Science MCQ, Class 9th, Chapter-3, Atoms and Molecules

This article covers NCERT based MCQ of Class 9th, Chapter-3, Atoms and Molecules with brief explanation for competitive exams like HTET, SSC and all other state competition exams. 

Along with MCQ, Chapter Key-Points for quick learning NCERT related facts are also discussed below. 

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Summary of the Chapter-3-Atoms and Molecules with Key-Points:

• The total mass of the reactants and products in a chemical reaction remains constant, a principle known as the Law of Conservation of Mass.
• In a pure compound, the elements are always combined in a fixed ratio by mass, referred to as the Law of Definite Proportions.
• An atom is the smallest unit of an element that typically cannot exist independently while retaining the element's chemical properties.
• A molecule is the smallest unit of an element or compound that can exist independently under normal conditions, exhibiting all the properties of the substance.
• A compound’s chemical formula represents its constituent elements and the number of atoms of each element involved.
• Groups of atoms that function as a single ion with a specific charge are called polyatomic ions.
• The chemical formula of a molecular compound is based on the valency of the elements involved.
• In ionic compounds, the charges on the ions are used to determine the compound’s chemical formula.

NCERT Based MCQ with Explanation:

1. Who proposed the concept of indivisible particles called Parmanu in ancient India?

A) Democritus

B) Leucippus

C) Maharishi Kanad

D) Antoine Lavoisier

Explanation: Maharishi Kanad, an ancient Indian philosopher, proposed the concept of indivisible particles called Parmanu, which are the smallest units of matter.

2. What did Democritus and Leucippus call the smallest indivisible particles of matter?

A) Molecules

B) Atoms

C) Ions

D) Elements

Explanation: Democritus and Leucippus, Greek philosophers, named the smallest indivisible particles "atoms," derived from the Greek word "atomos," meaning indivisible.

3. Which law states that mass is neither created nor destroyed in a chemical reaction?

A) Law of Definite Proportions

B) Law of Conservation of Mass

C) Law of Multiple Proportions

D) Dalton’s Atomic Theory

Explanation: The Law of Conservation of Mass, established by Antoine Lavoisier, states that the total mass of reactants equals the total mass of products in a chemical reaction.

4. What does the Law of Constant Proportions state?

A) Mass is conserved during chemical reactions.

B) Elements in a compound are present in a fixed ratio by mass.

C) Atoms cannot be created or destroyed.

D) Elements combine in different ratios to form different compounds.

Explanation: The Law of Constant Proportions, also known as the Law of Definite Proportions, states that a chemical compound always contains the same elements in the same proportions by mass.

5. Which scientist is credited with laying the foundation of chemical sciences with his work on chemical combinations?

A) John Dalton

B) Antoine Lavoisier

C) Joseph Proust

D) Dmitri Mendeleev

Explanation: Antoine Lavoisier is known as the father of modern chemistry for his foundational work, including establishing the Law of Conservation of Mass.

6. According to Dalton’s atomic theory, what are atoms?

A) Divisible particles that form molecules

B) Indivisible particles that participate in chemical reactions

C) Elements with identical masses and properties

D) Particles that can be seen under a microscope

Explanation: Dalton’s atomic theory states that atoms are indivisible particles that cannot be created or destroyed in a chemical reaction.

7. What key concept did Dalton's atomic theory help explain?

A) The periodic table

B) Chemical bonding

C) The laws of conservation of mass and constant proportions

D) Nuclear reactions

Explanation: Dalton's atomic theory explained how atoms combine in fixed ratios to form compounds, supporting the laws of conservation of mass and constant proportions.

8. What is the smallest unit of an element that retains its chemical properties?

A) Atom

B) Molecule

C) Ion

D) Compound

Explanation: An atom is the smallest unit of an element that retains all of its chemical properties and can participate in chemical reactions.

9. What is the mass of one atomic mass unit (amu) based on?

A) Mass of a hydrogen atom

B) Mass of an oxygen atom

C) 1/12th of the mass of a carbon-12 atom

D) Mass of a proton

Explanation: One atomic mass unit (amu) is defined as exactly 1/12th the mass of a carbon-12 atom, which serves as the standard reference.

10. Which of the following elements has the symbol Fe?

A) Lead

B) Fluorine

C) Iron

D) Silver

Explanation: The symbol Fe for iron is derived from its Latin name, "ferrum."

11. What does the atomic mass of an element represent?

A) The size of the atom

B) The number of protons in the nucleus

C) The mass of an atom compared to carbon-12

D) The number of neutrons in the atom

Explanation: Atomic mass is a relative measure that compares the mass of an atom to 1/12th of the mass of a carbon-12 atom.

12. Which gas was chosen as the standard for atomic mass before carbon-12?

A) Hydrogen

B) Oxygen

C) Helium

D) Nitrogen

Explanation: Before the adoption of carbon-12, oxygen was used as the reference for atomic mass due to its widespread reactivity and whole-number atomic mass values.

13. Which law can be explained by Dalton's postulate that atoms cannot be created or destroyed?

A) Law of Conservation of Mass

B) Law of Constant Proportions

C) Law of Multiple Proportions

D) Law of Periodicity

Explanation: Dalton’s atomic theory postulates that atoms cannot be created or destroyed in chemical reactions, directly supporting the Law of Conservation of Mass.

14. Which postulate of Dalton's atomic theory explains the Law of Definite Proportions?

A) Atoms are indivisible.

B) Atoms of the same element are identical.

C) Atoms combine in small whole-number ratios.

D) Atoms are the basic unit of matter.

Explanation: The postulate that atoms combine in simple whole-number ratios explains the Law of Definite Proportions, as elements form compounds with fixed mass ratios.

15. What is the term used for the average mass of the atom compared to 1/12th the mass of one carbon-12 atom?

A) Atomic weight

B) Atomic radius

C) Atomic mass

D) Atomic number

Explanation: Atomic mass is the average mass of an atom expressed relative to the mass of carbon-12.

16. Which element has an atomic mass of approximately 16 u?

A) Carbon

B) Nitrogen

C) Oxygen

D) Magnesium

Explanation: Oxygen has an atomic mass of approximately 16 u, based on its isotopic composition and relative atomic mass.

17. How many atoms are present in a diatomic molecule of oxygen?

A) One

B) Two

C) Three

D) Four

Explanation: A diatomic molecule of oxygen (O₂) consists of two oxygen atoms bonded together.

18. Which type of molecule consists of different atoms chemically bonded together?

A) Diatomic molecule

B) Monoatomic molecule

C) Elemental molecule

D) Compound molecule

Explanation: Compound molecules consist of different atoms chemically bonded in fixed proportions, such as H₂O or CO₂.

19. What is the simplest compound made up of carbon and oxygen?

A) Carbon dioxide

B) Carbon monoxide

C) Carbon tetrachloride

D) Carbonic acid

Explanation: Carbon monoxide (CO) is the simplest carbon-oxygen compound, formed by one carbon atom and one oxygen atom.

20. Which term refers to charged particles formed by atoms gaining or losing electrons?

A) Molecules

B) Ions

C) Atoms

D) Compounds

Explanation: Ions are charged particles that form when atoms lose or gain electrons, resulting in cations (positive) or anions (negative).