Structure Of The Atom

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Neils Bohr, a Danish physicist, in 1913 proposed model of the atom which rectified the problems left by Rutherford’s Model. He proposed that

1. Electrons revolve around the nucleus in a fixed orbit.
2. He called these orbits as ‘stationary orbit’.
3. Each stationary orbit is associated with the fixed amount of energy, thus electrons do not

Radiate energy as long as they keep on revolving around the nucleus in a fixed orbit.

The circular path around the nucleus is called orbit, energy level or shell. Energy level is represented by letter – K, L, M, N, .... and so on.

Therefore,

1st orbit is denoted by – K
2nd orbit is denoted by – L
3rd orbit is denoted by – M, and so on.

The orbits are denoted by 1, 2, 3, .... and so on.

Neils Bohr, a Danish physicist, in 1913 proposed model of the atom which rectified the problems left by Rutherford’s Model. He proposed that

1. Electrons revolve around the nucleus in a fixed orbit.
2. He called these orbits as ‘stationary orbit’.
3. Each stationary orbit is associated with the fixed amount of energy, thus electrons do not

Radiate energy as long as they keep on revolving around the nucleus in a fixed orbit.

The circular path around the nucleus is called orbit, energy level or shell. Energy level is represented by letter – K, L, M, N, .... and so on.

Therefore,

1st orbit is denoted by – K
2nd orbit is denoted by – L
3rd orbit is denoted by – M, and so on.

The orbits are denoted by 1, 2, 3, .... and so on.

1. In 1932, J. Chadwick discovered neutron.
2. Neutrons are present in the nucleus of all atoms, except hydrogen.
3. Neutron is denoted by ‘n’ and is a neutral particle.
4. The absolute mass of the neutron is 1.6 x 10-24 g. The relative mass of the neutron is equal to 1.
5. Neutron is present in the nucleus of an atom.
6. Nucleus – The centre of atom is called nucleus. Nucleus comprises of neutron and proton.
7. The nucleus of an atom contains the whole mass of an atom.

Rutherford was interested in knowing how the electrons are arranged within an atom.

Rutherford designed an experiment in this fast moving alpha (α)-particles were made to fall on a thin gold foil.

1. He selected a gold foil because he wanted as thin a layer as possible. This gold foil was about 1000 atoms thick.
2. α-particles are doubly-charged helium ions. Since they have a mass of 4 u, the fast-moving α-particles have a considerable amount of energy.
3. It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms. Since the α-particles were much heavier than the protons, he did not expect to see large deflections.

The following observations were made:

1. Most of the fast-moving α-particles passed straight through the gold foil.
2. Some of the α-particles were deflected by the foil by small angles.
3. Surprisingly one out of every 12000 particles appeared to rebound.

Rutherford concluded from the α-particle scattering experiment that:

1. Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
2. Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
3. A very small fraction of α-particles was deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

He also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.

Rutherford put forward the nuclear model of an atom:

1. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
2. The electrons revolve around the nucleus in well-defined orbits.
3. The size of the nucleus is very small as compared to the size of the atom.

Drawbacks of Rutherford’s model of the atom:

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

J.J Thomson proposed the model of atom similar to a Christmas pudding or similar to a watermelon. His model of the atom is generally called plum and pudding model of the atom.

He proposed that electron is embedded the way black seeds of watermelon are embedded; In the sphere of positive charge.
According to Thomson-
a) An atom consists of the positively charged sphere in which electrons are embedded.
b) The quanta of negative and positive charges are equal. Thus, the atom is electrically neutral.