Question Bank Atomic Structure

1. An atom is made of charged particles called protons and electrons. Why is an atom uncharged ? Ans. In an atom (i) the number of electrons is equal to the number of protons. (ii) the total negative charge on the electrons is equal to the total positive charge on the protons. Thus, an atom is electrically neutral. 2. (a) An atom usually contains three particles. Name the particles and state the charge they carry and their relative masses with respect to hydrogen. (b) Which element contains only two of these types of particles in its atom? Ans. (a) The particles are electrons, protons and neutrons. The mass of an electron is

1 or 0.0005 times the atomic mass of 1837

hydrogen. The charge is –1 elementary charge unit. The mass of a proton is equal to the atomic mass unit of hydrogen and charge is equal to +1 elementary charge unit. The mass of neutron is equal to the atomic mass unit of hydrogen, but it has no electric charge. (b) Hydrogen : It has one proton and one electron.

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3. What is meant by the following statements : (i) Atomic number of calcium is 20? (ii) Mass number of calcium is 40? Ans. (i) It means the nucleus of calcium has 20 protons and hence it has a charge +20 elementary charge units. Furthermore, there are 20 electrons revolving around the nucleus of calcium. (ii) It means the total number of protons and neutrons (each having a mass equivalent to 1 a.m.u) are 40 in the nucleus of calcium. 4. What is meant by the statement that atomic number of sodium is 11? Ans. It means : (i) There are 11 protons in the nucleus of sodium. (ii) The total atomic mass of protons is 11 a.m.u. (iii) The total elementary charge is +11 units. (iv) The total number of electrons revolving around the nucleus of sodium is 11. 5. What do you understand by the term electronic configuration? Write down the electronic configuration of the following elements. (i) Magnesium (At. No. 12). (ii) Chlorine (At. No. 17). Ans. Electronic configuration : The arrangement of electrons into various energy levels around the nucleus of an atom is called electronic configuration. Electronic configuration of magnesium is Electronic configuration of chlorine is

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K 2 K 2

L 8 L 8

M 2 M 7 Question Bank

6. Write down the names of the following particles and explain the meaning of their symbol, superscript, subscript, where P stands for the particle. 1 1

Ans.

p; 01 p;

1 –1

p

1 1p

Particle is proton. Superscript denotes its mass is 1 a.m.u Subscript denotes its elementary charge is +1 unit. 1 Particle is neutron. 0p Superscript denotes its mass is 1 a.m.u Subscript denotes its charge is zero. 0 Particle is electron. –1 p Superscript denotes its mass is 0 (zero) a.m.u, i.e., it has no mass. Subscript denotes its elementary charge is –1 unit. 7. An atom of fluorine may be written as 199 F . Write down : (1) the number of protons. (2) the number of neutrons. (3) the number of electrons in an atom of fluorine. Show the atom by geometric diagram.

Ans. 199 F

No. of protons = 9 No. of neutrons = Z – A = 19 – 9 = 10 No. of electrons = 9 (2,7). Geometric representation of fluorine atom.

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8. From the symbol

24 12 Mg

state :

(i) its mass number, (ii)its atomic number,

(iii) its electronic configuration.

Ans. (i) Mass number is 24. (ii) Atomic number is 12. (iii) Electronic configuration : K (2), L(8), M(2). 9. Draw the structures of (i) Sodium atom Ans. (i)

23 11

Na (ii) fluorine atom 199 F .

(ii)

Structure of

23 11 Na

Structure of 199 F 10. Element Potassium

Mass No. Atomic No. Number of protons Number of neutrons 39

19

35 17 Cl 37 17 Cl

(a) Complete the table given above. 35 (b) What is the name given to the pair of atoms, such as 17 Cl and

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37 17

Cl ?

Question Bank

Ans. (a) Element

Mass No. Atomic No. Number of protons

Number of neutrons

Potassium 35 17 Cl

39 35

19 17

19 17

39 – 19 = 20 35 – 17 = 18

37 17 Cl

37

17

17

37 – 17 = 20

(b) The pair of atoms

35 17

Cl and

11. By using isotopes of chlorine

37 17

35 17

Cl are called isotopes.

