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Isotopes

Atomic Structure II - Concepts

Bohr Model of The Atom Modern Atomic Theory

Electronic Configuration

Significance of Electronic Configuration

Subshells

Valence Shell and Valence Electrons

Combining Capacity or Valency of Elements

Formation of Ions

Class - 9th CBSE Subjects

Concept Explanation

Isotopes

Isotopes: These are defined as the atoms of the same element, having the same atomic number but different mass numbers. E.g. there are 3 isotopes of hydrogen atom, namely protium (1H), deuterium (2H) and tritium (3H).

In other words, it can be said that isotopes have same number of protons but differ in the number of neutrons. Each isotope of an element is a pure substance. Since, chemical properties of elements largely depend on their electronic configuration or outermost electrons and as the isotopes of an element have similar electronic configuration.Therefore, isotopes of an element have the same chemical properties. We know that, masses of isotopes of elements are different. Since, physical properties such as density, light scattering etc., depend on mass therefore, these are different for isotopes of an element.

Average Atomic Mass: If an element has no isotopes, the mass of its atom would be the same as the sum of protons and neutrons in it. But if an element occurs in isotopic forms, then from the percentage of each isotopic form, the average mass is calculated as:

Average atomic mass of an element:[(Atomic mass of isotope I x percentage of isotope I) + (Atomic mass of isotope II x percentage of isotope II) +...]

Example: The two isotopic forms of chlorine atom with masses 35u and 37u occur in the ratio of 3:1. Therefore, the average atomic mass of chlorine atom, can be calculated as:

The average atomic mass of chlorine atom:

The 3:1 ratio of isotopes mean 75% of $^3^5Cl_1_7$ and 25% of $^3^7Cl_1_7$

Therefore, average atomic mass of chlorine is $frac{35times 75+37times 25}{100}=35.5$

We can calculate the number of neutrons of different isotopes.

Number of Neutrons = Atomic mass - Atomic number

For eg: There are three isotopes of carbon(C) with same numbers of protons and electrons but differ in number of neutrons.

$_6^{12}C:::::::::_6^{13}C::::::::::_6^{14}C$

No. of neutrons 12-6 = 6 13-6 =7 14-6 = 8

Here, 35.5 u is not the atomic mass of any one atom of chlorine but it shows that its given amount contains both the isotopes and their average atomic mass is 35.5 u.

NOTE: The fractional atomic masses of elements are due to the fact that all elements have isotopes. The isotopes of an element have different atomic masses. Since the atomic mass of an element is the average atomic mass of all the natural isotopes of that element, most elements have fractional atomic masses.

Applications of Isotopes:

An isotope of uranium (U-235) is used as a fuel for the production of electricity in nuclear reactors.

U-238 is used to determine the age of very old rocks and even the age of the earth.

An isotope of cobalt (Co-60) is used to remove brain tumours and in the treatment of cancer.

An isotope of carbon (C-14) is used to determine the age of old specimen of wood or old bones of living organisms.

An isotope of iodine (I-131) is used in the treatment of goiter (thyroid complications).

Sample Questions

(More Questions for each concept available in Login)

Question : 1

Which of the following are correct : (a) An isotope of uranium is used as a fuel for the production of electricity in nuclear reactors. (b) An isotope of cobalt is used to remove brain tumours and in the treatment of cancer. (c) An isotope of carbon is used to determine the age of old specimen of wood or old bones of living organisms.
A. Only (a) is correct.		B. Only (b) is correct.
C. Only (b) and (c) are correct.		D. All of these.
Right Option : D
View Explanation

Question : 2

How many isotopes are there in hydrogen atom ?
A. 1	B. 2	C. 3	D. 4
Right Option : C
View Explanation

Question : 3

Which of the following isotope of uranium is used as a fuel for the production of electricity in nuclear reactors?
A. U-234	B. U-235	C. U-236	D. None of these.
Right Option : B
View Explanation