Structure of Atom Class 9 Notes
Charged particles in matter
The particles that carry an electric charge are called charged particles.
Discovery of electrons
It was known by 1900, that the atom was not a simple, indivisible particle but contained atleast one sub-atomic particle-the electron, which was identified by J.J. Thomson, when he performed cathode ray experiment using a discharge tube.
Electrons are negatively charged particles and are denoted by ‘e–‘. The charge present on an electron is equal to -1.6 X 10-19 coulomb. Since, this charge is considered to be the smallest, therefore, charge on e– is taken as -1. The mass of an electron is equal to 9.1 X 10-31 kg.
Discovery of protons
Before the identification of electron, E. Goldstein in 1886, discovered the presence of new radiations known as canal rays or anode rays. These are the positively charged rays which are seen moving from the anode towards cathode is specially designed discharge tube (with a porous cathode), when a high voltage is applied across the electrodes. Porous cathode is used to provide the path for passing anode rays. It led to the discovery of another sub-atomic particle, the proton.
Protons are positively charged particles and are denoted by ‘p+’. The charge present on proton is equal to +1.6 X 10-19 coulomb and it is considered as +1. The mass of a proton is equal to 1.6 X 10-27 kg. The mass of proton is approximately 2000 times as that of the electron.
Structure of an atom
According to Dalton’s atomic theory, atom was indivisible and indestructible. Now, the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s theory.
Thomson’s model of an atom
J.J. Thomson was the first scientist to purpose a model of an atom. Thomson’s model of an atom was similar to Christmas pudding. The electrons in a sphere of positive charge were like currants (dry fruits) in a spherical Christmas pudding.
It can also be compared to a watermelon, in which, the positive charge in an atom is spread all over like the red edible part, while the electrons studded in the positively charged sphere, like the seeds in the watermelon.
Following are the postulates of this model:
(i) Electrons are embedded in the sphere of positive charge.
(ii) The negative and positive charges are equal in magnitude. Therefore, the atom as a whole is electrically neutral.
(iii) The mass of an atom is assumed to be uniformly distributed throughout the atom.
Limitations of Thomson’s mode of an atom
Limitations of J.J. Thomson’s model of an atom are:
(i) J.J. Thomson’s model could not explain the experimental results of other scientists such as Rutherford, as there is no nucleus in the atomic model proposed by Thomson.
(ii) It could not explain the stability of an atom, i.e. how positive and negative charges could remain, so close together.
Rutherford’s model of an atom
He bombarded thin sheets of gold foil with fast-moving ∝-particles (these are doubly charged helium ions having a mass of 4u). He selected a gold foil because he wanted a layer as thin as possible. This gold foil was about 1000 atoms thick.
The following observations were made by Rutherford:
(i) Most of the fast-moving ∝-particles passed straight through the gold foil.
(ii) Some of the ∝-particles were deflected by the foil by small angles.
(iii) Very few ∝-particles (one out of 12000) appeared to rebound.
On the basis of his experiment, Rutherford concluded that:
(i) Most of the space inside the atom is empty because most of the ∝-particles passed through the gold foil without getting deflected.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
(iii) A very small fraction of a ∝-particles were deflected by 180O (i.e. they rebound), indicating that all the positive charge and mass of atom were concentrated in a very small volume within the atom.
On the basis of his experiment, Rutherford put forward the nuclear model of an atom, having the following features:
(i) There is a positively charged, highly dense centre in an atom, called nucleus. Nearly, the whole mass of the atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in a circular path.
(iii) The size of the nucleus (10-15m) is very small as compared to the size of the atom (10-10m).
Limitations of Rutherford’s model of an atom are:
(i) Any charged particle when accelerated is expected to radiate energy. To remain in a circular orbit, the electron would need to undergo acceleration. Therefore, it 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. Therefore, matter would not exist, but we know matter exists. It means that atoms are quite stable. Thus, it could not explain the stability of an atom when charged electrons are moving under the attractive force of positively charged nucleus.
(ii) Rutherford’s model could not explain the distribution of electrons in the extranuclear portion of the atom.
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