Samacheer Kalvi 9th Term I Science Book Back Question and Answers – Unit 5:
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Samacheer Kalvi 9th Science Book Back Unit 5 Term 1 Solutions Guide PDF:
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Term – 1
Unit – 5
Atomic Structure
I. Multiple-Choice Questions
1. Among the following the odd pair is
Ans: (c)
2. A change in the number of neutrons in an atom changes it to
a) an ion
b) an isotope.
c) an isobar.
d) another element.
3. The term nucleons refer to
a) Protons and electrons
b) only Neutrons
c) electrons and neutrons
d) Protons and neutrons
4. The number of protons, neutrons, and electrons present respectively in
a) 80, 80, 35
b) 35, 55,80
c) 35, 35, 80
d) 35, 45, 35
5. The correct electronic configuration of potassium is
a) 2,8,9
b) 2,8,1
c) 2,8,8,1
d) 2,8,8,3
II. True or false/if false give the correct answer
1. In an atom, electrons revolve around the nucleus in fixed orbits
Ans: True
2. Isotopes of an element have different atomic numbers
Ans: False – They have different mass numbers.
3. Electrons have negligible mass and charge.
Ans: True (but they have a negative charge).
4. Smaller the size of the orbit, the lower is the energy of the orbit.
Ans: True
5. The maximum number of electrons in L Shell is 10
Ans: False – the maximum number of electrons in L shell is 8.
III. Fill in the Blanks:-
1. Calcium and Argon are examples of a pair of Isobars
2. Total Number of electrons that can be accommodated in an orbit is given by 2n²
3. Iodine – [3] isotope is used in the treatment of goiter
4. The number of neutrons present in Li is 4
5. The valency of Argon is Zero
IV. Match the following:
a. Dalton – 1. Hydrogen atom model
b. Thomson – 2. Planetary model
c. Rutherford – 3. First atomic theory
d. Neils Bohr – 4. Plum pudding model
– 5. Discovery of neutrons
Ans: a -3, b -4, c – 2, d – 1
ii)
a. Mass of proton – 1) 1. 6 x10-19C
b. Planetary model – 2) -1. 6 x10-19C
c. Charge of electron – 3) 9.31 x 10-28 g
d. Charge of proton – 4) 1.67 x 10-24 g
Ans: a -4, b -3, c -2, d -1
V. Complete the following table:
| Atomic Number | Mass Number | Number of Neutrons | Number of Protons | Number of Electrons | Number of the Element |
| 9 | – | 10 | – | – | |
| 16 | – | 16 | – | – | – |
| – | 24 | – | – | 12 | Magnesium |
| – | 2 | – | 1 | – | – |
| – | 1 | 0 | 1 | 1 | – |
Ans:
| Atomic Number | Mass Number | Number of Neutrons | Number of Protons | Number of Electrons | Number of the Element |
| 9 | 19 | 10 | 9 | 9 | Fluorine |
| 16 | 32 | 16 | 16 | 16 | Sulphur |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 1 | 2 | 1 | 1 | 1 | Deuterium |
| 1 | 1 | 0 | 1 | 1 | Protium |
VI. Arrange the following in the increasing order of atomic number:
Calcium, Silicon, Boron, Magnesium, Oxygen, Helium, Neon, Sulphur, Fluorine, and Sodium
Ans: Helium < Boron < Oxygen < Fluorine < Neon < Sodium < Magnesium < Silicon < Sulphur < Calcium
VII. Short answer:
1. Name an element with the same number of electrons in its first and second shells.
Ans: Beryllium (2,2)
2. Write the electronic configuration of K+ and Cl-
Ans: The electronic configuration of K+ is (2,8,8)
The electronic configuration of Cl- is (2,8,8) So, they are Iso-electronic.
3. Compare the charge and mass of protons and electrons.
| Ans: Particle | Charge | Mass |
| Proton | 1.602 x 10-19C | 1.672 x 10-24g |
| Electron | 1.602 x 10-19C | 9.108 x10-24g |
4. For an atom ‘X’, K, L, and M shells are completely filled. How many electrons will be present in it?
Ans: Total number of electrons = K + L +M
= 2+8+18=28
28 S2 IS2 2P6 3S2 w3inm een.com P6 3d10 K L M
5. Ca2+ has completely filled the outer shell. Justify your answer.
Ans: Ca has electronic configuration 2,8,8,2. After losing 2 c-s it becomes 2,8,2 (completely filled outer shell.
Ca -2e Ca2+
2,8,8,2 2,8,8 (completely filled)
VIII. Short answer:
1. State the law of multiple proportions.
Ans: When two elements A and B combine together to form more than one compound, then masses of A which separately combine with a fixed mass of B are in a simple ratio.
2. List the uses of isotopes.
Ans: Cobalt-60 isotope is used in the treatment of Cancer. Uranium-235 acts as a fuel in nuclear reactors.
3. What is isotone? Give an example.
Ans: Atoms of different elements with
• Different atomic numbers
• Different mass numbers.
• a Same number of neutrons.
4. Calculate the number of neutrons, protons, and electrons.
i) atomic number 3 and mass number 7
ii) atomic number 92 and mass number 238
| S.No | Element | Neutrons | Protons | Electrons |
| i | 3X7 | 24 | 3 | 3 |
| Ii | 92Y238 | 145 | 92 | 92 |
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