Structure of Atom: Overview, Questions, Preparation

Isotopes are defined as atoms of the same element having the same atomic number but different mass numbers. The term isotope is derived from Greek words ‘isos’ meaning equal and ‘topos’ meaning the same place, which means that all the isotopes of an element occupy the same place on the periodic table. Alternatively, isotopes are also defined as variants of an element that differ in their neutron number but have a similar number of protons and electrons.   

Atomic number is defined as the total number of protons present in the nucleus of an atom. It is denoted by the alphabet ‘Z’. Mass number is defined as the sum total of protons and neutrons present in an atom’s nucleus. It is denoted by ‘A’.

Hence, in the AZE notation of an atom, these terms are represented as follows:

Isotopes can also be represented by the element’s name followed by a hyphen and the isotope mass number. For example, isotopes of carbon can be written as Carbon-12 and Carbon-14. 

Several elements have been identified in nature having the same atomic number but different mass numbers. The isotopes of hydrogen are a typical example. Hydrogen has three atomic species—Protium (11H), Deuterium (21H) and tritium (31H).

Each of these has its atomic number 1, but the mass numbers are 1, 2 and 3, respectively. These share the same place in the periodic table and are hence known as isotopes.

Several elements consist of a mixture of isotopes. Every isotope of an element is a pure substance. Isotopes differ in physical properties but have similar chemical properties.

Two major types of isotopes exist:

1) Stable isotopes—These have a stable proton–neutron combination due to which they are not subjected to decay. Oxygen isotopes are said to be stable isotopes. These pose no dangerous effects on living beings.

2) Radioactive isotopes—These undergo radioactive decay because they have an unstable combination of protons and neutrons. Examples include uranium. Due to the decay, they can emit alpha, beta or gamma rays.

Isobars have a similar number of nucleons or similar mass numbers and different atomic numbers, whereas isotopes have identical atomic numbers but different mass numbers. Examples of a group of isobars include Sulfur-40, Chlorine-40, Argon-40, Potassium-40 and Calcium-40. 

Applications

1) Isotopes of uranium are used as fuel in nuclear reactors.

2) Isotopes of cobalt are used in the treatment of cancer and irradiation of food products.

3) Isotopes of Iodine are used in the treatment of goitre.  

The topic ‘Isotopes’ is covered in the chapter ‘Structure of Atom’ in Class 11 Chemistry, which along with ‘Some Basic Concepts of Chemistry,’ has a total weightage of 11 marks.

1. How are isotopes represented? 

Answer: Isotopes are represented using the AZE notation where A is the mass number and Z is the atomic number. (AZE) For example- 11H

They can also be represented as (name of element- isotope) For example- Carbon-14. 

2. Name the three isotopes of hydrogen. 

Answer: a) Protium - 11H

b) Deuterium- 21H

c) Tritium- 31H

3. Name the two isotopes of chlorine.

Answer: a) 3517Cl

b) 3717Cl