9TH Class Chemistry Chapter - 2 Structure of Atoms ( SOLVED EXERCISE )

 Chapter - 2

Structure of Atoms

EXERCISE

 

Multiple Choice Questions

Put a (✓) on the correct answer

            1. Which one of the following results in the discovery of proton:

(a)    Cathode rays

(b)   Canal rays    ✓

(c)    X-rays

(d)   Alpha rays

2. Which one of the following is the most penetrating?

(a)    Protons

(b)   Electrons

(c)    Neutrons    ✓

(d)   Alpha particles

3. The concept of orbit was used by:

(a)    J.J Thomson

(b)   Rutherford

(c)    Bohr    ✓

(d)   Planck

4. Which one of the following shell consists of three subshells.

(a)     O shell

(b)   N shell

(c)    L shell

(d)   M shell    ✓

5. Which radioisotope is used for diagnosis of tumor in the body?

(a)    Cobalt-60

(b)   Iodine-131    ✓

(c)    Strontium-90

(d)   Phosphorus-30

6. When U-235 breaks up, it produces:

(a)    Electrons

(b)   Neutrons    ✓

(c)    Protons

(d)   Nothing

7. The p subshell has:

(a)    One orbital

(b)   Two orbitals

(c)    Three orbitals    ✓

(d)   Four orbitals

8. Deuterium is used to make:

(a)    Light water

(b)   Heavy water    ✓

(c)    Soft water

(d)   Hard water

9. The isotope C-12 is present in abundance of:

(a)    96.9 %

(b)   97.6 %

(c)    99.7 %

(d)   None of these    ✓

10. Who discovered the proton:

(a)   Goldstein    ✓

(b)   J.J Thomson

(c)    Neil Bohr

(d)   Rutherford Short

Short answer questions.

1. What is the nature of charge on cathode rays?

Ans.

    The cathode rays deflected towards positive plate in an electrical field showing that these rays are negatively charged.

2. Give five characteristics of cathode rays.

Ans. Characteristics of cathode rays:

i.                    These rays travel in straight lines perpendicular to the cathode surface.

ii.                  They can cast a sharp shadow of an opaque object if placed in their path.

iii.         They are deflected towards positive plate in an electric field showing that they are      negatively charged.

iv.                They raise temperature of the body on which they fall.

v.                  J.J. Thomson discovered their charge/mass (e/m) ratio.

vi.                Light is produced when these rays hit the walls of the discharge tube.

vii.       It was found that the same type of rays were emitted no matter which gas and which cathode was used in the discharge tube.

3. The atomic symbol of a phosphorus ion is given as ^P3~

    (a)   How many protons, electrons and neutrons are there in the ion?

    (b)   What is the name of the ion?

    (c)    Draw the electronic configuration of the ion.

    (d)   Name the noble gas which has the same electronic configuration as the phosphorus ion has.

Ans.

(a)    Protons = 15

Electrons = 18

Neutrons = 16

(b)   Phosphorous ion

(c)    1s², 2s², 2p⁶, 3s², 3p⁶

(d)   Argon

4. Differentiate between shell and subshell with examples of each.

Ans.

Shell	Sub-shell
The fixed circular orbit which is associated with a definite amount of energy is called shell or energy level.	Each shell is composed of one or more sub shells.                   OR The sub division of a shell in an atom is called sub shell.
Examples: K, L, M, N etc. are considered as the shell or energy levels of any atom.	Examples: s, p, d and f are considered as the sub shells of a shell.

5. An element has an atomic number 17. How many electrons are present in K, L and M shells of the atom?

Ans. K = 2

        L = 8

        M = 7

6. Write down the electronic configuration of Al³⁺. How many electrons are present in its outermost shell?

Ans.

        Electronic configuration of Al³⁺ = 1s², 2s², 2p⁶.

        8 electrons are present in its outermost shell.

7. Magnesium has electronic configuration 2, 8, 2.

    (a) How many electrons are in the outermost shell?

    (b) In which subshell of the outermost shell electrons are present?

    (c) Why magnesium tends to lose electrons?

Ans.

    (a)    There are two electrons are present in outermost shell.

    (b)   In s-subshell of the outermost shell electrons are present.

    (c)    It has two valence electrons. It has tendency to lose electrons to get noble gas configuration of               Neon (2,8).

8. What will be the nature of charge on an atom when it loses an electron or when it gains an electron?

Ans.

    When atom loses electron it will be positively charged while atom gains electron it will become negatively charged.

9. For what purpose U-235 is used?

Ans.

    When U-235 is bombarded with slow moving neutrons, the uranium nucleus breaks up to produce Ba-139 and Kr-94 and three neutrons.

    U-235 is used to get large amount of energy which can be used to drive the turbines to generate electricity.

10. A patient has goiter. How will it be detected?

Ans.

    The goiter in a patient can be detected by using Iodine-131 isotope.

11. Give three properties of positive rays.

Ans. Properties of positive rays:

        i.                    These rays travel in straight lines in a direction opposite to the cathode rays.

        ii.                  Their deflection in electric and magnetic field proved that these are positively charged.

        iii.                The nature of canal rays depends upon the nature of gas, present in the discharge tube.

        iv.                These rays do not originate from the anode. In fact these rays are produced when the                             cathode rays or electrons collide with the residual gas molecules present in the discharge                         tube and ionize them as follows:

M + e^- à M⁺ + 2e^-

v.                  Mass of these particles was found equal to that of a proton or simple multiple of it. The mass of a proton is 1840 times more than that of an electron.

12. What are the defects of Rutherford’s atomic model?

Ans. Defects of Rutherford’s atomic model:

        i.                    According to classical theory of radiation, electrons being the charged particles should                         release or emit energy continuously and they should ultimately fall into the nucleus.

        ii.                  If the electrons emit energy continuously, they should form a continuous spectrum but in                         fact, line spectrum was observed.

13. As long as electron remains in an orbit, it does not emit or absorb energy. When does it emit or absorb energy?

Ans.

    The energy is emitted or absorbed only when an electron jumps from one orbit to another. When an electron jumps from lower orbit to higher orbit, it absorbs energy and when it jumps back from higher orbit to lower orbit it radiates energy.