• Earlier scientists only believed that atom contained only positively charged protons and negatively charged electron

  • Later in 1932 J.Chadwick found out that there was another subatomic particle, this had mass nearly equal to proton (1u)  however, it did not have any charge .

  • This was later known as Neutron .

  • This Neutron was present in the nucleus of the atom along with protons .

  • Hence, the mass of a nucleus is given as a sum of mass of protons and mass of neutrons.



Note: Both protons and neutrons are present in the nucleus .

Hence, they are collectively called nucleons .

A neutron is represented by ‘ n ’.

Neutrons are present in the nucleus of all atoms except Hydrogen .

12. neutrons model of james chadwick-Teachoo-01.png

 

Protons 

Electrons

Neutrons

Protons are positively charged.

Electrons are negatively charged.

Neutrons do not have any charge.

Atomic mass is 1u.

Negligible atomic mass.

Atomic mass is 1u .

Located inside the nucleus.

Located outside the nucleus in orbits/shells .

Located inside the nucleus.

Charge of a proton is +1

Charge of an electron is -1

Charge of a neutron is 0

 

13. aluminium atom-Teachoo-01.png

Examples -

In text Question - Page 49 Q1, Q2

NCERT Back Exercise - Q1, 14(ii)  

  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