 protons and 45 neutrons. Identify its mass number and its atomic number.
c) Two common isotopes of copper are copper – 63 and copper – 65. Identify the numbers of protons, neutrons and electrons
for each isotope.
d) Determine the atomic number, mass number, number of protons, number of electrons and number of neutrons for the 2+
ion belonging to the copper – 63 isotope.
2. Do #10, 11 page 166 and #5 page 167.
NOTE: Most text questions have answers at the back of the book. Check your work.