## Important Questions for CBSE Class 9 Science Chapter 2 Is Matter Around us Pure

### IMPORTANT QUESTIONS

1 MARK QUESTIONS
Question.1.  Define atomic number. How it is denoted ?
Answer.  Number of protons of an atom determines its atomic number. It is denoted by ‘Z’.

Question.2.  What are nucleons ?
Answer.  Protons and neutrons present in the nucleus of an atom are called nucleons.

Question.3.  Which study led to the conclusion that atoms are not indivisible ?
Answer.Study of static electricity and the condition under which electricity is conducted by different substances.

Question.4.Name the particles which determine the mass of an atom. [SAII – 2014]

Question.5.  Electron attributes negative charge, protons attribute positive charge. An atom has both but why there is no charge ? [SAII- 2014]
Answer.  The negative and positive charges of electrons and protons respectively are equal in magnitude. So, the atom as a whole is electrically neutral.

Question.6.  Write the charge and mass of an electron. [SAII – 2014]
Answer.  Its mass is 1/2000 mes that of proton and it is negatively charged.

Question.7.  What type of charge is present on the nucleus of an atom ?

Question.8.  Name one element, the nucleus of which does not have any neutron.

Question.9.  Who discovered neutron ?

Question.10.  What is the charge and mass of alpha particle ?
Charge = + 2 units
Mass = 4 units

Question.11.  Why did Rutherford select a gold foil for his experiment ? [SAII – 2014]
Answer.  Rutherford selected a gold foil for his experiment as he wanted a very thin layer and gold is highly malleable.

Question.12.  An uncharged particle is found in the nucleus of an atom. Identify this uncharged particle.  [SAII -2014]

Question.13.  Name the scientist who concluded That the size of nucleus is very small as compared to the size of atom.

Question.14.Write the names of three elementary particles which constitute an atom.

Question.15.  Helium atom has two electrons in its valence shell but its valency is not two. Explain.  [SAII -2014]
Answer.  Since its outermost-shell is completely filled, its combining capacity or valency is zero.

Question.16.  What do you understand by an octet of electrons in the valence shell ?
Answer.  When the outermost-shell of an atom of an element Has eight electrons, it is said to possess an octet.

Question.17.  The sulphate of aluminium has a chemical formula AI2(S04)3. State the valency of aluminium.  [SAII-2014]

Question.18.  Write the symbols of two isotopes of uranium.

Question.19.  Write any two observations which support the fact that atoms are divisible.
Answer.  Discovery of electrons and protons.

Question.20.  If an atom contains one electron and one proton, will it carry any charge or not ?  [SAII-2011]
Answer.  It will not carry any charge because the positive charge on the proton neutralise the negative charge on the electron.

Question.21.  On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

2 MARKS QUESTIONS
Question.1.Describe briefly Thomson’s model of an atom. [SAII – 2014]
Answer.  Thomson’s model of an atom :

1. An atom consists of a positively charged sphere and the electrons are embedded in it.
2. The negative and positive charges are equal in magnitude. So the atom as a whole is electrically neutral.

Question.2
(a) (i) Name the scientist who discovered neutrons.
(ii) State the charge and mass of a neutron.
(iii)Where is neutron located in an atom ?
(b) Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have? [SAII – 2014]
(ii) Neutron has no charge and its mass is equal to that of a proton.
(iii) It is located in the nucleus of an atom.
(b) Two.

Question.3.Mention two uses of isotopes in the field of medicine.

1. An isotope of uranium is used as a fuel in nuclear reactors.
2. An isotope of cobalt is used in the treatment of cancer.
3. An isotope of iodine is used in the treatment of goitre.

Question.4.  Write two differences between isobars and isotopes.

Question.5.Give four characteristics of Isotopes.

1.  All isotopes of an element consist of the same number of protons inside their nuclei. Hence, they have the same atomic number.
2. All isotopes of an element consist of different number of neutrons in their nuclei.  Hence, they have different mass number.
3.  All isotopes of an element give identical chemical reactions.
4. Isotopes of an element have same electronic configuration.

