Compare all the proposed models of an atom given in this chapter .
Answers
In this model, an atom consists of a positively charged sphere and the electrons embedded in it. The number of electrons is equal to the number of protons so the atom is electrically neutral. In this model, there is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus. The electrons revolve around the nucleus in circular paths. The size of the nucleus is very small as compared to the size of the atom.
In this model, only certain special orbits known as the discrete orbit of electrons, are allowed inside the atom. While revolving the electrons do not radiate energy. These orbits are called energy levels or shells.
Answer: Thomson's model of atom:
J. J. Thomson proposed the model of atom similar to a Christmas Pudding or similar to a water melon. His model of atom is generally called plum and pudding model of atom.
He proposed that electrons are embedded the way black seeds of water melon are embedded; in the sphere of positive charge. According to Thomson
(a) An atom consists of positively charged sphere in which electrons are embedded.
(b) The quanta of negative and positive charges are equal. The equal number of negative charge and positive charge makes an atom electrically neutral.
Rutherford's model of atom:
Ernest Rutherford in 1909 with his team bombarded very thin gold foil with α – particles. He found that
Most of the α – particles passed without any hindrance.
Some of the α – particles deflected from their original path at noticeable angle.
Very few of the α – particles bounced back at their original path.
On the basis of his observation, he proposed the model of atom. The Rutherford's Model of Atom is as follows:
Most of the part in an atom is empty.
There is a positively charged center in atom, which contains nearly the whole mass of atom. The centre is called nucleus.
* The size of nucleus is very small compared to an atom.
* Electrons revolve round the nucleus.
The Rutherford’s Experiment is also known as Geiger-Marsden Experiment.
Bohr's model of atom:
According to Bohr’s theory:
i. The atom consists of a small positively charged nucleus at its centre.
ii. The whole mass of the atom is concentrated at the nucleus and the volume of nucleus is smaller than the volume of the atom by a ratio of about 1:10.
iii. The nucleus contains all the protons and neutrons of the atom.
iv. The electrons of the atom revolve round the nucleus in definite circular paths known as orbits which are designated as K, L, M, N or numbered as n=1,2,3,4 outward from the nucleus.
v. Each orbit is associated with a fixed amount of energy. Therefore, these orbits are also known as energy levels or energy shells.