Intext Questions

Q.1     What are canal rays?
Sol.      Canal rays are positively charged radiations. These rays consist of positively charged particles known as protons. They were discovered by Gold stein in 1886.

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Q.2     If an atom contains one electron and one proton, will it carry any charge or not?
Sol.      An electron is a negatively charged particle, whereas a proton is a positively charged particle. The magnitude of their charges is equal. Therefore, an atom containing one electron and one proton will not carry any charge. Thus, it will be a neutral atom.

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Page 49

Q.1    On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Sol.    According to Thomson’s model of the atom, an atom consists of both negatively and positively charged particles. The negatively charged particles are embedded in the positively charged sphere. These
negative and positive charges are equal in magnitude. Thus, by counterbalancing each other’s effect, they make an atom neutral.

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Q.2     On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Sol.      On the basis of Rutherford’s model of an atom, protons (positively-charged particles) are present in the nucleus of an atom.

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Q.3     Draw a sketch of Bohr’s model of an atom with three shells.
Sol.

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Q.4    What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?
Sol.     If the α-scattering experiment is carried out using a foil of a metal rather than gold, there would be no change in the observation. In the α-scattering experiment, a gold foil was taken because gold is malleable and a thin foil of gold can be easily made. It is difficult to make such foils from other metals.

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Page 49

Q.1     Name the three sub-atomic particles of an atom.
Sol.     The three sub-atomic particles of an atom are:
           (i) Protons
           (ii) Electrons, and
           (iii) Neutrons

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Q.2     Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Sol.      Helium atom has two neutrons. The mass of an atom is the sum of the masses of protons and neutrons present in its nucleus. Since helium atom has two protons, mass contributed by the two protons is (2 × 1) u = 2 u. Then, the remaining mass (4 − 2) u = 2 u is contributed by 2u /1u = 2 neutrons.

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Page 50

Q.1     Write the distribution of electrons in carbon and sodium atoms?
Sol.     The total number of electrons in a carbon atom is 6. The distribution of electrons in carbon atom is given by: First orbit or K-shell = 2 electrons
Second orbit or L-shell = 4 electrons
Or, we can write the distribution of electrons in a carbon atom as 2, 4.
The total number of electrons in a sodium atom is 11. The distribution of electrons in sodium atom is given by:
First orbit or K-shell = 2 electrons
Second orbit or L-shell = 8 electrons
Third orbit or M-shell = 1 electron
Or, we can write distribution of electrons in a sodium atom as 2, 8, 1.

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Q.2     If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Sol.      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.

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Page 52

Q.1     How will you find the valency of chlorine, sulphur and magnesium?
Sol.      If the number of electrons in the outermost shell of the atom of an element is less than or equal to 4, then the valency of the element is equal to the number of electrons in the outermost shell . On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than 4, then the valency of that element is determined by subtracting the number of electrons in the outermost shell from 8. The distribution of electrons in chlorine, sulphur, and magnesium atoms are 2, 8, 7; 2, 8, 6 and 2, 8, 2 respectively. Therefore, the number of electrons in the outer most shell of chlorine, sulphur, and magnesium atoms are 7, 6, and 2 respectively. Thus, the valency of chlorine = 8 −7 = 1 The valency of sulphur = 8 − 6 = 2 The valency of magnesium = 2

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Q.1     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?
Sol.      (i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.
(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is 0.

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Q.2     With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Sol.     Mass number of oxygen = Number of protons + Number of neutrons = 8 + 8 = 16
Mass number of sulphur = Number of protons + Number of neutrons
= 16 +16
= 32

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Page 53 

Q.1     For the symbol H, D and T  tabulate three sub – atomic particles found each of them.
Sol.    

Symbol	Proton	Neutron	Electron
H	1	0	1
D	1	1	1

 

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Q.2     Write the electronic configuration of any pair of isotopes and isobars.
Sol.     Two isotopes of carbon are  126Cand146C
           The electronic configuration  of 126C is 2, 4.
           The electronic configuration  of 146C  is 2, 4.
[Isotopes have same electronic configuration ]
4029Caand4018Ar are a pair of  isobars
The electronic configuration  of 4029Ca 2, 8, 8, 2
The electronic configuration  of 4018Ar 2, 8, 8.

Exercise

Q.1     Compare the properties of electrons, protons and neutrons.
Sol.

Electron	Proton	Neutron
(i) Electron are present outside the nucleus of an atom.	(i) Proton are present in the nucleus of an atom.	(i) Neutron are present in the nucleus of an atom.
(ii) Electron are negatively charged	(ii) Proton are positively charged.	(ii) Neutron are neutral.
(iii) The mass of an electron is considered to negligible.	(iii) The mass of a proton is approximately 2000 times as the mass of an electron	(iii) The mass of neutron is nearly equal to the mass  of a proton.

 

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Q.2     What are the limitations of J.J. Thomson’s model of the atom?
Sol.      According to J.J. Thomson’s model of an atom, an atom consists of a positively charged sphere with electrons embedded in it. However, it was later found that the positively charged particles reside at the center of the atom called the nucleus, and the electrons revolve around the nucleus.

