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Class IX – Structure of Atom

Important Questions

1. (a) What are canal rays?

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Ans. Canal rays are positively charged radiations which led to the discovery of positively charged subatomic particle called proton.


(b) How are canal rays different from electrons in terms of charge and mass?

View Answer

Ans. Canal rays is a stream of positively charged particles whereas electrons are negatively charged. In terms of mass canal ray’s particles are almost 2000 times as compared to electron.


2. J. Chadwick discovered a subatomic particle which has no charge but has mass nearly equal to that of a proton. Name the particle and give its location in an atom.

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Ans. Neutron

It is located in the nucleus of an atom along with proton.


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

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Ans. According to Thomson’s model of an atom

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude. So the atom is electrically neutral.


4. What are the limitations of J.J Thomson’s model of the atom?

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Ans. According to J.J Thomson’s model of an atom, the electrons are embedded all over in the positively charged spheres. But experiments done by other scientists showed that protons are present only in the centre of the atom and electrons are distributed around it.


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

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Ans. Protons


6. In Rutherford’s model of an atom, fast moving alpha (α)- particles were made to fall on a thin gold foil. State two properties of (α)- particles.

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Ans. (a) Alpha particles are positively charged particles.

(b) They are doubly charged helium ions having a mass number of 4 (consists of 2 protons and 2 neutrons)


7. List three observations of the experiment performed by Rutherford for his model of an atom.

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Ans. From the scattering experiment, Rutherford made the following observations:

1. Most of the alpha particles were able to pass through the gold foil undeflected.

2. Some of these particles were deflected by small angles.

3. A very few (one out of approx. 12000) alpha particles suffered major deflections and even came back in the same direction.


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

How many electrons L-shell can accommodate?

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Ans.

Number of electrons in L-shell

=2n2

=2 x 22

=8


9. Describe Bohr’s model of the atom.

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Ans. Bohr’s model of the atom:

1) Atom has nucleus in the centre

2) Electrons revolve around the nucleus

3) Certain special orbits known as discrete orbits of electrons are allowed inside the atom.

4) While revolving in discrete orbits the electrons do not radiate energy.

5) These orbits or shells are called energy levels

6) These orbits or shells are represented by the letters K, L, M, N or the numbers n – 1, 2, 3, 4


10. For the following statement, write T for True and F for False

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

(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutron – F

(c) The mass of an electron is about 1/2000 times of that of proton. – T

11. What are the limitations of Rutherford’s model of the atom?

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Ans. Following are the limitations of Rutherford’s model of the atom.

1) There is no expected stability in the revolution of the electron in a circular orbit.

2) Charged particles radiate energy when accelerated thus causing the revolving electrons to lose energy and would fall into the nucleus.

3) Hence atoms must be highly unstable. Matter would not exist in their known form which clearly is an assumption as atoms are highly stable.


12. Justify the given statements:

(a) Most of the space in an atom is empty.

View Answer

This can be justified by Rutherford’s α- scattering experiment. Since most of the alpha particles could pass through the sheet, made up of atoms of gold, undeflected, this means that they did not come across any obstruction.

Thus, most of the space in an atom is empty or hollow.


(b) The elements are identified by their atomic numbers and not by their mass numbers.

View Answer

In the study of the atomic structure, we have seen that the isotopes of an element have different mass numbers. In isobars, the atoms of different elements have same mass numbers. However, the atomic numbers of no two elements can be the same. Therefore, the elements are identified by their atomic numbers and not by their mass numbers.


13. (a) Who discovered electron, proton and neutron?

(b) Compare the properties of electrons, protons and neutrons. Write five properties.

View Answer

Ans. Electron – J. J Thomson

Proton – Goldstein

Neutron – Chadwick

PropertyElectronProtonNeutron
1NatureNegatively chargedPositively chargedNeutral
2Charge-1 unit or -1.6 x 10-19 C+1 unit or -1.6 x 10-19 CZero
3Mass9.1 x 10-11 kg1.67 x 10-27 kg1.675 x 10-27 kg
4LocationAround the nucleusIn the nucleusIn the nucleus
5Relative mass1/1838 u1u1u

14. Observe the given figure and answer the questions that follow:

(a) Which experiment is represented by the given figure?

View Answer

Rutherford’s α-ray scattering/Gold foil experiment


(b) State conclusions drawn from each observation of this experiment

View Answer

Rutherford explained his observations as follows:

(i) Since most the α-particles passed through the foil undeflected, it indicates that the most the space in an atom is empty.

(ii) α-particles being positively charged and having considerable mass, could be deflected only by some heavy, positively charged centre. The small angle of deflection of α-particles indicated the presence of a heavy positive centre in the atom. Rutherford named this positive centre as nucleus.

(iii) α-particles which made head on collision with heavy positive centre are deflected through large angles. Since the number of such α-particle is very small, thus the space occupied by the heavy positive centre must be very small.


(c) Write the features of the nuclear model of an atom.

View Answer

The nuclear model of atom had following features:

(i) There is a positively charged centre in an atom called the nucleus. Nearly whole the mass of an atom resides in the nucleus.