Question.6.Give drawbacks of Thomson’s model.

1.  It could not explain about the stability of an atom, i.e., how both positive and negative charges could remain so close together.
2. It could not explain the results of experiments (such as alpha ray scattering experiment) carried out by other scientists.

Question.7.  Draw a sketch of Bohr’s model of an atom with three shells. [SAII  – 2017]

Question.8.What do you think would be the observation if the a-particle scattering experiment is carried out using a foil of a metal other than gold ?
If experiment is carried out by using a foil of some other metal, then results would not have been such as most of the of particles will be deflected back. Gold can be beaten to an extremely thin sheet as it is extremely malleable metal. With the use of extremely thin gold foil, Rutherford could predict that, “Most of the space inside the atom is empty and nucleus is positively charged occupying a small volume within the atom”.

Question.9.  Name the three subatomic particles of an atom. [SAII -2011]

1. Electrons [negatively charged particles] which revolve around the nucleus.
2. Protons [positively charged particles] which are present in the nucleus.
3. Neutrons [having no charge] which are present in the nucleus.

Question.10.  Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons it have ? [SAII – 2011]
Mass number = Number of protons + Number of neutrons 4 = 2 + No. of neutrons
Number of neutrons = (4 – 2) = 2.

Question.11.  Write the distribution of electrons in carbon and sodium atoms. [SAII -2011]

Question.12.  If K and L shells of an atom are full, then what would be the total number of electrons in the atom? [SAII-2011]
Number of electrons in full K-shell = 2 Number of electrons in full L-shell = 8
... Total number of electrons in an atom is 2 + 8 = 10 electrons.

Question.13.  If number of electrons in an Mom 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 ? [SAII -2011]
(i) Atomic number
= Number of Protons = 8
(ii) The charge of the atom is zero, as total number of positive charge is equal to total number of negative charge.
Number of Protons = Number of Electrons
8 = 8 ,

Question.14.  If Z = 3, what would be the valency of the element ? Also, name the element [SAII – 2011]
Atomic number Z = 3
Electronic configuration = 2(K), T(L)
Valence shell has 1 electron in the outermost shell, so valency of element is 1. The element is Lithium.

Question.15
Composition of the nuclei of two atomic species X and Y are given as under :

Give the mass numbers of X and Y, What is the relation between the two species ?
Mass number of X = No. of Protons + No. of Neutrons !
= 6 + 6 = 12 u
Mass number of Y = No. of Protons + No. of Neutrons
= 6 + 8 = 14 u
The species X and Y are isotopes, as they have same atomic number but different mass number.

3 MARKS QUESTIONS

Question.1.  Elaborate the postulates put forward by E. Rutherford about the structure of atom based on the a-particle scattering experiment. [SAII – 2014]

1. Most of the space inside the atom is empty because most of the alpha particles passed through the gold foil without getting deflected.
2. Very few particles are deflected from their path, indicating that positive charge of the atom occupies very little space.
3. 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.2

1. What are canal rays ? Who discovered them ? What is the charge and mass of canal ray ?
2. How are the canal rays different from electrons in terms of charge and mass ?  [SAII -2012]

1. 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.
2. Electrons are negatively charged particles, mass of which is approximately 1/2000 that of – canal rays.

Question.3.  List three main features of Rutherford’s nucleus model of an atom. [SAII – 2012]
Answer.  On the basis of a-particle scattering experiment, Rutherford proposed a model of atom. According to him :

1. There is positively charged centre in an atom called the nucleus which contains the whole mass of the atom.
2. The electrons revolve around the nucleus.
3. The size of the nucleus is very small as compared to the size of the atom.

Question.4.  List three subatomic particles of an atom. Compare them on the basis of relative mass and charge in a tabular form.4 [SAII -2014]

Question.5.  List the three observations made by Rutherford in the experiment of scattering of a-particles . by a gold foil.

1. Most of the fast moving a-particles passed straight through the gold foil.
2. Some of the a-particles were deflected by the foil by small angles. .
3. Surprisingly one out of every 12000 particles appeared to rebound.

