SCIENCE - CLASS IX - ATOMS AND MOLECULES
Chapter-wise Multiple Choice Questions with Instant Feedback
Quick Revision Box
- Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction. (Total mass of reactants = Total mass of products).
- Law of Constant Proportions: A pure chemical compound always contains the same elements combined together in the same fixed proportion by mass.
- Dalton's Atomic Theory: All matter is made of tiny, indivisible particles called atoms. Atoms of a given element are identical.
- Atom: The smallest particle of an element that can exist and take part in a chemical reaction.
- Atomic Mass Unit (u): A mass unit equal to exactly one-twelfth (1/12) the mass of one carbon-12 atom.
- Molecule: The smallest particle of an element or compound that can exist independently and shows all the properties of that substance.
- Atomicity: The number of atoms present in one molecule of an element (e.g., O₂ is diatomic, O₃ is triatomic).
- Ion: A charged particle (cation = positive, anion = negative). Formed by loss or gain of electrons.
- Valency: The combining capacity of an element. It is equal to the number of electrons an atom can lose, gain, or share.
- Chemical Formula: A symbolic representation of a compound that shows the elements present and the number of their atoms (e.g., H₂O).
- Mole Concept: 1 mole is the amount of substance containing 6.022 × 10²³ particles (atoms, molecules, ions).
- Avogadro Constant (N₀): 6.022 × 10²³, the number of particles in one mole of a substance.
- Molar Mass: The mass of 1 mole of a substance (equal to atomic/molecular mass in grams).
- Key Formula (Mole Concept): Number of Moles (n) = Given Mass (m) / Molar Mass (M). Number of Particles (N) = n × N₀.
Basic Level Questions
Chapter Summary
"Atoms and Molecules," unveils the fascinating invisible world that constitutes everything around us. This chapter is the foundation of chemistry, taking you from the macroscopic laws governing chemical reactions to the microscopic particles responsible for them. By exploring this chapter through targeted MCQs, you will move beyond rote memorization to develop a deep, conceptual understanding essential for acing your Class 9 Science board exams and building a strong base for competitive tests.
What You Will Learn from This Chapter & How This Website Helps: This chapter equips you with the fundamental laws of chemical combination—Conservation of Mass and Constant Proportions—and introduces you to Dalton's revolutionary atomic theory. You will learn to differentiate between atoms, molecules, and ions, understand the concept of valency, and master the skill of writing chemical formulae for countless compounds. The pivotal mole concept, often a challenging topic, is broken down to help you seamlessly connect the mass of a substance to the number of particles it contains. Our website reinforces this learning by providing a vast repository of practice questions. Each MCQ is designed to test your application of these concepts, from simple calculations of molecular mass to complex problems involving the mole concept and chemical reactions. This rigorous practice ensures you are not just familiar with the definitions but are exam-ready, capable of solving numerical problems accurately and understanding the 'why' behind every chemical principle.