ISOTOPY /ISOTOPES
Isotopy is the existence of atoms of the same element having the same atomic number but different mass number or atomic mass.
The different atoms are called isotopes. Hence
Isotopes are atoms of a particular element with the same atomic number but different mass number.
Examples of isotopes are
iii. 11H, 21H and 31H
Relative Abundance: - when atoms exist naturally in isotopic mixture, they do so in a particular amount, when this amount is expressed in percentage, it is known as relative abundance.
For example, chlorine is found always in two isotopic mixtures out of 100, 75 is Cl-35 and 25 is Cl-37. so, 75 and 25 when expresses in percentage are the relative abundance of the two isotopes.
The relative abundance of an element is the amount (expressed in %) in which a particular isotope occurs in nature
Examples of isotopes are
i. 3517Cl with relative abundance of 75% and 3717Cl with relative abundance of 25%.
ii. 158O with relative abundance of 0.17, 168O with relative abundance of 99.76%, 178O with abundance of 0.05% and 188O with abundance of 0.02%
RELATIVE ATOMIC MASS: - The relative atomic mass of an element is the number of times the average mass of one atom of the element is as heavy as 8one-twelfth the mass of carbon-12.
DETERMINATION OF RELATIVE ATOMIC MASS OF AN ELEMENT GIVEN THE RELATIVE ABUNDANCE
The relative abundance of an element can be calculated from their various isotopic masses, and their relative abundance as follows: -
1. Naturally occurring exist in two isotopic mixtures 3517Cl with relative abundance of 75% and 3717Cl with relative abundance of 25% respectively. Calculate the relative atomic mass of Cl
SOLUTION
75 x 35 + 25 x 37 =100 100
26.25 + 9.25
= 35.5
2. An element Q has two isotopes 6329Q and 6529Q with relative abundance of 70% and 30% respectively. Calculate the relative atomic mass of Q
SOLUTION
70 x 63 + 30 x 65 =100 100
44.2 + 19.5
= 63.70
3. X is an element which exists as an isotopic mixture containing 90% of 3919Xand 10% of 4119X
a. How many neutrons are present in 4119X isotope
b. Calculate the mean relative atomic mass of X
Solution
a. Neutrons in 4119X
= 41-19 = 22
b. R.A.M =
90 x 39 + 10 x 41 =100 100
CALCULATIONS
1. The following are more examples of calculations of relative atomic masses of elements.
2. An element Y exist in two isotopic forms 3918R and 4018R in the ratio 3:2 respectively. What is the relative atomic mass of the element?
SOLUTION
First you add the ratio together, that is, 3+2 =5
R.A.M of Y= 3 x 39 + 2 x40 =
5 5
0.6 x 39 + 0.4 x 40 =
23.4 + 16 =
= 39.4
3. An element with relative atomic mass 16.2 contains two isotopes 168R with relative abundance 90% and m8R with relative abundance 10%. What is the value of m?
SOLUTION
16.2 = 90 x 16 + 10 x m100 100
16.2 = 0.9 x 16 + 0.1m
16.2 = 14.4 + 0.1m
16.2 – 14.4 = 0.1m
1.8 = 0.1 m
m = 1.8 = 18
0.1
The value of m is 18
OBJECTIVE QUESTIONS
1. The atomic number of an element is precisely
(a) the number of protons in the atom
(b) the number of electrons in the atom
(c) the number of neutrons in the atom
(d) the number of protons and neutrons in an atom
2. An atom can be defined more accurately as
(a) the smallest indivisible particle of an element that can take part in a chemical reaction
(b) the smallest part of an element that can take part in a chemical reaction
(c) a combination of protons and neutrons
(d) is the simplest unit of an element
3. The mass number is
(a) proton number + neutron number
(b) electron number + proton number
(c) neutron number + electron number
(d) electron number + atomic number
4. Calculate the relative atomic mass of an element having two isotopes 107Ag and 109Ag in the ratio 1:1
(a) 106
(b) 107
(c) 108
(d) 109
5. An element X has two isotopes 18.8X and 15.8X in the proportion of 1:9 respectively. Find the relative atomic mass of X
(a) 16.1
(b) 13.6
(c) 16.8
(d) 17.0
THEORY
1. (a) Define the term isotopy.
(b) Determine the number of electrons, protons and neutrons in each of the following:
i.3919K ii. 6326Cu iii. 2311Na
2. If an element R has isotopes 60% of 126Qand 40% x6Q and the relative atomic mass is 12.4, find x.
3. Consider the atoms represented below: qrX and srX
a. State the relationship between the two atoms.
b. What is the difference between them?
c. Give two examples of other elements which exhibit the phenomenon illustrated.
4. State the number of electrons, protons and neutrons present in the following atoms/ions
(a) 20Ca
(b) 16S2-
(c) 13Al3+
(d) 15P
Calculate the relative atomic mass of Z
Give a reason why the relative stomach mass of Z is not a whole number
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