AP Board Class 9 Physical Science Chapter 5 Atoms And Molecules

AP Board Class 9 Physical Science syllabus is designed in an interesting way as most of the chapters and the concepts discussed in them are inter-related. The good thing is that it makes studying easy and students will be able to master concepts fully. As such, while studying Chapter 5, students will use the acquired knowledge from the previous chapters to understand as well as find answers to various observations. The chapter discusses important topics like;

  • Atoms and Molecules
  • Atomicity
  • Law of constant proportions
  • Law of conservation of mass
  • Avogadro constant


The smallest particle of an element is known as an atom. They can take part in a chemical reaction and retain all its properties.


A molecule is the smallest physical unit of a compound or an element that further consists of two or more atoms which are chemically bonded together.


The total number of atoms present in a molecule is known as its atomicity.

Law Of Constant Proportions

This law states that every pure substance contains the same elements combined in the fixed proportions by weight.

Law Of Conservation Of Mass

The law states that mass or matter can neither be created nor destroyed during chemical reactions in an isolated system. The total mass of the products is always equal to the mass of the reactants.

Avogadro Constant

Avogardo constant is the number of particles present in one mole of any substance. It is a fixed a value of 6.022 X 1023.

Students can have a look at some of the chapter questions given below to further understand some of the topics.

Question 1: Define molar mass.

Answer: Molar mass is defined as the mass of one mole of a substance.

Question 2: What as symbols and formulas used for?

Answer: The main uses of symbols is that it is used to represent atoms, while formulas are basically used to represent molecules and compounds.

Question 3: Write down the chemical names of these common household substances.

a) common salt b) baking soda c) washing soda d) vinegar


a. Sodium Chloride

b. Sodium Hydrogen Carbonate

c. Sodium Carbonate

d. Acetic acid

Question 4: Define Atomicity.

Answer: The number of atoms constituting a molecule is known as its atomicity.

Question 5: What is an atom?

Answer: An atom is the smallest particle of an element that can participate in chemical reaction and retain all its properties.

Keep visiting BYJU’s to learn more about Class 9 Science topics and access various Andhra board study materials,  including syllabus, exam timetable and more.

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