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Chapter Analysis
Beginner12 pages • EnglishQuick Summary
The chapter 'Atoms and Molecules' for Class 9 Science introduces students to the fundamental concepts of atomism, the basic laws of chemical combinations, and Dalton's atomic theory. It covers the definitions and distinctions between atoms, molecules, and ions, explaining how they combine to form chemical compounds with examples of forming chemical formulas. The chapter also touches on the concept of the atomic mass and how it is determined, emphasizing the importance of atomic theory in understanding matter.
Key Topics
- •Dalton's Atomic Theory
- •Law of Conservation of Mass
- •Law of Constant Proportions
- •Concept of Molecules
- •Formation of Ions
- •Chemical Formulas
- •Atomic Mass
- •Polyatomic Ions
Learning Objectives
- ✓Understand and explain Dalton's Atomic Theory.
- ✓Define and illustrate the laws of chemical combination.
- ✓Distinguish between atoms, molecules, and ions.
- ✓Master the writing of chemical formulas for compounds.
- ✓Apply the concept of atomic mass in calculations.
- ✓Differentiate between monoatomic, diatomic, and polyatomic structures.
Questions in Chapter
Define the atomic mass unit.
Page 30
Why is it not possible to see an atom with naked eyes?
Page 30
What are polyatomic ions? Give examples.
Page 35
Write the chemical formulae of the following compounds: (a) Magnesium chloride, (b) Calcium oxide, (c) Copper nitrate, (d) Aluminium chloride, (e) Calcium carbonate.
Page 35
Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.
Page 35
Additional Practice Questions
Explain Dalton’s atomic theory in detail.
mediumAnswer: Dalton's atomic theory states that all matter is composed of atoms, which are indivisible and indestructible particles. Atoms of a given element are identical in mass and properties, and atoms of different elements differ in these attributes. Chemical reactions involve the rearrangement of atoms, not their creation or destruction, and compounds are formed by a combination of two or more different types of atoms.
What is the law of conservation of mass? Provide an example to illustrate it.
easyAnswer: The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. For example, when hydrogen burns in oxygen to form water, the total mass of water produced is equal to the sum of the masses of hydrogen and oxygen that reacted.
Describe the difference between an atom, a molecule, and an ion with examples.
easyAnswer: An atom is the smallest unit of an element, like an O atom in oxygen. A molecule is a group of atoms bonded together, like O₂, which contains two oxygen atoms. An ion is an atom or group of atoms with a net electric charge due to the loss or gain of one or more electrons, like Na⁺ or Cl⁻.
How can you determine the formula of an ionic compound?
mediumAnswer: The formula of an ionic compound is determined by the charges on the ions; the total positive charge must balance the total negative charge to give a neutral compound. For example, to form NaCl, sodium ion Na⁺ and chloride ion Cl⁻ combine in a 1:1 ratio.
Explain how atomic masses are determined and the importance of using a standard unit.
hardAnswer: Atomic masses are determined relative to the 1/12th mass of a carbon-12 atom, which is defined as one atomic mass unit (u). Using a standard unit allows for consistency in comparing the mass of atoms across different elements.
NCERT Exemplar
AvailablePractice with NCERT Exemplar problems and solutions for this chapter. Exemplar problems are designed to help you master the concepts with advanced-level questions.
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