KSEEB Solutions for Class 9 Science Chapter 4 Structure of The Atom

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Karnataka State Syllabus Class 9 Science Chapter 4 Structure of The Atom

KSEEB Class 9 Science Structure of The Atom Intext Questions and Answers

Question 1.
What are the canal rays?
Answer :
Positively charged radiations whose magnitude equal to that of electron discovered by Goldstern.

Question 2.
If an atom contains one electron and one proton. Will it carry any charge or not?
Answer :
No. charge

Question 3.
On the basis of Thomson’s model of an atom explain how the atom is neutral as a whole?
Answer :
Every atom consists of an equal number of protons and electrons. Hence the net charge carried by an atom is neutral.

Question 4.
On the basis of Rutherford’s atomic model of an atom. Which subatomic particle is present in the nucleus of an atom?
Answer :
Proton

Question 5.
Draw the sketch of Bohr’s model of an atom with three shells?
Answer :

Question 6.
What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of metal other than gold?
Answer :
Gold is stable to the charged particles.

Question 7.
Name the three sub-atomic particles of an atom.
Answer :
Proton, Electrons, and Neutrons.

Question 8.
A helium atom has an atomic mass of 44 and two protons in its nucleus. How many neutrons does it have?
Answer :
₂He⁴
Number of neutrons = 4 – 2 = 2

Question 9.
Write the distribution of electrons in carbon and sodium atom.
Answer :
Carbon: 1s², 2s², 2p²
Sodium: 1s², 2s², 2p⁶ 3s¹

Question 10.
If K and L shells of an atom arc full then what would be the total number of electrons in the atom.
Answer :
K = 1 = 2n² = 2(1)² = 2 x 1 = 2
L = 2 = 2(2)² = 2(2)² =2 x 4 = 8
Electrons = 10

Question 11.
How will you find the valency of chlorine, sulfur, and magnesium?
Answer :

Question 12.
If the number of electrons in an atom is 8 and the number of protons is also 8. Then (i) What is the atomic number of the atom? and (ii) What is the charge of an atom?
Answer :
(1) Atomic number is 8
(2) 1s² 2s² 2p⁴, charge is 2

Question 13.
Write the atomic number and mass number for the following:
Answer :
(1) ₈O¹⁶ (Atomic number is 8) (the Mass number is 16)
(2) ₁₆S³² (Atomic number is 6) (the Mass number is 32)

Question 14.
For the symbol, H.D. and T tabulate three subatomic particles found in each of them.
Answer :

Question 15.
Write the electronic configuration of any one pair of isotopes and isobars.
Answer :
Isotopes :
(a) ₁₇CI³⁵ : Is² 2s² 2p⁶ 3s² 3p⁵
(b) ₁₇C1³⁷ : Is² 2s² 2p⁶ 3s² 3p⁵

Isobars :
(a) Calcium (Z = 20) : Is² 2s² 2p⁶ 3s² 3p⁶ 4s²
(b) Argon (Z = 18) : Is² 2s² 2p⁶ 3s² 3p⁶

KSEEB Class 9 Science Structure of The Atom Textbook Exercise Questions and Answers

Question 1.
Compare the properties of electrons, protons, and neutrons.
Answer :

Question 2.
What are the limitations of J.J. Thomson’s model of the atom?
Answer :
(1) Atom is electrically neutral.
(2) Charged sphere

Question 3.
What are the limitations of Rutherford’s model of the atom?
Answer :
(a) Fast-moving a-particles passed straight through the gold foil.
(b) Some of the a-particles were deflected by the foil by small angles.

Question 4.
Describe Bohr’s model of the atom.
Answer :
Only certain special orbits are known as
discrete orbits of electrons are allowed inside the atom.

While revolving in discrete orbits the electrons do not radiate energy orbits are called Main energy level, shells called energy levels.

Question 5.
Compare all the proposed models of an atom given in this chapter.
Answer :
Thomson model :

An atom consists of a positively charged sphere and the electrons are embedded in It.
The negative and positive charges are equal in magnitude so the atom is electrically neutral.

Rutherford model :

Centremost region of an atom made up of positively charged particles called protons.
Most of the space inside the atom is empty because most of the a- particles passed through the gold foil. Electrons revolve around the nucleus in circular paths the size of the nucleus is very small as compared to the size of the atom.

Bohr model :

Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
While revolving in discrete orbits the electrons do not radiate energy.

