STRUCTURE OF THE ATOM, CLASS 9 TH CBSE, NCERT SOLUTION

                              Page no 47

1-   What are canal rays ?

Ans- Canal rays is a beam of positive charged particles (positive ions),which originates from anode and directed towards cathode under the influence of high voltage.It is discovered by Goldstein, it lead him to discover protons after this experiment.

2-   If an atom contains one electron and one proton. Will it carry any charge or not ?

 Ans- charge in one electron = - 1.6 x 10^-19C

Charge in one proton  = +1.6 x 10^-19C

Net charge of atom  = 0, so an atom containg same number of  protons and electrons will always be neutral.

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3-   On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Ans- According to J.J Thomson’s model, atom is spherical in shape and  negative charge ,electrons are embedded on positive charge  on the same way as seeds are on the edible part of water melon ,In atom total positive charge is equal to total negative charge,so net charge on atom is zero or atom is neutral.

4-   On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nuleus of an atom ?

Ans- On the basis of Rutherford’s model protons are present in the nucleus of an atom.

5-   Draw a sketch of Bohr’s model of an atom with three shells.

Ans-

6-   What do you think would  be the observation of the α- particle scattering experiment is carried out using a foil of a metal other than gold.

Ans- If the observation of the α particle scattering experiment carried out using a foil of a metal other than gold , there wouldn’t be any change in the observation .As it is required thinest foil so that α particles could be penetrated.Gold has the greatest malleability so such a thin foil which is equivalent to the width of few layers of atoms can be made by the gold only.
NCERT Solution of class IX Science 'The Cell'

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7-   Name the three sub-atomic particles of an atom.

Ans- Three sub-atomic particles of an atom are

(i)Protons (ii) Neutron (iii) Electron

8-   Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have ?

Ans- Atomic mass  = number of protons + number of electrons

Atomic mass of He = 4 u

Number of protons in it =2

So, number of neutrons = atomic mass – number of protons  = 4 – 2 = 2

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9-   Write the distribution of electrons in carbon and sodium atoms.

Ans- No of electrons in carbon  and sodium atom have 6 and  11 respectively.

So,electrons distribution of them will be as follows

C → 2, 4

Na →2, 8, 1

10- If K and L shells of an atom are full , then what would be the total number of electrons in the atom ?

Ans- Maximum electrons K shell can accommodate = 2n² = 2 x1² = 2

Maximum electrons L shell can hold = 2 x 2²=8

So, total number of electrons the atom will have =2 + 8 = 10

11- Have will you find the valency of chlorine , sulphur and magnesium ?

Ans- Atomic numbers of chlorine, sulphur and magnesium are 17, 16 and 12 respectively ,so their electronic structure will be as follows. All the elements have a tendency of making an octet in their outermost orbit and whatever exchange of electrons is taken place in making it is known as valency.

Cl → 2,8,7

The valency of chlorine = - 1

S →2, 8, 6

The valency of sulphur = - 2

Mg → 2, 8, 2

The valency of magnesium = +2

12- 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 of the atom = number of protons= number of electrons in it

So, atomic number of the given atom =8

(ii) Charges on one electron and on one protons are equal and opposite to each other,there numbers in atom are equal so net charge on atom will be zero.

13- With the help of Table 4.1. Find out  the mass number of oxygen and sulphur atom.

Ans- In the table the  number of protons in oxygen=8 and number of neutrons = 8, so mass number of oxygen = 8 + 8 = 16 u. Number of protons and neutrons in sulphur are each of 16, so mass number of sulphur = 16 + 16 = 32 u

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14- For the symbol H,D and T tabulate three sub – atomic particles found in each of them.

Ans- H, D and T are  the symbols used for three isotopes of the hydrogen of the name protium, duetirium and tritium respectively, The atomic number of each one is 1 and their atomic masses are 1,2 and 3 respectively . So number of subatomic particles in them will be as following.

H→ neutrons = 1 – 1 = 0, protons =1, electrons=1

D→ neutrons = 2- 1 = 1, protons = 1, electrons =1

T→nuetrons = 3 – 1 = 2, protons = 1, electrons =1

15- Write the electronic configuration of any one pair of isotopes and isobars.

Ans- Electronic configuration of two isotopes of chlorine are

(35)Cl(17) →2, 8, 7 and (37)Cl(17)→2,8,7

Electronic configuration of a pair of isobars (Ca and Ar) are

(40)Ca(20)→2,8,8,2 and (40) Ar(18)→ 2, 8, 8

EXERCISES

  1-Compare the properties of electrons, protons and neutrons.

Ans-

2-What are the limitations of J.J.Thompson’s model of the atom ?

Ans- According to J.J Thomson’s atomic model electrons are embedded on the positive charge but later it was found that electrons revolves around the nucleus and positive charge protons resides in the nucleus of the atom.

3-What are the limitations of Rutherford’s model of the atom ?

 Ans- According to Rutherford,s model of the atom, the electrons revolves around the nucleus in fixed and circular orbital where it will be accelerated continuously ,it will loss energy in the form of radiations as electron is a charged particle  as a result  ultimately  will collapse at the nucleus and atom will be finished,while atom is a stabilized particle.

4-Describe Bohr’s model of the atom.

Ans- According to Bohr’s model all the electrons revolves in discrete orbitals, the electrons filled up in these orbits as per the rule 2n², where n is the number of orbital. He named these orbitals , first orbital,K, second orbital, L , third orbital, M and so on.., the number of electrons in these orbitals are fixed.These are discrete orbitals where electrons revolves but does not radiate its energy.