Question.6.  Write the maximum number of electrons which can be accommodated in K, L, M, N shells and give the rule on basis of which it is decided. [SAII -2011]

Question.7.  (a) Which of the following electronic configurations are not possible ? Give reasons.  (i) X: 2, 8, 4 (ii) Y : 3, 8, 2 (iii) Z : 2, 8, 9
(b) Write electronic configurations of the following elements and predict their valencies :
Fluorine : 9, Aluminium : 13, Argon : 18. [SAII – 2014]

Question.8.  Give drawbacks of Rutherford’s model.

1.  It could not explain the source of energy required for movement of electrons.
2.  Rutherford’s model could not explain as to why a moving charge (charged electron) does not lose energy and fail into the nucleus.
3. It could not explain about the emission of radiations of different frequencies by different atoms when heated.
4. It could not explain the stability of an atom when charged electrons are moving under the attractive force of positively charged nucleus.

Question.9.  On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.[SAII – 2011]

Thomson proposed that:

1. An atom consists of a positively charged sphere and the electrons are embedded in it.
2. The negative and positive charges are equal in magnitude. So the atom as a whole is electrically neutral.

Question.10.  How will you find the valency of chlorine, sulphur and magnesium ? [SAII -2011]

Question.11.Valency of magnesium = Number of electrons in the valence shell = 2 Write the electronic configuration of any one pair of isotopes and isobars. [SAII – 2011]

Question.12.  Compare all the proposed models of an atom given in this Chapter.

Question.13

Question.14

5 MARKS QUESTIONS

Question.1.  Write the postulates of Bohr’s model of atom.
Answer.  A Danish physicist, Neil’s Bohr proposed an atomic model in 1913. This model of atom is called Bohr’s model of atom.
Basic postulates of the Bohr’9 atomic model are :

1. In an atom, the electrons revolve around the nucleus in certain definite circular orbits.
These circular orbits are also called energy shells or energy levels.
2. An electron revolving in a particular orbit has a fixed energy. While revolving in discrete orbits the electrons do not radiate energy.
3. The orbits or the energy shells are numbered as n = 1, 2, 3, 4, … etc. or K, L, M, N,… etc. shells starting from the nucleus. These integers 1, 2, 3, 4, … etc. are known as quantum numbers of the orbits.
4. An electron can lose or gain only certain definite energies.

Question. 2.  Number of electrons, protons and neutrons in chemical species A, B, C and D is given below:

Now, answer the following questions :
(a) What is the mass number of A and B ?
(b) What is the atomic number of B ?
(c) Which two elements represent a pair of isotopes and why ?
(d) What is the valency of element C ?
(a) Mass number of A=3 + 4 = 7 Mass number of B = 9 + 8 = 17
(b) Atomic number of B = Number of protons  = 9
(c) Elements C and D represent a pair of isotopes because their atomic numbers are the same.
(d) Electronic configuration of C (8) = 2, 6.  So its valency is 2.

Question.3.  Read the information given in the table and answer the following questions :

(i) How many pairs of isotopes are there amongst the given atoms ?
(ii) Find a pair of isobars.
(iii) Find an element with valency 2.
(iv) Find an element with 5 valence electrons.
(v) Calculate the number of different subatomic particles in atom C.
(i) Two pairs : (a) Elements B and C (b) Elements D and E.
(ii) Elements A and C

(iii) Elements D and E are having valency 2.
(iv)Element A has 5 valence electrons.
(v) Number of protons : 6 Number of neutrons : 8 Number of electrons : 6

Question.4.With the help of labelled diagram describe in brief the Rutherford’s alpha-particle scattering experiment. Write any three important conclusions drawn from the experiment.  [SAII – 2012, 2014]
Rutherford took a very thin gold foil and born bared it with a-particles.He observed that:

1. Most of the fast moving a-particles passed straight through the gold foil.
2. Some of the alpha-particles were deflected by the foil by small angle.
3. Out of every 12000 particles, one appeared to rebound.