Cl and

37 17

Cl , give reasons (or explain):

(i) Two isotopes have identical properties. (ii) The relative mass of chlorine is not a whole number. Ans.(i)

35 17 Cl

and

37 17 Cl

have the same number of protons and hence electrons. Thus,

they have the same electronic configuration, i.e., same number of electrons in the valence shell. It is on account of the same number of electrons in their valence shell, that they have similar chemical properties. (ii) Natural chlorine contains isotopes

35 17 Cl

and

37 17 Cl

in the ratio of 3:1. Thus,

their relative atomic mass is an average of the atomic masses of the isotopes and hence it is not a whole number. 12. (a) Three different atoms of hydrogen are represented as 11 H, 21 H, 31 H. What do the subscripts and superscripts represent? (b) Which factor is responsible for the change in the superscripts, 1, 2 and 3 though the element is the same?

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(c) What is the appropriate term for the elements having such a property? (d) Draw the atomic structure of 31 H. Ans.(a) The superscripts represent mass number. The subscripts represent atomic number. (b) It is the excess neutrons, which are responsible for the change in mass number. In 11 H there is no neutron, in 21 H there is one neutron and in 31 H there are two neutrons in the nucleus of hydrogen. (c) Isotopes is the appropriate term. (d) Structure of 31 H is as shown alongside. 13.

26 12 Mg

and

24 12 Mg

are the symbols of two isotopes of magnesium. Compare the

atoms of these isotopes with respect to : (i) Composition of their nuclei. (ii) Electronic configuration. (iii) Give reason why the two isotopes of magnesium have different mass numbers. Ans. (i) The nucleus of

26 12 Mg

has 12 protons and 14 neutrons.

The nucleus of

24 12 Mg

has 12 protons and 12 neutrons.

(ii) Electronic configuration of

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26 12 Mg

6

and

24 12 Mg

⎡K L M⎤ is ⎢ ⎥ ⎣2 8 2 ⎦

Question Bank

(iii) The difference in the mass number is due to two extra neutrons in

26 12 Mg .

14. Chlorine is an element with atomic number 17. It is a mixture of two isotopes of mass number 35 and 37 in the ratio of 3:1. (i) What is meant by the atomic number of an element? (ii) Write down the electronic configuration of chlorine. (iii) Calculate the average relative mass of chlorine. Ans. (i) The number of protons present in the nucleus of an atom is called its atomic number. (ii) The electronic configuration of chlorine is

(iii) Relative atomic mass of chlorine Ar =

=

K L M 2 8 7

xA1 + yA 2 x+ y

3 × 35 + 1 × 37 105 + 37 142 = = = 35.5. 3 +1 4 4

15. Calculate the relative atomic masses of the following elements : (a) Copper, which consists of 69% of copper–63 and 31% of copper–65. (b) Gallium, which consists of 60% of Gallium–69 and 40% of Gallium – 71. Ans. (a) Relative atomic mass of copper Ar =

=

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xA1 + yA 2 x+ y 69 × 63 + 31 × 65 69 + 31

Question Bank

=

4347 + 2015 100

=

6362 = 63.62. 100

(b)Relative atomic mass of Gallium Ar = =

xA1 + yA 2 60 × 69 + 40 × 71 = x+ y 60 + 40 4140 + 2840 6980 = 69.80. = 100 100

16. Write the symbols with mass number and atomic number of the following: (a) Arsenic (atomic number 33, mass number 75). (b) Uranium–238(atomic number 92). (c) Uranium –235(atomic number 92). (d) Uranium – 239(atomic number 92). Ans. (a) Arsenic [At. No. 33, Mass No. 75]

75 33 As

(b) Uranium [At. No. 92, Mass No. 238]

238 92 U

(c) Uranium [At. No. 92, Mass No. 235]

235 92 U

(d) Uranium [At. No. 92, Mass No. 239]

239 92 U

17. Complete the following table : Element Carbon Aluminium Fluorine Argon Helium Sodium

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Mass No. 12 ----19 ----4 23

Atomic No. ----13 ------------11

Neutrons ----14 ----22 ---------

Protons ------------18 ---------

8

Electrons 6 ------------2 -----

Electronic configuration --------2, 7 -------------

Question Bank

Ans. Element

Mass No.

