Extra Questions for Class 9th: Ch 4 Structure of the Atom (Science) Important Questions Answer Included

Very Short Answer Questions (VSAQs): 1 Mark

Q1. Define atomic number.
It is the number of protons in the nucleus of an atom.

Q2. What is an electron ? Who discovered it ?
Electron is a negatively charged particle present outside the nucleus of an atom. It was discovered by J. J. Thomson in 1897.

Q3. Name the particles which determine the mass of an atom.
Protons and Neutrons.

Q4. What will happen to an element ‘Z’ if its atoms gain three electrons?
It will change into trivalent anion i.e. Z3−

Q5. There are 15 protons and 16 neutrons in the nucleus of an element. Calculate its atomic number and its atomic mass.
Atomic number (Z) = Number of protons = 15
Atomic mass (A) = Number of protons + Number of neutrons = 15 + 16 = 31

Short Answer Questions-I (SAQs-I) : 2 Marks

Q1. Give one word for the following:
(i) Positively charged atom.
(ii) A group of atoms carrying a charge.


(i) Positively charged atom: Cation
(ii) Group of atoms carrying a charge: Polyatomic ion.

Q2. Which atom, Na+ or He, has completely filled K and L shells? Give reason to support your answer.
Na+ has completely filled K and L shells. Explain.


Na: K=2, L=8, M=1
Na+: K=2, L=8
He: K=2
Na+ has completely filled K and L shells. Na atom gets converted into Na+ by losing one electron from its outermost shell. He atom has only K shell.

Q3. An atom of an element has three electrons in its 3rd orbit, which is the outermost shell. Write:
(i) The electronic configuration
(ii) Atomic number
(iii) Number of protons
(iv) Valency


(i) Electronic configuration : 2, 8, 3
(ii) Atomic number : 13
(iii) Number of protons – 13
(iv) Valency – 3

Q4. How many electrons, protons and neutrons will be there in an element 9𝑿19? What will be the valency of the element?


No. of protons =Atomic number = 9
No. of (protons + neutrons) = Mass number = 19
Number of electrons = 9
Number of protons = 9
Number of neutrons = Mass number–Atomic number = 19 – 9 = 10
Electronic configuration of X = 2, 7
Valency of X = 1 (since it requires one electron to complete its octet)

Q5. (a) J. Chadwick discovered a sub-atomic particle that has no charge and has mass nearly equal to that of a proton. Name the particle and give its location in the atom.
(b) If ‘K’ and ‘L’ shells of an atom are completely filled, then what would be: (i) the total number of electrons in the atom and (ii) its valency ?


(a) Particle is neutron. Neutron is present in the nucleus of an atom.
(b) (i) Total number of electrons = 10
(ii) Valency is zero.

Short Answer Questions-II (SAQs-II) : 3 Marks

Q1. The discovery of subatomic particles led to a revolution in the study of matter. Name the scientists who discovered these sub-atomic particles.


Electron: by J.J Thomson in 1897
Proton: by E. Goldstein in 1886
Neutron: by James Chadwick

Q2. Illustrate that Na atom has completely filled K and L shells.


Electronic configuration of Na is: 2, 8, 1
Since, K can have Maximum of 2 electrons using 2𝑛2 we can say that, it is filled completely. L can have maximum of 8 electrons, so, we can say it is filled completely.

Q3. Write the electronic configuration and valency of the following:
(i) Chlorine
(ii) Sodium and
(iii) Silicon


(i) E.C. of Cl = 2, 8, 7 Valency = – 1
(ii) E.C. of Na = 2, 8, 1 Valency = 1
(iii) E.C. of Si = 2, 8, 4 Valency = 4

Q4. There are two elements 26A13 and  26Al14. Find the number of sub-atomic particles in each of these elements. What is the relationship between the two?


26 = 13 protons + 13 neutrons,
Atomic number = 26 – 13 = 13

26 = 14 protons + 14 neutrons
Atomic number = 26 – 14 = 12
They are isobars as they have same mass number but different atomic number.

Q5. (i) Why chemical properties of all the isotopes of an element are same?
(ii) Name the isotopes used in the treatment of goitre and cancer.
(iii) An element ‘X’ has 2 electrons in its M shell. What is its atomic number?


(i) This is because isotopes have same atomic number, so the number of valence electrons present in them are same and it is the valence electrons which take part in chemical reactions. So the isotopes of an element have same chemical properties.
(ii) Goitre – Isotope of iodine
Cancer – Isotope of cobalt

(iii) Atomic number of X = 12.

Long Answer Questions (LAQs) : 5 Marks

Q1. Hydrogen has three isotopes. State the composition of their nuclei and write their mass numbers. Also represent them in the form of symbols.


Isotopes of hydrogen are: Protium 1H1 (1 proton, 0 neutron), Mass no. :1,
Deuterium 1H2 (1 proton, 1 neutron), Mass no.: 2
Tritium 1H3 (1 proton, 2 neutrons), Mass no.: 3

Q 2. (i) Calculate the number of electrons, protons and neutrons in an atom of an element with atomic number 20 and mass number 40. Write electronic configuration and draw the structure of the atom.
(ii) An atom has complete K and L shells. Is this an atom of a metal, non-metal or noble gas? Justify.


(i) e = 20,
(ii) p = 20,
(iii) n = 40 – 20 = 20,
Electronic Configuration = 2, 8, 8, 2.

(ii) Atom has K = 2e; L = 8e
∴ Total electrons = 2 + 8 = 10e −
Complete shell with 8e −
∴ Noble gas

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