class 9 chapter : - structure of atom (ncert exercise solution)

Chapter 4 ;- Structure of Atom

     Questions and answer 

1    Draw a sketch of Bohr’s model of an atom with three shells.
Answer:

2    What do you think would be the observation if the a-particle scattering experiment is carried out using a foil of a metal other than gold?
Answer: On using any metal foil, the observations of the Rutherford  alpha-particle scattering experiment would remain the same because all atoms of element  would have same structure

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

Answer 

1 ELECTRON :- Negative charge 

2 PROTON :- positive charge

3 NEUTRON :- Neutral charge

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

Answer :- atomic mass oh helium =4 amu

atomic number = no. of proton = no. of electron = 2

no. of neutron = atomic mass - atomic no

no. of neutron = atomic mass -  no. of electron

no. of neutron = atomic mass -  no. of proton

no. of neutron  = 4-2 = 2

5  Write the distribution of electrons in carbon and sodium atoms.

ANSWER :- The distribution of electrons in carbon and sodium atoms.

carbon atomic number = 6

Sodium atomic number = 11

Name of element	Atomic number	No. of electron	K  (1)	L (2)	M (3)
CARBON (C)	6	6	2	4
SODIUM (Na)	11	11	2	8	1

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

Answer: K (1) shell can hold 2 electrons and L (2) shell can hold 8 electrons.When both the shells are full, there will be (8 + 2) =  10 electrons in the atom. The total electron number 10

Atomic number = no. of electron = 10
Than   Name of element is Neon (Ne)

7   How will you find the valency of chlorine, sulphur and magnesium?

Answer:

Name of element	Symbol	Atomic number	K First shell	L second shell	M Third  shell	No . of valency electron	VALENCY
chlorine	Cl	17	2	8	7	7	-1
sulpher	S	16	2	8	6	6	-2
manesium	Mg	12	2	8	2	2	+2

8  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?
Answer :- Atomic number equal to no. of proton or no. of electron

according to question no of electron 8

therefor (1) atomic number of the atom 8

(2) no of electron (8) equal to no of proton (8) than atom always neutral

9 With the help of given Table 4.1, find out the mass number of oxygen and sulphur atom.

ELEMENT	SYMBOL	NO. OF ELECTRON	NO. OF PROTON	NO OF NEUTRON	K	L	M	Valency electron	VALENCY
HYDROGEN	H	1	1	1	1				1
OXYGEN	O	8	8	8	2	6			2
SULPHER	S	16	16	16	2	8	6		2,4,6

ANSWER :- atomic mass of oxygen 

atomic mass = atomic number + no of neutron  or
atomic mass = no of electron + no of neutron   or
atomic mass = number of proton  + no of neutron

atomic mass  = 8  +    8          =  16

atomic mass of sulpher 

atomic mass = atomic number + no of neutron  or
atomic mass = no of electron + no of neutron   or
atomic mass = number of proton  + no of neutron
atomic mass  = 16  +    16          =  32

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

Name of atom	symbol	Atomic no	Atomic mass	No .of electron	No .of proton	No.of neutron	remark
protIum	H	1	1	1	1	0	ISOTOPES
deuterium	D	1	2	1	1	1	OF
tritium	T	1	3	1	1	2	HYDROGEN

11 :- Write the electronic configuration of any one pair of isotopes and isobar.

12        Compare the properties of electrons, protons and neutrons

ANSWER 

SL NO	ELECTRON	PROTON	NEUTRON
1	negative charged particle	positive charged particle	no charge
2	neglible mass	mass =  1amu	mass = 1 amu
3	Attracted towards + ve charge	Attracted toward – ve charge	Its neutral
4	Electron revolve around nucleus	Proton present in side nucleus with neutron	Neutron  present in side nucleus with proton

13        write the limitations of J.J. Thomson’s model of the atom?
Answer: According to J.J. Thomson’s model that whole atom is positive , the electrons are embedded all over in the positively charged spheres. But according Rutherford experiments  and other scientist suggest  that protons are present only in the center of the atom and electrons are distributed around it.

14     What are the limitations of Rutherford’s model of the atom?
Answer: According to Rutherford’s model of an atom the electrons are revolving in a circular orbit around the nucleus. Any such particle that revolves would undergo acceleration and radiate energy. The revolving electron would lose its energy and finally fall into the nucleus, the atom would be highly unstable. But we know that atoms are quite stable.

15    Describe Bohr’s model of the atom.
Answer: Bohr model, description of the structure of atoms proposed in 1913 by the Danish physicist Niels Bohr

Main postulates are:-
(1) Nucleus in the centre of the atom
(2) Electrons revolve around the nucleus in certain energy level.
(3) Certain special energy level known as discrete orbits of electrons

(4) While revolving in discrete orbits the electrons do not lose  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

16      Summaries the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Answer: The rules for writing of distribution of electrons in various energy level (orbital) for the first eighteen elements are:
(i) The maximum number of electrons present in a shell is given by the formula
∵ n = orbit number i.e., 1, 2, 3
∵ Maximum number of electrons in different shells are:
K shell n = 1 2n² => 2(1)² = 2
L shell n = 2 2n² => 2(2)² = 8
M shell n = 3 2n² => 2(3)² = 18
N shell n = 4 2n² => 2(4)² = 32
(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.
(iii) Electrons are not accommodated in a given shell unless the inner shells are filled. (Shells are filled step-wise).

