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Question
(a) Describe the Rutherford's model of an atom. State one drawback of Rutherford's model of the atom.
(b) The mass number of an element is 23 and it contains 11 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?
(a) Describe the Rutherford's model of an atom. State one drawback of Rutherford's model of the atom.
(b) The mass number of an element is 23 and it contains 11 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?
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Solution
(a) Rutherford's model of atom-
Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. He conducted an experiment in which a thin gold foil was bombarded with fast moving alpha (α) particles.
• He selected a gold foil because he wanted a layer as thin as possible. The gold foil was about 1000 atoms thick.
• The particles used for bombarding the gold foil were doubly-charged helium ions. Since, they have a mass of 4 u, the fast-moving α particles have a considerable amount of energy.
• 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.
But the α particle scattering experiment produced totally unexpected results.
The following observations were made:
(i) Most of the fast moving α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by the foil by small angles.
(iii) Surprisingly, one out of every 12000 particles appeared to rebound.
Conclusions:
(i) The first observation led to the conclusion that most of the atom is hollow.
(ii) The small angle deviation of the α particles confirms the presence of a positively charged centre called nucleus.
(iii) The third observation confirms that the nucleus of an atom is solid as the ray returns on its path and is very small in size because 1 out of 12000 rays returns.
Drawbacks of Rutherford’s model of the atom
The major drawback of Rutherford’s model is that it does not explain the stability of an atom. The orbital revolution of an electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles 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 in the form that we know. However, we know that atoms are stable. So, this model could not explain this reasoning.
(b) Given:
Mass no. = 23
No. of electrons = 11,
No of protons = No. of electrons = 11
No. of neutrons = 23 – 11 = 12
Atomic number of the element = 11
(a) Rutherford's model of atom-
Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. He conducted an experiment in which a thin gold foil was bombarded with fast moving alpha (α) particles.
• He selected a gold foil because he wanted a layer as thin as possible. The gold foil was about 1000 atoms thick.
• The particles used for bombarding the gold foil were doubly-charged helium ions. Since, they have a mass of 4 u, the fast-moving α particles have a considerable amount of energy.
• 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.
But the α particle scattering experiment produced totally unexpected results.
The following observations were made:
(i) Most of the fast moving α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by the foil by small angles.
(iii) Surprisingly, one out of every 12000 particles appeared to rebound.
Conclusions:
(i) The first observation led to the conclusion that most of the atom is hollow.
(ii) The small angle deviation of the α particles confirms the presence of a positively charged centre called nucleus.
(iii) The third observation confirms that the nucleus of an atom is solid as the ray returns on its path and is very small in size because 1 out of 12000 rays returns.
Drawbacks of Rutherford’s model of the atom
The major drawback of Rutherford’s model is that it does not explain the stability of an atom. The orbital revolution of an electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles 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 in the form that we know. However, we know that atoms are stable. So, this model could not explain this reasoning.
(b) Given:
Mass no. = 23
No. of electrons = 11,
No of protons = No. of electrons = 11
No. of neutrons = 23 – 11 = 12
Atomic number of the element = 11
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