Question 6.
Summarise the rules for the writing of the distribution of electrons in various shells for the first eighteen elements.
Answer :

The maximum number of electrons present in a shell is given by the formula 2n2 [n indicates shell number]
The maximum number of electrons that can be accommodated in outermost orbit is 8.
Electrons are not accommodated in a given shell. Unless the inner shells are filled that is the shells are filled in a stepwise manner.

Question 7.
Define valency by taking examples of silicon and oxygen.
Answer :
Combining the capacity of an atom is called valency.

Silicon :
1s² 2s² 2p⁶ 3s² 3p² Valency : 4

Oxygen :
Is² 2s² 2p⁴ Valency : 2

Question 8.
Explain with examples
(i) Atomic number,
(ii) Mass number,
(iii) Isotopes and
(iv) Isobars. Give any two uses of isotopes.
Answer :
(a) Elements: Sodium, silver, tin,
Answer :

Atomic number: Number of protons or electrons in an atom denoted by Z.
Mass number: The total number of protons and neutrons denoted by A.
Isotopes: Atoms of the species having the same atomic number but different mass numbers.

Ex :
(a) ₁₇CI³⁵, 17Cl³⁷
(b) ₂₁H¹, ₁H², ₁H³

Uses :

Used in agricultural research
Cobalt – 60 used in the treatment of cancer.

(iv) Isobars: Atoms of different elements with different atomic numbers and same mass numbers.

Question 9.
Na⁺ has completely tilled K and L shells. Explain.
Answer :
Sodium valency is 1. The atomic number is 11 and the Mass number is 23.
\(\frac{1 \mathrm{s}^{2}}{\mathrm{K}}, \frac{2 \mathrm{s}^{2} 2 \mathrm{p}^{6}}{\mathrm{L}}, \frac{3 \mathrm{s}^{1}}{\mathrm{M}}\)
Na⁺ → Is² 2s² 2p⁶ 3s² Outermost Orbital is completely filled

Question 10.
If bromine atom is available in the form of say, two isotopes
\(\begin{array}{llll} {\text { 79 } \mathrm{Br}(49.7 \%)} & {\text { and }} & {81} & {\text { Br }} & {(50.3 \%)} \end{array}\) calculate the average atomic mass of bromine atom.
Answer :
\(\left(79 \times \frac{49 \cdot 7}{100}\right)+\left(\frac{81 \times 50 \cdot 3}{100}\right)\) = 80.006 units

Question 11.
The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes j6X and l*X and in the sample?
Answer :

Question 12.
If Z = 3, what would be the valency of the element? Also, name the element.
Answer :
Z = 3, 1s² – 2s¹
Valency is 1 Atom is lithium

Question 13.
Composition of the nuclei of two atomic species X and Y are given as under
X – Y
Protons = 6 – 6
Neutrons = 6 – 8
Give the mass numbers of X and Y. What is the relation between the two species?
Answer :
X and Y are isotopes

Question 14.
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 times that of \(\frac{1}{2000}\) protons.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Put a tick (✓) against correct choice and cross (✗) against the wrong choice in questions 15, 16 and 17.
Answer :
(a) False
(b) False
(c) True
(d) True

Question 15.
Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron
Answer :
(c) Proton

Question 16.
Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.
Answer :
(c) different number of neutrons

Question 17.
Number of valence electrons in CP^– ion are:
(a) 16
(b) 8
(c) 17
(d) 18
Answer :
(d) 18

Question 18.
Which one of the following is a correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1.
Answer :
(d) 2, 8, 1

Question 19.
Complete the following table.

Answer :

KSEEB Class 9 Science Structure of The Atom Additional Questions and Answers

Question 1.
(a) Study the data given below and answer the questions which follow.

(b) Write the mass number and an atomic number of 41w particles A, B, C, and D.

Particle	Atomic number	Mass number
A	2	7
B	10	17
C	8	16
D	8	18

(b) Write the mass number and the atomic number of 41w particles A, B, C, and D.

Question 2.
Mention the drawbacks of Rutherford’s model.
Answer :
Any particles under continuous acceleration should radiate energy and finally fall into the nucleus resulting in instability of atom. But we know that atoms are quite stable.

Question 3.
Out of the ‘three sub-atomic particles known. Which is called the universal particle?
Answer :
Electron

Question 4.
Is the atomic number of an atom always equal to the number of electrons?
Answer :
Atomic number represents the number of protons and electrons.

Question 5.
Out of the proton and neutrons. Which is heavier?
Answer :
Neutrons are slightly heavier than protons.