Question

Compare all the proposed model of an atom give in this chapter.

Chapter: Structure of atom
Class: 9​

Answer 1

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A comparison between the models of an atom proposed by JJ Thomson, Rutherford and Bohr is given below:-

★Thomson's model of an atom

$\longrightarrow$ An atom consists of sphere (or ball) of positive charge with negatively charged electrons. embedded in it (like the seeds in a watermelon).

$\longrightarrow$ The total positive charge on the sphere is equal to the total negative charge on the electrons due to which an atom is electrically neutral. It has no overall positive or negative electric charge.

Limitation. Thomson's model of an atom could not explain the results of alpha particle scattering experiment carried out by Rutherford.

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★ Rutherford's model of an atom

$\longrightarrow$ Anatomy consists of a very small positively charged nucleus (containing all the protons and neutrons) at its centre with negatively charged electrons revolving around it in circular paths with high speeds

$\longrightarrow$ The electrostatic attraction between the positively charged nucleus and the negatively charged electrons holds the atom together.

$\longrightarrow$ The number of positive protons is equal to the number of negative electrons due to which an atom is electrically neutral. (w) Almost the entire mass of an atom is concentrated in the

nucleus.

$\longrightarrow$ Most of the atom is empty space

Limitation. Rutherford's model of an atom could not explain the stability of the atom because the accelerated circular motion of revolving electrons should make them lose energy by radiation fall into the nucleus and make the atom to collapse quickly (which does not happen at all)

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★ Bohr's model of an atom

$\longrightarrow$ An atom is made up of three particles: electrons, protons and neutrons Electrons have negative charge. protons have positive charge whereas neutrons have no charge they are neutral.

$\longrightarrow$ Due to the presence of equal number of negative electrons and positive protons the atom on the whole is electrically nutral.

$\longrightarrow$ The protons and neutrons a located in a small nucleus at the centre of the atom. Due to the presence of protons, nucleus s positively charged.

$\longrightarrow$ The electrons revolve rapidly around the nucleus in fised circular paths called energy levels of shells (denoted either by the numbers 1,2,3,4,5 aud 6or by the letters K, L, M, N, O, and P) counted from the centre outwands

$\longrightarrow$ Each energy level (or shell) is associatest with a fived amount of energy (the energy increaning from the centre outwards).

$\longrightarrow$ There is no change in the energy of electrons as long as they keep nvolving in the same energy levels.

Advantage. Bohr's model of an atom explains the staletity of stoms by saying that the no loss in the energy revolving electrons of the an as long as they herp revolving in the site cog level due to which the atom

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Answer 2

Compare all the proposed models of an atom.

J. Thomson: An atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude. J. J. Thomson suggested that electrons are embedded in a sphere of a positive charge. Thomson’s model of the atom fails to explain Rutherford’s α-particle scattering experiment in which most of the fast-moving α-particles passed straight through the gold foil.

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E. Rutherford: The gold foil experiments of Rutherford suggested that all the positive charge is located in a very small space which is 10 times the radius of an atom. Therefore, Rutherford gave a model in which electrons are revolving around the nucleus. He found that the size of the nucleus is very small as compared to the size of the atom. Rutherford’s model of the atom failed to explain the stability of the atom.

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Neils Bohr: To explain the stability of an atom and atomic spectra, Bohr suggested that electrons are moving around the nucleus in orbits which have fixed energy shells. There is a loss or gain in energy of an electron when it moves from one orbit to the other. Only certain special orbits known as discrete orbits of electrons are allowed inside the atom. These orbits or shells are represented by the letters K, L, M, N,… or the numbers, n=1,2,3,4,…

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It helps uhh. . . . ♡