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Q.3    What are the limitations of Rutherford’s model of the atom?
Sol.     According to Rutherford’s model of an atom, electrons revolve around the nucleus in fixed orbits. But, an electron revolving in circular orbits will not be stable because during revolution, it will experience
acceleration. Due to acceleration, the electrons will lose energy in the form of radiation and fall into the
nucleus. In such a case, the atom would be highly unstable and collapse.

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Q.4     Describe Bohr’s model of the atom.
Sol.     Bohr’s model of the atom Niels Bohr proposed the following postulates regarding the model of the atom.
           (i) Only certain orbits known as discrete orbits of electrons are allowed inside the atom.
           (ii) While revolving in these discrete orbits, the electrons do not radiate energy.These discrete orbits or
shells are shown in the following diagram.

The first orbit (i.e., for n = 1) is represented by letter K. Similarly, for n = 2, it is L − shell, for n = 3, it is M − shell and for n = 4, it is N − shell. These orbits or shells are also called energy levels.

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Q.5     Compare all the proposed models of an atom given in this chapter
 Sol.

Thomson’s model	Rutherford’s model	Bohr’s model
An atom consists of a positively charged sphere with electrons embedded in it.	An atom consists of a positively charged particles concentrated at the center known as the nucleus. The size of the nucleus is very small as compared to the size of the atom. The electron revolve around the nucleus in well – defined orbits.	There are only certain orbits know as discrete orbits inside the atom in which electrons revolve around the nucleus. Electrons do not radiate energy while revolving.

 

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Q.6     summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Sol.     The rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below.
(i) The maximum number of electrons that a shell can accommodate is given by the formula  ‘2n2‘, where ‘n’ is the orbit number or energy level index (n = 1, 2, 3…).
The maximum number of electrons present in an orbit  of n = 1 given by
2n2=2×12=2
Similarly, for second orbit, it is  2n2=2×22=8
For third orbit, it is 2n2=2×32=18
And so on …..
(ii) The outermost orbit can be accommodated by a maximum number of  8 electrons.
(iii) Shells are filled with electrons in a step wise manner i.e., the outer shell is not  occupied with electrons unless the inner shells are completely filled with electrons.

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Q.7     Define valency by taking examples of silicon and oxygen.
Sol.      The valency of an element is the combining capacity of that element. The valency of an element is determined by the number of valence electrons present in the atom of that element. If the number of
valence electrons of the atom of an element is less than or equal to four, then the valency of that element
is equal to the number of valence electrons. For example, the atom of silicon has four valence electrons.
Thus, the valency of silicon is four. On the other hand, if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is obtained by subtracting the number of valence electrons from eight. For example, the atom of oxygen has six valence electrons. Thus, the valency of oxygen is (8 − 6) i.e., two.

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Q.8     Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
Sol.      (i) Atomic number
The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.
           (ii) Mass number
The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.
           (iii) Isotopes And, the percentage of isotope is (100 − 10) % = 90%.

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Q.12     If Z = 3, what would be the valency of the element? Also, name the element.
Sol.        By Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron).
Therefore, the element with Z = 3 is lithium.

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Q.13     Composition of the nuclei of two atomic species X and Y are given as under X Y
Protons = 6 6
Neutrons = 6 8
Give the mass numbers of X and Y. What is the relation between the two species?
Sol.        Mass number of X = Number of protons + Number of neutrons
= 6 + 6
= 12
Mass number of Y = Number of protons + Number of neutrons
= 6 + 8
= 14
These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.

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Q.14     For the following statements, write T for ‘True’ and F for ‘False’.
             (a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
             (b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
             (c) The mass of an electron is about 1/2000 times that of proton.
             (d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Sol.       (a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (F)
(b) A neutron is formed by an electron and a proton combining together. Therefore, itis neutral. (F)
(c) The mass of an electron is about 1/2000 times that of proton. (T)
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (T)

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Q.15     Put tick ( – ) against correct choice and cross ( x ) against wrong choice in the following question: Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
             (a) Atomic nucleus
             (b) Electron
             (c) Proton
             (d) Neutron
Sol.       Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus ( – )
(b) Electron ( x )
(c) Proton ( x )
(d) Neutron ( x )

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Q.16     Put tick ( – ) against correct choice and cross ( x ) against wrong choice in the following
question: Isotopes of an element have
             (a) the same physical properties
             (b) different chemical properties
             (c) different number of neutrons
             (d) different atomic numbers
Sol.       Isotopes of an element have
(a) the same physical properties ( x )
(b) different chemical properties ( x )
(c) different number of neutrons ( – )
(d) different atomic numbers ( x )

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Q.17     Put tick ( – ) against correct choice and cross ( x ) against wrong choice in the
following question:
Number of valence electrons in Cl− ion are:
             (a) 16
             (b) 8
             (c) 17
             (d) 18
Sol.       Number of valence electrons in Cl− ion are:
(a) 16 ( x )
(b) 8 ( – )
(c) 17 ( x )
(d) 18 ( x )

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Q.18     Which one of the following is a correct electronic configuration of sodium?
              (a) 2, 8
              (b) 8, 2, 1
              (c) 2, 1, 8
              (d) 2, 8, 1
Sol.        (d) The correct electronic configuration of sodium is 2, 8, 1.
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Exemplar

Multiple Choice Questions 

Q.1 Which of the following correctly represent the electronic distribution in the Mg atom?