(ii) The electrons revolve around the nucleus in circular paths.

(iii) The size of the nucleus is very small as compared to the size of the atom


15. Write properties of Cathode rays.

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Ans. 1) Cathode rays are negatively charged

2) They are produced at the cathode (negatively charged electrode) and travel towards the anode (positively charged electrode) in a vacuum tube.

3) They produce fluorescence when they strike the glass wall of the discharge tube.

4) The properties of the cathode rays do not depend on the electrodes and the gas used in the vacuum tube.

5) Cathode rays travel in a straight line and can cast sharp shadows.


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

View Answer

Ans. Yes, it is true for hydrogen atom which is represented as 1H1


17. Write the electronic configuration of an element whose atomic number is 12.

View Answer

p+ = p = 12

            K         L          M

12Mg = 2          8          2


18. (i) Why do helium, neon and argon have a zero valency?

(ii) What will be the valency of an atom if it contains 3 protons and 4 neutrons?

View Answer

(i) Helium has two electrons in its valence shell, while argon and neon have 8 electrons in their valence shells. As they 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.

(ii) 3p = 3e => 2, 1

The valency of the atom will be one


19. An element ‘A’ has valency +3, while another element B has valency -2. Give the formula of their compound formed when ‘A’ reacts with ‘B’

View Answer

A         B

+3        -2

A2B3


20. 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?

View Answer

Valence electron = 6

Gain 2e => to complete an octet

X2-

The charge would -2


21. Which of the following are isotopes and which are isobars?

Argon, Protium, Calcium, Deuterium

40Ar18, 1H1, 40Ca20, 2H1

View Answer

Isotopes – 1H1, 2H1

Isobars – 40Ar18, 40Ca20


Explain why the isotopes have similar chemical properties but they differ in physical properties?

View Answer

Since the isotopes of an element have same atomic number, they have same electronic configuration and also similar chemical properties. They have different physical since their mass number are different.

Chemical Properties => valence e=> Elements same = p+ = e

Physical Properties => Mass dependent


22. The electronic configuration of phosphorus atom is 2, 8, 5. Give the electronic configuration of P3- ion.

View Answer

P = 2, 8, 5

P3- = 2, 8, 8 (gain 3 electrons to get negatively charged ion- to complete its octet)


23. (i) Why do isotopes show similar chemical properties?

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Isotopes have same atomic numbers and thus same number of electrons. Therefore, they have the same electronic configuration which provides them similar chemical properties.


(ii) Give two important applications of radioactive isotopes.

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An isotope of uranium is used in fuel reactor

An isotope of cobalt is used in treatment of cancer

An isotope of iodine is used in treatment of goitre


24. The atomic number and mass number of an element are 16 and 32 respectively. Find the number of protons, electrons and neutrons in it. State its valency. Is this element a metal or a non-metal, Justify your answer.

View Answer

(Z) Atomic number => 16 = p+ = e

(A) Mass number => 32 = n + p

n = A – Z = 32 – 16 = 16

Electronic configuration = 2, 8, 6 = gain 2e

Valency = -2

Gain electron = Non-metal


25. Define valency by taking examples of silicon and oxygen.

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Ans. Valency is defined as the combining capacity of an atom of an element. If an atom has four or less than four electrons in its valence shell, then valency is equal to the no. of valence electrons. But if it has more than 4 valence electrons, then valency is equal to 8 – no. of valence electrons.

Si (Z= atomic number = 14) = 2, 8, 4

Valency of silicon = 8 – 4 = 4 (lose electron)

Oxygen (Z= atomic number = 8) = 2, 6

Valency of oxygen = 2 (gain electron)


26. Explain with examples: Isotopes and Isobars

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Isotopes – Same element, same atomic number but different mass number.

Chemical properties same, physical properties different.

Eg. 12C6, 13C6, 14C6

Isobars – Different element, different atomic number but same mass number

Chemical properties different, physical properties may or may not be same

Eg. 40Ar18 and 40Ca20


27. 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?

View Answer

p+ = e= 8

Atomic number = 8


(ii) What is the charge on the atom?

View Answer

p+(+8) = e(-8)

No charge – Neutral


28. Explain with examples:

(i) Atomic number (Z) = p+ = e

(ii) Mass Number (A) = n + p

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29. The atomic number of Al and Cl are 13 and 17, respectively. What will be the number of electrons in Al3+ and Cl ?

View Answer

Ans. 13Al => 2, 8, 3 = lose 3 electrons => 2, 8

17Cl => 2, 8, 7 = gain 1 electron => 2, 8, 8


30. If bromine atom is in the form of say two isotopes 35Br79 (49.7%) and 35Br81(50.3%), then calculate the average bromine atom.

35Br79              35Br81

View Answer

Average atomic mass = (m1 x a1 + m2 x a2)/(a1 + a2)

a1 = 49.7%

a2 = 50.3%

Avg atomic mass = (79 x 49.7 + 81 x 50.3)/(49.7 + 50.3)

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

                        = (3926.3 + 4074.3)/100

                        = 80.006 u