From the above observations he concluded :

1. There is a positively charged centre in an atom called the nucleus. Nearly all mass of an
atom resides in the nucleus.
2. The electrons revolve around the nucleus in well defined orbits.
3. The size of the nucleus is very small as compared with the size of the atom.

Question.5 Complete the following table :

APPLICATION BASED QUESTIONS

Question.1.  State the major drawback in Rutherford’s model of an atom. Mention two features of Bohr’s model which helped compensate this drawback. [SAII – 2014]
Answer.  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. Thus, the revolving electron would lose energy and finally fall into the nucleus.
Two features of Bohr’s model which helped compensate this drawback :

1. Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
2.  While revolving in these discrete orbits, the electrons do not radiate energy.

Question.2
.
(a) 8.
(b) 16.
(c) Number of neutrons = Mass number – Atomic number
= 16 â€” 8 = 8.

Question.3.  Define isotopes. Why do isotopes have same atomic number but different mass number ?  Explain with the help of an example. [SAII – 2014]

Question.4
(a) What is the relationship between two elements X and Y whose atomic numbers are 18 and 20 respectively but their mass numbers remain same as 40 ?
(b) Are their chemical properties same or different? Explain.
(c) Which has more number of electrons Na or Na+ ? Why ? [SAII – 2014]
(a) Elements X and Y are isobars.
(b) Chemical properties of an element depends upon its atomic number. Since both the
elements have different atomic numbers, their chemical properties will be different.
(c) Na has more number of electrons because Na+ has lost an electron.

Question.5.  In the Gold foil experiment of Geiger and Marsden, that paved the way for Rutherford’s model of an atom, âˆ¼  1.00% of the a-particles were found to deflect at angles > 50 °. If one mole of a-particles were bombarded on the gold foil, compute the Humber of a-particles that would deflect at angles less than 50 °.

Question.6.  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. [SAII – 2012]
Answer.  The maximum number 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).

Higher Order Thinking Skills (HOTS) Questions

Question.1.  One electron is present in the outermost shell of the atom of an element X. What would be the nature and value of charge on the ion formed if this electron is removed from the outermost shell ?

Question.2.  Find put the valency of the atom represented by the fig. (a) and (b).

Question.3.  Write down the electron distribution of chlorine atom. How many electrons are there in the L-shell ? (Atomic number of chlorine is 17).
Answer. 2, 8, 7. The L-shell has eight electrons.

Question.4.  In the atom of an element X, 6 electrons are present in the outermost shell. If it acquires noble gas configuration by accepting requisite number of electrons, then what would be the charge on the ion so formed ?

Question. 5.  What information do you get from the fig. about the atomic number, mass number and valency of atoms X, Y and Z ? Give your answer in a tabular form.

Question. 6.  The ratio of the radii of hydrogen atom and its nucleus is  105. Assuming the atom and the nucleus to be spherical, (i) What will be the ratio of their sizes ? (ii) If atom is represented by planet earth ‘  Re‘ = 6.4 x  106  m, estimate the size of the nucleus.

Question.7.  In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model ?
Answer.  Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits. There is a positively charged center 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 nuclei is. Whereas Thomson proposed the model of an atom to be similar to a Christmas pudding. The electrons are studded like currants in a positively charged sphere like Christmas pudding and the mass of the atom was supposed to be uniformly distributed.

Question.8.  In response to a question, a student stated that in an atom, the number of protons is greater than the number of neutrons, which in turn is greater than the number of electrons. Do you agree with the statement ? Justify your answer.
Answer.  No, the statement is incorrect. In an atom, the number of protons and electrons is always equal.

Reasoning Questions

Question.1.  Give reason for the following :
(Ð°) Ions are more stable than atoms.
(b) Noble gases show low reactivity.
(c) An atom is electrically neutral. [SAII -2014]
(a) Ions are more stable than atoms because except the atoms of inert gases, the outermost shell of the atoms is incomplete. In order to complete octet, they lose or gain electrons.
(b) The outermost shell of the atoms of noble gases are complete. So they show little chemical
activity. . .
(c) Because number of protons (+ve charge) inside the nucleus are equal to the number of electrons (-ve charge) outside the nucleus.

