Download Ans. - Testlabz.com

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.
Class-IX
1
Question Bank
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
Class-IX
2
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.
Class-IX
3
Question Bank
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
Class-IX
4
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?
Class-IX
5
Question Bank
(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
Class-IX
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 =
=
Class-IX
7
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
Class-IX
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.
Class-IX
9
Question Bank
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.
Class-IX
10
Question Bank
(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.
Class-IX
11
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.
Class-IX
12
Question Bank
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.
Class-IX
13
Question Bank