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

6-Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Ans-The electrons accommodate in the shells of atom as per the rule 2n²(n=1,2,3,4,5….),where n is the number of orbitals or energy levels ,electrons are filled up in the orbitals in stepwise ways. Outer shell is not occupied by the electrons unless inner orbital is filled. The electrons are filled in first, second, third orbital…..and so on as per following ways.

First orbital→n=1, 2n² = 2 x 1² = 2

Second orbital→n=2, 2n² =2 x 2²= 8

Third orbital→n=3, 2n² = 2 x 3²= 18

→on the same way other orbitals also filled.

7-Define valency by taking examples of silicon and oxygen.

Ans-

Valency- The tendency of atoms of an element to make octet in its outer most orbit and transaction of electrons in making this octet is known as valency of element.The valency of element is decided by the electrons available in the outermost orbit known as valence electrons,if the valence electrons are more than 4 then valency is calculated by subtracting these from 8 and if valence electrons are less than 4 or equal to 4 then the valency will be the same as the valence electrons.

As an example silicon(Si) has atomic number of 14,so in its outermost orbit there will be 4 electrons, so its valency will be 4.

Oxygen has atomic number of 8,so its valence electrons are 6, then its valency = 8 -6 =2.

8-Explain with examples (i) Atomic number (ii) Mass number (iii) Isotopes and (iv)Isobars . Give any two uses of isotopes .

Ans-

(i)Atomic number-The total number of positive charge at the nucleus is known as atomic number which is equivalent to the number of protons,as an example atomic number of hydrogen is 1 and atomic number of oxygen is .

(ii)Mass number- The sum of number of protons and neutrons at the nucleus is known as mass number,as an example oxygen number of protons =8 and number of neutrons =8, so the mass number of oxygen atom = 8 +8 =16 u.

(iii)Isotops- The pair of elements which has same atomic numbers  but have different atomic masses are known as isotopes, as an example hydrogen has three isotopes ,protium(1H1), duetirium (2H1) and tritium (3H1) which have the same atomic number,1 and different atomic masses 1,2 and 3 respectively.

(iv)Isobars-The pair of elements which have the same atomic masses but different atomic number,as an example argon,40Ar18 and calcium, 40Ca20.

Uses of Isotopes- One of the isotopes of iodine is used for the treatment of goiter.One of the isotopes of uranium i.e U(235) is used for the treatment of cancer.

9-Na⁺ has completely filed K and L shells . Explain .

Ans –The electronic distribution of Na atom

→K= 2, L= 8, M= 1

Na⁺ ion is formed after freeing one electron which exist in the outermost orbital(M)

So,Na⁺ has  K and L shells  completely filed

10-If bromine atom is available in the form of ,say, two isotopes 79Br35(49.7%) and 81Br35(50.3%), calculate the average atomic mass of bromine atom.

Ans –

Average atomic mass of bromine

=49.7 % of 79 + 50.3% of 81

=(49.7 x 79)/100 + (50.3 x 81)/100

=(3926.3 +4074.3)/100

= 80.006 u

11-The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 16X8 and18X8 in the sample ?

Ans-Let the percentage of 16X8  = a %

Pecentage of another isotope 18X8 = (100 – a)%

Average atomic mass of X

= a % of 16 X 8 + (100 – a)% of 18 X 8

a % of 16 X 8 + (100 – a)% of 18 X 8 = 16.2

a % of 16 + (100 – a)% of 18 = 16.2

16a/100 – 18(100 – a)/100 = 16.2

→a = 90

So ,percentage of 16 X 8 in the sample = 90 %

And percentage of 18 X 8 = 100 – 90 = 10 %

12-If Z = 3,what would be the valency of the element ? Also , name the element.

Ans- Atomic number,Z =3 So, number of electrons =3

Its electronic structure→2,1

So, its valency = 1

The element is lithium(Li)

13-Composition of the nuclei of two atomic species X and Y are given as under.

                              X        Y

Protons =            6        6

Neutron =           6        8

Give the mass numbers of X and Y .What is the relation between the two species ?

Ans- Mass number of X = Protons + Nuetrons

                                           = 6 + 6 = 12 u

          Mass number of Y = 6 + 8 = 14 u

14-For the following stataments,write T for True and F for false .

(a)J.J Thomson proposed that the nucleus of an atom contains only nucleons.

(b)A neutrons 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 for making tincture iodine. Which is used as a medicine.

Ans-(a)False  (b)False (c) True (d) True

Put tick(√) against correct choice and cross (x) against wrong choice in questions 15, 16 and 17.

15-Rutherford’s alpha-particle scattering experiment was responsible for the discovery of

(a)Atomic nucleus                 (b)Electron

(c)Proton                                 (d)Neutron

Ans-(a) Atomic nucleus

16-Isotopes of an element have

(a)the same physical properties

(b)different chemical properties

(c)different number of neutrons

(d)different  atomic numbers

Ans- (c)different number of neutrons

17-Number of valence electrons in Cl^- ion are:

(a)16           (b)8             (c)17           (d)18

Ans-(b)

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

Ans-(d)

STUDY NCERT SOLUTION OF THE LESSON "GRAVITATION"

https://futurepointstudy.blogspot.com/2018/11/gravitation-class-9-th-cbse-ncert.html

19-Complete the following table.

Ans-

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