CBSE Class 9 Chemistry Structure Of Atom Worksheet Set B

CBSE Class 9 Chemistry Worksheet - Structure of Atom (1)

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 Exam Questions NCERT Class 9 Science Chapter 4 Structure of the Atom

Question. What kind of elements have a tendency to gain electrons? What are they commonly called?
Ans : The elements having 5, 6 or 7 valence electrons have the tendency to gain electrons. They are commonly called non-metals.

Question. In what way the Rutherford proposed atomic model?
Ans :Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits.
There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centered in the nucleus.

Question. Give reasons :
(i) Mass number of an atom excludes the mass of an electron.
(ii) Nucleus of an atom is charged.
(iii) Alpha-particle scattering experiment was possible by using gold foil only and not by foil of any other metal.
Ans :
(i) Mass number of an atom excludes the mass of an electron because electrons have negligible mass in comparison to protons and neutrons.
(ii) Nucleus of an atom consists of protons and neutrons. Protons are positively charged particles. So, the nucleus of an atom is charged.
(iii) Because an extremely thin film was required for the experiment and it was only possible by using gold, as gold is a highly malleable metal.

Question. Is it possible for the atom of an element to have one electron, one proton and no neutron? If so, name the element.
Ans : Yes, it is true for hydrogen atom which is represented as 11 H.

Question. Which particles were discovered by Chadwick, Thomson and Goldstein?
Ans :
(i) Chadwick – Neutron
(ii) Thomson – Electron
(iii) Goldstein – Proton

Question. Is an atom electrically neutral?
Ans : Yes, because number of protons (+ve charge) inside the nucleus are equal to the number of electrons (–ve charge) outside the nucleus.

Question. Why do some elements possess fractional atomic mass?
Ans : Some elements possess fractional atomic mass because they occur in nature in different isotopic forms. So, accordingly their average mass is calculated.

Question. How was the neutron discovered?
Ans : Atom was considered to have electrons and protons only till 1920. But electrons have negligible mass. Then entire mass of the atom was considered to be only due to the protons present in it. In 1920, Rutherford found that atomic masses of all elements are higher than the mass of all protons and electrons in their atoms. Chadwick discovered the presence of an electrically neutral particle inside the atom in 1932.

Question. Define isotopes. Why do isotopes have same atomic number but different mass number? Explain with the help of an example.
Ans : Atoms of the same element, having the same atomic number but different mass numbers are called isotopes. Isotopes have same atomic number but different mass number because they contain different number of neutrons.
For example : In nature chlorine occurs in two isotopic forms; 17Cl35 and 17Cl37.
Here atomic number of both the atoms is same but due to the difference in the number of neutrons, their mass number is different.

Question. What type of charge is present on the nucleus of an atom?
Ans : Positive charge.

Question. Explain the essential features of experiment that led to the discovery of isotopes.
Ans : The m e values of the ionized atoms were determined by studying their deflections under the influence of electrical and magnetic fields. Ionised atoms of an element do not give a unique mass and chemical properties of these atoms are identical.

Question. State the major drawback in Rutherford’s model of an atom. Mention two features of Bohr’s model which helped compensate this drawback.
Ans : The major drawback of Rutherford’s model of an atom is that it does not explain the stability of an atom. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. So, revolving electron wouldlose energy and finally fall into the nucleus.
Two features of Bohr’s model which helped to resolve this drawback : Only certain special orbits known as discrete orbits of electrons are allowed inside the atom. While revolving in these discrete orbits, the electrons do not radiate energy.

Question. What ar canal rays? Who discovered them? What is the charge and mass of canal ray?
Ans : New radiations in a gas discharge tube which are positively charged are known as canal rays. They were discovered by E. Goldstein. Charge on canal rays is positive and its mass is one unit.

Question. Define the following terms :
(i) Electronic configuration
(ii) Valence shell
(iii) Valency
Ans :
(i) The distribution of electrons amongst different orbits of an atom is known as electronic configuration.
(ii) The outermost shell of an atom is called its valence shell.
(iii) The combining capacity of an atom is called its valency or the number of electrons lost or gained by an atom to acquire noble gas configuration.

Question. If K and L shells of an atom are full, then what would be the total number of electrons in the atom? What is the valency of this element? Name the element.
Ans : The maximum numbers of electrons that can occupy K and L shells of an atom are 2 and 8 respectively. Therefore, if K and L shells of an atom are full then the total number of electrons in the atom would be 2 + 8 = 10 electrons. So, the valency of this element is zero. The element is neon (Ne).

Question. State the charge and mass of a neutron.
Ans : Neutron has no charge and its mass is equal to that of a proton.
Ans :
(i) Most of the space inside the atom is empty because most of the a -particles passed through the gold foil without getting deflected.
(ii) Very few particles are deflected from their path, indicating that positive charge of the atom occupies very little space.
(iii) A very small fraction of particles was deflected by 180°, indicating that all the positive charge and mass of the gold atom were concentrated in a small volume within the atom.

Question. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom? And (ii) what is the charge on the atom?
Ans : (i) Atomic number = Number of protons = 8
(ii) The charge of the atom is zero, as total numbern of positive charge is equal to total number of negative charge. Number of protons = Number of electrons 8 = 8

Question. What is the limitation of J.J. Thomson’s model of an atom?
Ans : The major limitation of J.J. Thomson’s model is that it does not explain how positively charged particles are shielded from negatively charged particles, without getting neutralized.

Question. Name an element which has one electron, one proton and no neutron.
Ans : Hydrogen atom (1H1) has one electron, one proton and no neutron.

Question. Write drawbacks of Thomson’s model.
Ans : Drawbacks of Thomson’s Model : It could not explain about the stability of an atom, i.e., how both positive and negative charges could remain so close together. It could not explain the results of experiments (such as alpha ray scattering experiment) carried out by other scientists.

Question. Where is neutron located in an atom?
Ans : It is located in the nucleus of an atom.

Question. What is discharge tube?
Ans : A discharge tube is a glass tube about 70 cm long and 5 cm of diameter. Two metal electrodes are sealed at the two ends, one is connected to negative terminal of battery and other to the positive terminal. A side tube is fused at the centre of the glass tube which serves to pump out air from it, using a suction pump.

Question. Who discovered neutron?
Ans : James Chadwick.

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