Question.2.  Why atomic number is more fundamental attribute of an element ?
Answer.Atomic number is more fundamental attribute of element. In fact, elements are defined by the  number of protons they possess. Chemical properties of an element change with the change in  atomic number.

Question.3.  Give reasons:
(Ð°) Mass number of an atom excludes the mass of an electron.
(b) Nucleus of art atom is charged.
(c) Alpha-particle scattering experiment was possible by using gold foil only and not by foil bf any other mfetal. 4 [SAII – 2014]
(a) Mass number of an atom excludes the mass of an electron because electrons have negligible mass in comparison to protons and neutrons.
(b) Nucleus of an atom consists orprotons and neutrons. Protons are positively charged particles. So the nucleus of an atom is charged.
(c) 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.4
(a) An ion M2+ contains 10 electrons and 12 neutrons. What is the atomic number and mass number of the element M ?
(b) Is it possible in an atom to have 12 protons and 13 electrons ? Explain. [SAII -2014]
(c) Why helium gas is inert ?
(a) Atomic number = 12,
Mass number = 24.
(b) No, it is not possible. An atom is electrically neutral. The number of positively charged particles (protons) is always equal to the number of negatively charged particles (electrons).
(c) HeliUm atom has completely filled outermost-shell. Thus, it is inert.

Question.5.  State reason for the following statements:

1.  Some elements possess fractional atomic mass.
2. Isotopes of an element have similar chemical properties.
3.  Noble gases are inert. [SAII – 2012]

1. Some elements possess fractional atomic mass because they occur in nature in different isotopic forms. So accordingly their average mass is calculated.
2. Isotopes of an element have similar chemical properties because they have the same atomic number and valence electrons.
3. The outermost-shell of the atoms of noble gases are completely filled up. So they do not show affinity towards chemical reactions.

Question.6.  Will  35cI and 37cI  have different valencies ? Justify your answer.
Answer.  No,  35cI and 37cI are isotopes of an element.

Question.7.  Why did Rutherford select a gold foil in his a-ray scattering experiment ?
Answer.  Because gold has high malleability.

Question.8.  Is it possible for the atom of an element to have one electron, one proton and no neutron. If so, name the element.

Question.9.  Why do Helium, Neon and Argon have a zero valency ?
Answer.  Helium has two electrons in its only energy shell while Argon and Neon have eight electrons in their valence shells. As these have maximum number of electrons in their valence shells, they do not have any tendency to combine with other elements. Hence, they have a valency equal to zero.

Question.10.  Give reasons for the following :
(Ð°) The valency of Na is 1 and not 7.
(b) When an atom loses or gains electrons it becomes stable.
(a) it is easy for Na atom to lose one electron instead of gaining seven electrons. So valency of Na is one and not seven
(b) For stability of an atom it must have either 2 or 8 electrons in the outermost orbit. So by losing or gaining electrons it reaches to the electronic configuration of the nearest noble gas.

### Important Topics/Areas/Questions which, are frequently asked in the examination

Question.1

(a) Find the number of electrons present in A and B.
(b) Find the number of nucleons present in A and B.
(c) Explain the special*termaused to represent A and B. [SAII – 2014]
(a) The elements A and B have 20 and 18 electrons respectively.
(b) Both have 40 nucleons. ‘
(c) Isobars.

Question. 2.  Write the conclusions drawn by Rutherford for the following observing during his a-scattering experiment:
(a) Most of the alpha-particles passed straight through the gold foil.
(b) Some alpha-particles getting deflected from their path.
(c) Very small fraction of alpha-particles getting deflected by 180 °. [SAII -2014]
(a) Most of the space inside the atom is empty.
(b) It indicates that the positive charge of the atom occupies a very little space.
(c) All the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

Question.3.  Define the following terms:
(a) Electronic configuration
(b) Valence shell
(c) Valency
(a) The distribution of electrons amongst different orbits of an atom is known as electronic configuration.
(b) The outermost-shell of an atom is called its valence shell.
(c) The combining capacity of an atom is called its valency.

Question. 4