What are Nucleons?

  • We know that every atom contains protons, neutrons and electrons
  • Out of this, Protons and Neutrons are present in the nucleus of an atom.
  • They are together called Nucleons .

What is an atomic number?

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

31. composition of nucleon-Teachoo-01.png

Example - 1

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

Example - 2

  • For Carbon, Atomic Number (Z) = 6

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

 

 

What is 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
  • Mass number (A) = 12

 

Example - 2

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

 

How is Atomic and Mass Number Represented?

  • Atomic Number is represented as Subscript
  • Mass Number is represented as Superscript

33. atomic no. and mass no-Teachoo-01.png

Example

  • Nitrogen has 7 Protons and 7 Neutrons
  • Hence Atomic Number = Z = No of Protons = 7

Mass Number = A = Number of Protons + Number of Neutrons = 7 + 7 = 14

34. Nitrogen has 7 Protons and 7 Neutrons-Teachoo-01.png

Examples -

In text Question -

NCERT Back Exercise - Q8 (i) (ii), Q19

  1. Class 9
  2. Chapter 4 Class 9 - Structure of Atom (Term 2)

About the Author

CA Maninder Singh's photo - Founder at Teachoo
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