MCQs For Class 9 Science- Structure Of Atom

Structure Of Atom

1. Who discovered canal rays or anode rays?
   a) E.Goldstein
   b) J.J Thomson
   c) Rutherford
   d) Dalton

Ans: a

Explanation: In 1886  E. Goldstein discovered the presence of new radiation in a gas discharge and called them canal rays or anode rays. These rays are positively charged radiation. They are seen moving from the anode towards the cathode in a specially designed discharge tube.

2. According to Dalton’s atomic theory matter consist of indivisible matter:
   a) Atom
   b) Ion
   c) Molecule
   d) Mixtures

Ans: a

Explanation: Dalton hypothesized that the law of conservation of mass and the law of definite proportions could be explained using the idea of atoms. He proposed that all matter is made of tiny indivisible particles called atoms, which he imagined as “solid, massy, hard, impenetrable movable particles.” Atoms can not be subdivided, created or destroyed.

3. Two atoms are said to be Isobars if……
   a)They have the same number of electrons but different numbers of neutrons.
   b)They have the same number of neutrons but different numbers of electrons.
   c)They have the same atomic number but different mass numbers.
   d) None of the above

Ans: d

Explanation: Atoms of different elements with different atomic numbers but the same mass number are known as isobars.

4. An alpha particle is also known as:
   a)A neutral particle
   b) A doubly-charged helium ion
   c)Subatomic particle
   d) An unionised helium atom

Ans: b

Explanation: Alpha particles are doubly charged helium ions having a mass of 4 u.

5. Which of the following statement about the electron is incorrect?
   a)The mass of the electron is equal to the mass of the neutron
   b) It is a basic constituent of all atoms
   c)It is a constituent of cathode rays
   d)It is a negatively charged particles

Ans: a

Explanation: The mass of an electron is 1/1840 times, which is nearly about 1/2000 times that of the proton. The mass of the electron is equal to 9.1 x 10^-31 kg.

6. In Thomson’s model of the atom, which of the following statements is correct?
   1)The mass of the atom is assumed to be uniformly distributed over the atom.
   2)The electrons are uniformly distributed in the positively charged sphere.
   3)The positive charge is assumed to be uniformly distributed over the atom.
   4)The electron attracts each other to stabilise the atom.
   a)1), 2) and 3)
   b) 1) and 4)
   c) 1) and 3)
   d) 1),3) and 4)

Ans: a

Explanation: According to Thomson’s model of the atom, an atom consists of a sphere of positive charge with negatively charged electrons embedded in it. These negative and positive charges in an atom are equal in magnitude, due to which an atom is electrically neutral.

7. Who has given the first model of the atom:
   a) Rutherford
   b) Goldstein
   c) Bohr
   d) J.J Thomson

Ans: d

Explanation: J.J Thomson was the scientist who proposed a model for the structure of an atom. Thomson’s model of an atom was similar to Christmas pudding. The electron in a sphere of positive charge was like dry fruit in a spherical Christmas pudding.

8. Which cell is nearest to the nucleus of an atom?
   a) K -Class (Shell)
   b)L-Class (Shell)
   c) M-Class (Shell)
   d) N-Class (Shell)

Ans: a

Explanation:

9. If the number of electrons in an atom is 8 and the number of protons is also 8, then
   1) What is the atomic number of the atom?
   2) What is the charge on the atom? respectively
   a) 8,0
   b) 9,0
   c) 0,8
   d) 0,9

Ans: a

Explanation: 1) Atomic number=Number of protons=8

• The charge on the atom is zero because the number of protons and electrons is equal.

10. Isotopes of an element have:
a) The same physical properties
b) Different numbers of neutrons
c) Different atomic numbers
d)Different chemical properties

Ans: b

Explanation: Isotopes have different physical properties, similar chemical properties, and the same atomic number but they have different mass numbers. Hence isotopes of an element have different numbers of neutrons.

11. Which of the following statement is always correct?
    a) An atom has an equal number of electrons and protons
    b) An atom has an equal number of electrons and neutrons
    c) An atom has an equal number of protons and neutrons
    d) An atom has an equal number of electrons, protons and neutrons

Ans: a

Explanation: Atom is electrically neutral. In a neutral atom, atomic number = number of protons = number of electrons

12. An atom of element A may be written as ₁₂A²⁴. Then the electronic configuration of the given atom is:
    a) 2,8,2
    b) 2,8,1
    c) 2,8,8
    d) None of the above

Ans:b

Explanation: Electronic Configuration of the given atom – K    L     M

2,    8,    2

13. The number of valence electrons determines:
    a) Physical properties of elements
    b) Chemical properties of the element
    c) Both physical and chemical properties of the element
    d) Neither physical nor chemical properties of the element

Ans: b

Explanation: The electrons present in the outermost shell of an atom are known as the valence electrons. They govern the chemical properties of the atom.

14. Which one of the following elements has a 2,8,8,2 electronic configuration?
    a) Calcium
    b) Palladium
    c) Copper
    d) Silver

Ans: a

Explanation: The given electronic configuration contains 20 electrons. Therefore it is the electronic configuration of calcium (Z=20).

15. Number of electrons in Cl^– ion is:
    a) 16
    b) 8
    c)18
    d) 17

Ans: b

Explanation: Valence electrons are the electrons present in the outermost shell of an atom. Cl^– ion is formed when Cl atoms gain an electron. The atomic number of Cl is 17, it has 17e^-‑, when it gains one e^‑, it becomes Cl^– ion and has 18 e^–.

Thus, the electronic configuration of the Cl^– atom and Cl^– ion are as follows:

Cl (atom)=2,8,7; Valence electrons=7

Cl^– (ion)=2,8,8;Valence electrons=8

16. Element with valency 1 are:
    a) Always metals
    b) Always metalloids
    c) Either metals or non-metals
    d) Always non-metals

Ans: c

Explanation: Metals and Non-Metal both can have valency 1. Metals have 1 valence electron and non-metals which have 7 valence electrons have valency 1. It is because metals lose their 1 e^– and non-metals gain 1e^– to complete their octet.

17. Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order.
    1) Rutherford’s atomic model
    2) Thomson’s atomic model
    3) Bohr’s atomic model

1. 1),2) and 3)
2. 2),3)and 1)
3. 2), 1) and 3)
4. 3),2) and 1)

Ans: c

Explanation: The correct order of the improvements in the atomic model is as Thomson’s atomic model, Rutherford’s atomic model and then Bohr’s atomic model.

18. Which of the following correctly represents the electronic distribution in the Mg atom?
    a) 3,8,1
    b) 2,8,2
    c) 1,8,3
    d) 8,2,2

Ans: b

Explanation: Atomic number and the number of electrons in the magnesium atom is 12. So electronic configuration is 2,8,2 (because 12=2-8-2)

19. Elements ₂₀Ca⁴⁰ and ₁₈Ar⁴⁰ are:
    a) Isobars
    b) Isotopes

Ans: a

Explanation: Isotopes-Same atomic number and different atomic mass

Isobars- Different atomic numbers and the same mass number

20. The ion of an element has 3 positive charges. The mass number of atoms is 27 and the number of neutrons is 14. What is the number of electrons in the ion?
a) 13
b) 10
c) 14
d) 16

Ans: b

Explanation: Mass number of ions = 27

No. of neutron = 14

Mass Number= No. of protons + No. of Neutron

27 = No. of proton + 14

No. of proton= 27-14

= 13

If the ion has 3 positive charges then no. of electrons on the ion =10

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