17 Define valency by taking examples of silicon and oxygen.
Answer: Valency is the bonding capacity of an isolated atom.
Atomic number of oxygen = 8  AND Atomic number of silicon = 14    K L M
Electronic configuration of oxygen = 2 6 –
Electronic configuration of silicon =2 8 4

Name of atom	symbol	Atomic number	K	L	M	Valency	Example
OXYGEN	O	8	2	6		-2	H2O
SILICON	Si	14	2	8	4	4	SiH4

In the atoms of oxygen the valence electrons are 6, 2 electrons are required. In the atom of silicon, the valence electrons are 4. To fill this orbit 4 electrons are required.
Hence, the combining capacity of oxygen is 2 and of silicon is 4.
i.e., Valency of oxygen = 2
Valency of silicon = 4

18 Na⁺ has completely filled K and L shells. Explain.
Answer: Sodium atom (Na), has atomic number =11
No of protons =11
No of electrons = 11
Electronic configuration of Na = K L M  = 2 8 1
Sodium atom (Na) looses 1 electron to become stable and form Na⁺ ion. Hence it has completely filled K and L shells.

19   If bromine atom is available in the form of say, two isotopes ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of bromine atom.
Answer:

20  The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ¹⁶₈X and ¹⁸₈X in the sample?

ANSWER:-  Let the percentage of ¹⁶₈X be x and the percentage of ¹⁶₈X be 100 – x.

21    If Z = 3, what would be the valency of the element? Also, name the element.
Answer : -  Z( Atomic number ) = 3,
Electronic configuration = 2, 1
Valency = 1
Name of the element is lithium.

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

Atomic species	proton	neutron
X	6	6
Y	6	8

Give the mass number of X and Y. What is the relation between the two species?
Answer: Mass number of X  atom = Protons + Neutrons
= 6 + 6 = 12
Mass number of Y atom  = Protons + Neutrons = 6 + 8 = 14
As the atomic number is same i.e., = 6.
[atomic number = number of protons].
Both X and Y are isotopes of same element.

22  Complete the following table.

Atomic no	Atomic mass	No of neutron	No. of proton	No . of electron	Name of atom
9		10	-	-
16	32	-	-	-	sulpher
-	24	-	12	-
-	2	-	1	-
-	1	0	1	0

answer

Atomic no	Atomic mass	No of neutron	No. of proton	No . of electron	Name of atom
9	19	10	9	9	Fluorine
16	32	-	-	-	sulpher
12	24	12	12	12	magnesium
1	2	1	1	1	deutirium
1	1	0	1	0	Hydrogen ion

Chapter 4 ;- Structure of Atom

short question and answer

Q 1 For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore,it is neutral.
(c) The mass of an electron is about 1/1836 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Answer: (a) FALSE
b) false 
c) true
d) true

Question 2. Explain with examples:

(i) Atomic number (ii) Mass number,

(iii) Isotopes and (iv) Isobars.

Answer:

(i) Atomic number: The atomic number of an element is equal to the number of protons in the nucleus of its atom. e.g., nitrogen has 7 protons hence atomic no. = 7.

(ii) Mass number: The mass number of an atom is equal to the number of protons and neutrons in its nucleus.

example
Nucleons = number of protons + number of neutrons Example: Protons + Neutrons = Nucleus = Mass number  7 + 7 = 14

(iii) Isotopes:-  Isotopes are atoms of the same element which have different mass number but same atomic number.

example :- carbon-12, carbon-13, and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13, and 14, respectively.

(iv) Isobars :- Isobars are atoms having the same mass number but different atomic numbers.

An example of a series of isobars would be ⁴⁰S, ⁴⁰Cl, ⁴⁰Ar, ⁴⁰K, and ⁴⁰Ca.

Question 3 What are canal rays?

When an electron is removed from a hydrogen atom, a proton remains. Goldstein observed rays travelling in the opposite direction of the cathode rays in a cathode ray tube. He demonstrated that these rays were positive particles and called the canal rays

Question 4. If an atom contains one electron and one proton, will it carry any charge or not?
Answer: The Hydrogen atom will be electrically neutral as one – ve charge balances one + ve charge

Question 5. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer: According to Thomson’s model of an atom
(i) An  whole 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

Question 6. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?
Answer: As per Rutherford’s model of an atom, the protons which are positively charged are present in the nucleus of an atom.

question 7 write two use of isotopes 

Two uses of isotopes are:
(i) An isotope of iodine is used in the treatment of goitre.
(ii) An isotope of uranium is used as a fuel in nuclear reactors. or
(III) C-14 isotope Use in radiocarbon dating