Rutherford proposed a model in which electrons revolve around the nucleus in welldefined orbits. There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nucleus. Whereas, Thomson proposed the model of an atom to be similar to a christmas pudding. The electrons are studded like currants in a positively charged sphere like christmas pudding and the mass of the atom was supposed to be uniformly distributed.

The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo a acceleration and the charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know

The postulates put forth by Neils Bohr’s about the model of an atom:
(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
These orbits are called energy levels. Energy levels in an atom are shown by circles.
These orbits are represented by the letters K, L, M, N, … or the numbers, n = 1, 2, 3, 4, …. 

Since the atomic number of sodium atom is 11, it has 11 electrons. A positively charged sodium ion (Na+) is formed by the removal of one electron from a sodium atom. So, a sodium ion has 11–1 = 10 electrons in it. Thus, electronic distribution of sodium ion will be 2, 8. The atomic number of an element is equal to the number of protons in its atom. Since, sodium atom and sodium ion contain the same number of protons, therefore, the atomic number of both is 11.

% of α-particles deflected more than 50⁰ =1% of α-particles.
 % of α-particles deflected less than 50⁰ =100–1 = 99%
 Number of α-particles bombarded = 1 mole = 6.022 ×10²³ particles
 Number of particles that deflected at an angles less than 50⁰ 

1. Electron was discovered by J.J.Thom son and proton by Goldstein.
2. The mass of proton was 2000 times as that of electron.
3. The mass of proton is taken as unit charge and its charge is +1, mass of electron is negligible and its charge is minus one.
4. J.J.Thomson proposed a model which states that an atom consist of a positively charged sphere and electrons are embedded in it.
5. He also stated that +ve and —ve charges are equal in magnitude and atom as a whole is electrically neutral.
6. Rutherford’s scattering experiment discovered fast moving (a) particles and also atomic nucleus.
7. Rutherford’s model states that atomic nucleus is +vely charge and lies in the centre, and electrons revolve around nucleus in well-defined orbits, size of nucleus is very small.
8. Rutherford could not explain the stability of an atom.
9. Neils Bohr suggested that electrons revolve in discrete orbits and does not radiate energy. If the atomic shells are complete then atoms are stable and less reactive.
10. J. Chadwick discovered subatomic particle with no charge called neutron.
11. Maximum number of electron present in a shell is given by formula 2n².
12. The maximum number of electrons that can be accommodated in outermost orbit is 8 is called octet.
13. To maintain octet atoms share, lose or gain the electrons.
14. Valency is the combining capacity of an atom.
15. Atomic number is the total number of protons present in nucleus of an atom.Represented by Z.
16. Mass number ¡s sum of total number of protons and neutrons present in nucleus of atom.
17. Isotopes are atoms of same elements which have different mass number, and isobars are atoms with some mass number but different atomic number.