Structure of Atom

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Atomic models were proposed to explain the distributions of these charged particles in an atom.

The charge on the proton is equal but opposite to that of the electron. The number of protons present in the nucleus is equal to the atomic number (Z )

For example, the number of protons in the hydrogen nucleus is 1, in sodium atom it is
11, therefore their atomic numbers are 1 and 11 respectively.

Atomic number (Z) = Number of protons in the nucleus of an atom or it can be defined number of electrons present in a neutral atom.

Mass number (A): Atomic mass can be defined as, the sum of a number of protons (Z) and a number of neutrons (n).

Isobars are the atoms with the same mass number but a different atomic number.

For example,${}_6{}^{14}C and {}_7{}^{14}N$.

Isotopes are the atoms with an identical atomic number but a different atomic mass number is known as Isotopes.

For example, ${}_1{}^{1}H , {}_1{}^{2}D , {}_1{}^{3}T$

The general representation of the atomic number and atomic mass can be given as,${}_Z{}^{A}X$

Element symbol (X) with super-script on the left-hand side as the atomic mass number (A) and subscript (Z) on the left-hand side as the atomic number.

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, in 1898, proposed that an atom possesses a spherical shape (radius approximately 10^–10 m) in which the positive charge is uniformly distributed.

It is also known as Thomson’s plum pudding model.

Thomson model of atom was discarded because it could not explain the overall neutrality of the atom.