The existence of isotopes was first suggested in 1913 by the radiochemist Frederick Soddy, based on studies of radioactive decay chains that indicated about 40 different species referred to as radioelements (i.e. radioactive elements) between uranium and lead, although the periodic table only allowed for 11 elements. *In nature, most of the elements have many atoms that have the same atomic number but different mass numbers such atoms of an element are called isotopes. This concept will be more clear from this ppt. And also radioactive isotopes applications and calculation of average atomic mass are explained. Hope it helps! And this is mainly for class ix

1. What are isotopes?🤔
-krishnapriya

2. Definition
➔Isotopes are atoms of the same element having the
same atomic number but different mass
numbers(atomic mass).

3. Examples of Isotopes:-
1.Isotopes of Hydrogen:-The hydrogen element has three isotopes, having the same
atomic number of 1 but different mass numbers of 1(Hydrogen-1), 2(hydrogen-2),
and 3(hydrogen-3) respectively. These are called Protium ,Deuterium and tritium
respectively.

4. 2.Isotopes of Carbon:-The carbon element has three isotopes having the
same atomic number of 6 but different mass numbers of 12,13 and 14 .

5. Reason for fractional atomic masses
And Calculation of average atomic masses
● The fractional atomic masses of elements are due to the existence of
their isotopes having different masses.
● For example, chlorine has two isotopes : chlorine-35 and chlorine-37 with
an abundance of 75% and 25% respectively.Thus the average mass of a
Chlorine atom is 35.5u

6. Eg:-chlorine has two isotopes : chlorine-35 and chlorine-37 with an
abundance of 75% and 25% respectively.Calculate the average atomic mass
of chlorine.
To find:Average atomic mass = ?
Formula to be used: Average atomic mass = [Mass1 × % of abundance]+[Mass2×
% of abundance]
Calculation:Average atomic mass two chlorine isotopes
= (35×75/100) + (37×25/100)
= 26.25+9.25
= 35.50 amu.
Conclusion:
The average atomic mass of chlorine is 35.5amu.

7. General characteristics of isotopes :-
❖ Same atomic number
❖ Same chemical properties
❖ Different mass numbers
❖ Different physical properties
❖ Different Nuclear properties

8. Radioisotopes
The isotopes which unstable due to the presence of extra
neutrons in their nuclei and emit various types of
radiations are called radioactive isotope (or just
radioisotopes) .

9. Applications of isotopes:-
Some isotopes(radioisotopes)have special properties which find them useful
in various fields. Some of them are :
(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of cancer.
(iii) An isotope of iodine is used in treatment of goitre.

10. Q1.Will 35Cl and 37Cl have different valencies?Justify your answer.
Q2.Hydrogen has three isotopes 1
1 H,1
2H,3
1H explain why:-
(i)these isotopes have almost identical chemical properties?
(ii)they are electrically neutral?
Q3.Given that the percentage abundance of the isotope neon-20 is 90 % and that of the
isotope neon-22 is 10% . Calculate the average atomic mass of neon.
Q4.The average atomic mass of a sample of an element X is 16.2u.What are percentages of
Isotopes 16
8X and 18
8X in the sample?