Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and iv) Isobars. Give any two uses of isotopes.


Answer

(i) Atomic number

  • It is the number of protons present in the nucleus of an atom
  • It is denoted by Z  

 

Example - 1

  • For Hydrogen, Z = 1
  • It means that in a hydrogen atom, there are 1 proton present in the nucleus.

 

Example - 2

  • For Carbon, Z = 6

It means that in a carbon atom, there are 6 protons in the nucleus.

 

ii) Mass number  

  • Mass number is the total number of protons and neutrons present in the nucleus of an atom.
  • It is denoted by A .

Example - 1

  • Carbon atom has 6 protons and 6 neutrons
  • So, Mass number is 6 + 6 = 12
  • A = 12

 

Example - 2

  • An aluminium atom has 13 protons and 14 neutrons
  • So, Mass number = 13 + 14 = 27
  • A = 27



iii) Isotopes

  • Isotopes are elements having the same atomic number but different mass numbers.
  • That is, they’ve equal number of protons/electrons but different numbers of neutrons.
  • Because they’ve same number of electrons, their electronic configuration is the same.
  • Example, and .
  • Atomic number of both the elements is 17 but mass numbers are different.

 

iv) Isobars

 

  • Isobars are elements having different atomic numbers but same mass numbers.
  • That is, they’ve same number of nucleons (protons+neutrons) but different numbers of protons/electrons.
  • Because they’ve different numbers of electrons, their electronic configuration is different.
  • For example, and  .
  • Mass number of both the elements is 40 but atomic numbers are different.
  1. Class 9
  2. Chapter 4 Class 9 - Structure of Atom (Term 2)

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CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 11 years and a teacher from the past 11 years. He teaches Science, Accounts and English at Teachoo