At. No.

Neutrons

Protons

Electrons

Electronic configuration K, L, M

Carbon

12

6

6

6

6

2, 4

Aluminium

27

13

14

13

13

2, 8, 3

Flourine

19

9

10

9

9

2, 7

Argon

40

18

22

18

18

2, 8, 8

Helium

4

2

2

2

2

2

Sodium

23

11

12

11

11

2, 8, 1

18. (i) What do you understand by the term valence electrons? (ii) Why are valence electrons named as such, rather than only electrons? Ans. (i) The electrons present in the outermost shell of an atom are called valence

electrons. (ii) They are named as valence electrons, because it is these electrons which take part in a chemical reaction either by actual transfer or by sharing. 19. An element is represented as 40 20 X . Name the shell which has valence electrons and the number of valence electrons in it. Ans. Electronic configuration of

40 20

X is 2(K), 8(L), 8(M), 2(N).

The valence shell is the N-shell. It has 2 valence electrons.

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20. Amongst the electrons revolving around the nucleus, which electrons (i) determine the chemical properties of an element, (ii) do not determine the chemical properties of an element? Give reasons for your answer. Ans. (i) The electrons in the valence shell determine the chemical properties of an

element. (ii) The electrons other than the electrons in the valence shell, do not determine the chemical properties of an element. Reason : The electrons in the valence shell are weakly held by the nucleus.

Thus, an element can donate or accept or share these electrons with the atoms of other elements and hence enters into a chemical reaction. 21. Fill in the blank spaces with appropriate words : (i) The electrons present in the _____(1)_____ shell of an atom are called valence electrons. (ii) The electrons present in a shell, other than the _____(2)_____ shell do not take part in a _____(3)_____ reaction. (iii) The nature of an element is _____(4)_____ , if it has one to three _____(5)_____ electrons. (iv) The element is a _____(6)_____ gas, if it has _____(7)_____ valence electrons. (v) The element _____(8)_____ has one electron in the K-shell, but is a non-metal.

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(vi)The element _____(9)_____ has two electrons in the K-shell, but is a noble gas. Ans. (1) valence (2) valence (3) chemical (4) metallic (5) valence (6) noble

(7) eight (8) hydrogen (9) helium. 21. Elements A, B and C have atomic numbers 9, 20 and 10 respectively. State which one is : (a) metal,

(b) non-metal,

(c) inert gas?

Ans. Electronic configuration of A(9) is (2, 7). As there are 7 electrons in the

valence shell, hence A is a non- metal. Electronic configuration of B(20) is (2, 8, 8, 2). As there are 2 electrons in the valence shell, hence B is a metal. Electronic configuration of C(10) is (2, 8). As there are 8 electrons in the valence shell, hence C is an inert gas. 22. A, B, C and D are the elements whose atomic numbers are 16,19,18 and 13 respectively. Write down the electronic configuration of each element and classify them as metal/non-metal/ noble gas. Ans. A (atomic no. 16) has electronic configuration of (2, 8, 6) and hence is a non-metal.

B (atomic no. 19) has electronic configuration of (2, 8, 8, 1) and hence is a metal.

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Question Bank

C (atomic no. 18] has electronic configuration of (2, 8, 8) and hence is a noble gas.

D (atomic no. 13] has electronic configuration of (2, 8, 3) and hence is a metal. 23. Complete the table below, with the elements E, F, G, H, such that element E is an inert gas having electronic configuration 2,8. Elements E F G H

Atomic number Z Z–1 Z+1 Z+4

Electronic configuration 2, 8 ---------------------

Which of the elements in the table are (i) metals,(ii) non-metals? Ans. Elements

Atomic number

Electronic configuration

E

Z

2, 8

F

Z–1

2, 7

G

Z+1

2, 8, 1

H

Z+4

2, 8, 4

(i) G is a metal. (ii) F and H are non-metals.

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24. Which of the following elements would you expect to be most stable and why? 4 27 23 40 2 A, 13 B, 11 C, 18 D

Ans. Elements 42 A [2(K)] and

40 18 D

[2(K), 8(L), 8(M)] are most stable, because

they have 2 and 8 electrons in their valence shell.

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