Discovery of Neutrons (By J. Chadwick)

J. Chadwick bombarded lighter elements (like lithium, boron etc.) with α-particles and observes

emission of new particles having zero charge but having mass equal to that of proton.

→ These particles were called ‘Neutron’ i.e., neutral particle of the atom.

→ Neutron are absent in Protium isotope of hydrogen atom.(1H1)

→ Since, mass of electrons are negligible as compared to that of proton and neutrons hence, sum

of masses of protons and neutrons in an atom will compose its atomic mass.

Bohr Model of an atom

Bohr suggested that –

•    Electrons spin around the nucleus in an individualized separate path or unattached          orbit.
•    The electrons do not emit any energy while moving Indies special orbits.
•    These orbits are also called as Energy Levels.
•    They are represented using letters or numbers as shown in the figure below –


 

 

 

 

 

 

 

The Neutrons

J. Chadwick discovered that there is another sub-atomic particle present in the atom. This particle carries no charge and is known as a Neutron. Therefore, we can conclude that atom consists of three types of particles –

Electrons

which carry a negative charge

Protons

which carry a positive charge

Neutrons

they are neutral 

 

The distribution of electrons in different shells or orbits

•    If Orbit number = n
•    Then number of electrons present in an Orbit = 2n2
•    So, for n =1
•    Maximum electrons present in shell – K = 2 * (1)2 = 2
•    The outermost shell can contain at most 8 electrons.
•    The shells in an atom are filled in sequence.
•    Thus, until the inner shells of an atom are filled completely the outer shells cannot         contain any electrons.

 

Discovery of Neutrons

Discovery of neutron

It was discovered by Chadwick in the year 1932. It was actually discovered while considering the mass of atomic particles. It was seen that the whole mass of an atom is due to the nucleus and as far we know, the nucleus is positively charged. That means it has only protons in it. But we also know that the mass of an atom is never equal to the number of protons. This shows that the nucleus contains some other particles also that contribute towards mass only and not towards charge. Therefore, the particles were called neutrons (as they possess no charge).

Bohr’s theory

To overcome the limitations of Rutherford model, the new concept and picture of atom was given by Neil Bohr which made a great contribution in knowing the structure of an atom. According to it:

  • Atom is electrically neutral i.e. number of Protons = number of Electrons.
  • In the centre of an atom, nucleus is present which is positively charged.
  • In the nucleus, protons and neutrons are present.

Protons possess = positive

Neutron possess = no charge

  • Outside the nucleus, shells or energy levels designated as K,L,M,N and so on  are present.
  • In shells, electrons revolve.
  • Electrons are negatively charged.
  • Each shell has a fixed amount of energy. So, as long as an electron remains in the same shell, it never loses or gains energy.
  • Number of electrons  in each shell is determined by Bohr Bury rule i.e. 2n2.