(a) 3, 8, 1

(b) 2, 8, 2

(c) 1, 8, 3

(d) 8, 2, 2

Sol. (b)

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Q.2 Rutherford’s ‘alpha (α) particles scattering experiment’ resulted in to discovery of

(a) Electron

(b) Proton

(c) Nucleus in the atom

(d) Atomic mass

Sol. (c)

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Q.3 The number of electrons in an element x is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?

(a) ³¹ ₁₅ x

(b) ³¹ ₁₆ x

(c) ¹⁶ ₁₅ x

(d) ¹⁵ ₁₆ x

Sol. (a)

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Q.4 Dalton’s atomic theory successfully explained

(i) Law of conservation of mass

(ii) Law of constant composition

(iii) Law of radioactivity

(iv) Law of multiple proportion

(a) (i), (ii) and (iii)

(b) (i), (iii) and (iv)

(c) (ii), (iii) and (iv)

(d) (i), (ii) and (iv)

Sol. (d)

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Q.5 Which of the following statements about Rutherford’s model of atom are correct?

(i) Considered the nucleus as positively charged

(ii) Established that the α–particles are four times as heavy as a hydrogen atom

(iii) Can be compared to solar system

(iv) Was in agreement with Thomson’s model

(a) (i) and (iii)

(b) (ii) and (iii)

(c) (i) and (iv)

(d) Only (i)

Sol. (a)

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Q.6 Which of the following are true for an element?

(i) Atomic number = number of protons + number of electrons

(ii) Mass number = number of protons + number of neutrons

(iii) Atomic mass = number of protons = number of neutrons

(iv) Atomic number = number of protons = number of electrons

(a) (i) and (ii)

(b) (i) and (iii)

(c) (ii) and (iii)

(d) (ii) and (iv)

Sol. (d)

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Q.7 In the Thomson’s model of atom, which of the following statements are correct?

(i) The mass of the atom is assumed to be uniformaly distributed over the atom

(ii) The positive charge is assumed to be uniformaly distributed over the atom

(iii) The electrons are uniformaly distributed in the positively charged sphere

(iv) The electrons attract each other to stabilise the atom

(a) (i), (ii) and (iii)

(b) (i) and (iii)

(c) (i) and (iv)

(d) (i), (iii) and (iv)

Sol. (a)

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Q.8 Rutherford’s α–particle scattering experiment showed that

(i) Electrons have negative charge

(ii) The mass and positive charge of the atom is concentrated in the nucleus

(iii) Neutron exists in the nucleus

(iv) Most of the space in atom is empty

Which of the above statements are correct?

(a) (i) and (iii)

(b) (ii) and (iv)

(c) (i) and (iv)

(d) (iii) and (iv)

Sol. (b)

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Q.9 The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?

(a) 13

(b) 10

(c) 14

(d) 16

Sol. (b)

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Q.10 Identify the Mg²⁺ ion from the Fig.where, n and p represent the number of neutrons and protons respectively

Sol.(d)

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Q.11 In a sample of ethyl ethanoate (CH₃COOC₂H₅) the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?

(a) One of the oxygen atoms has gained electrons

(b) One of the oxygen atoms has gained two neutrons

(c) The two oxygen atoms are isotopes

(d) The two oxygen atoms are isobars.

Sol. (c)

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Q.12 Elements with valency 1 are

(a) Always metals

(b) Always metalloids

(c) Either metals or non-metals

(d) Always non-metals

Sol. (c)

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Q.13 The first model of an atom was given by

(a) N. Bohr

(b) E. Goldstein

(c) Rutherford

(d) J.J. Thomson

Sol. (d)

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Q.14 An atom with 3 protons and 4 neutrons will have a valency of

(a) 3

(b) 7

(c) 1

(d) 4

Sol. (c)

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Q.15 The electron distribution in an aluminium atom is

(a) 2, 8, 3

(b) 2, 8, 2

(c) 8, 2, 3

(d) 2, 3, 8

Sol. (a)

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Q.16 Which of the following in Fig. do not represent Bohr’s model of an atom correctly?

(a) (i) and (ii)

(b) (ii) and (iii)

(c) (ii) and (iv)

(d) (i) and (iv)

Sol. (c)

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Q.17 Which of the following statement is always correct?

(a) An atom has equal number of electrons and protons.

(b) An atom has equal number of electrons and neutrons.

(c) An atom has equal number of protons and neutrons.

(d) An atom has equal number of electrons, protons and neutrons

Sol. (a)

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Q.18 Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order

(i) Rutherford’s atomic model

(ii) Thomson’s atomic model

(iii) Bohr’s atomic model

(a) (i), (ii) and (iii)

(b) (ii), (iii) and (i)

(c) (ii), (i) and (iii)

(d) (iii), (ii) and (i)

Sol. (c)