Class 9 Atoms And Molecules Study Notes

5 minute read
Class 9 Atoms and Molecules

The matter that surrounds us is made up of tiny particles. The class 9 syllabus for science entails a very interesting chapter that enlightens us with the actual phenomenon behind the composition of different substances. In addition to that, you will learn about Dalton’s atomic theory, atomic and molecular masses, ions, mole concept and many other interesting topics. Here are the complete study notes for Class 9 Atoms and Molecules to make this chapter easy to study and remember for you!

Laws of Chemical Combination

The chapter begins with a thorough explanation of the two laws of chemical combination. Great minds like Kannad (600 BC), Democritus, and Leucippus (400 BC), as well as Antoine Lavoisier and John Dalton, made some observations, based upon which these laws have been devised. After a detailed and logical analysis, they all came to some conclusions regarding the formation of compounds and laid down the below-mentioned laws- 

Law of Conservation of Mass
According to this law, for a general chemical reaction, there is no loss or gain in the total mass of substances. It means that, in a chemical reaction, there is no change in the total mass as the mass of the product is always equal to the mass of the reactant. 

Law of Constant or Definite Proportion 
This law explains that all pure elements of the same compound contain the same elements in the same proportion of weight. In simpler words, every element combines with another in a definite or constant ratio, i.e., depending on their atomic masses.

Check Out Study Notes for Class 9: Work, Energy and Power

Dalton’s Atomic Theory

The famous chemist, John Dalton, carried out a random experiment which later on became the famous Atomic Theory. He analysed it critically and then presented his insightful observations in the form of postulates of atomic theory. Dalton’s postulates regarding the atomic theory as per class 9 atoms and molecules are:

  • Atoms make up all the matter in this world
  • Atoms of a given element have exactly the same chemical properties and mass
  • Atoms are indivisible particles which can neither be created nor destroyed
  • Atoms of different elements have varied chemical properties and mass
  • Atoms combine in a ratio of small natural numbers to form compounds.
  • A given compound will always have determined kinds and number of atoms


Now we will explore the most vital topic of this chapter. The smallest and the tiniest unit that can be a part of a chemical reaction is known as an Atom. Atoms tend to have some general properties which they showcase, let us have a look at them-

Atoms and Molecules

Atomic Size

Atoms are invisible to the naked eye. Their shape is spherical and is not visible by an ordinary microscope. The diameter of an atom is about 1 × 10-10 m. Even on the tip of a pencil, there are thousands, if not millions of atoms.

Atomic Mass

Every element has a set atomic mass. This was a direct postulate from Dalton’s Theory on atoms. Determining the mass of such a tiny particle was a daunting task. Hence, this is where the concept of Relative Atomic Mass comes in.

What is Motion in a Straight Line?

Symbols for Elements

As the discovery of elements went on for years and years, every element was given a particular symbol either based on its initials or on the name of the scientist who discovered it. Tabulated below are some important elements from the class 9 atoms and molecules chapter. 



The very next important topic of this chapter is of molecules. A molecule is the smallest particle of matter (element or compound) which can exist in a free state. Like atoms, molecules of any given substance are all similar. Consequently, molecules of different substances are different.

Atoms and Molecules

Atomicity is the number of atoms contained in a molecule of a substance (element or compound). Based on atomicity, the types of molecules are:

  • Monoatomic Molecules
  • Diatomic Molecules
  • Triatomic Molecules
  • Tetratomic Molecules
  • Polyatomic Molecules

What is the Difference Between Acid and Base?


The class 9 atoms and molecules chapter include another integral topic called Ions. The compounds formed by charged particles are called ions. Ions carrying positive charges are called Cations, and the ones with negative charges are called Anions. For Example: NaCl is an ionic compound. That means that it is made up of Sodium ion (Na+) and Chlorine ion (Cl).

Rules for writing Chemical Formulae

Writing a chemical formula has an intricate science that runs behind the process. Till now, you must have learnt them without knowing the actual reason for their placement. But in the class 9 atoms and molecules, you will also get to know that the chemical formula denotes the ratio of elements used to form the given compound. The correct formula can be determined if we know the respective valencies of the given elements.

Before we go on to exploring the rules, the combining power of an element or ion as per which it can react is called Valency. The higher the valency, the more the number of atoms or ions it can accumulate. Mentioned below are some common valencies:

Iron (III)Fe+3





The formula of a compound is given by writing, side by side, the symbols of the constituent elements. The symbol of more a metallic element is written first in the formula, for example, HCl, NaCl, CaCO3, etc. Subscripts indicate the number of atoms in each of the constituent elements present. For example, in H2O, there are 2 Hydrogen and 1 Oxygen atom. Furthermore, the simple binary compounds are made by crisscrossing the valencies present in the molecules of a compound.

Mole Concept

One of the most important concepts of class 9 atoms and molecules is the explanation of the mole concept. A mole of atoms is a collection of atoms whose total mass is the number of grams equal to the atomic mass. A mole (mol) is defined as the amount of substance that contains as many atoms, molecules, ions, electrons, etc., as there are atoms in exactly 12 g of carbon 12 (12C). A mole of a substance represents 6.022 × 1023 particles of the substance.

Number of Moles = (mass of the given element in grams) ÷ (gram-atomic mass of the element)

Download the PDF for Class 9 Atoms and Molecules.

Class 9 Atoms and Molecules: Important Questions

Some conceptual questions to clarify any doubts about atoms and molecules class 9 are:

  1. What is the mass of 5 mol of Ammonia?
  2. Explain ‘relative atomic mass.’
  3. Write the correct chemical formula of (i) Sodium Carbonate (ii) Magnesium Oxide.
  4. Define atomicity? What is the atomicity of phosphorus and nitrogen?
  5. Explain the difference between cation and anion?
  6. What is polyatomic ion? Give one example.
  7. What is meant by valency. What is the valency for magnesium and copper?
  8. Explain the types of ion with examples.
  9. What is formula unit of mass? Explain how is it different from molecular mass?
  10. What is Dalton’s Atomic Theory?

Class 9 Atoms and Molecules PPT

Slide Share: Shiva Prasad

These were some essential points for class 9 atoms and molecules. Get in touch with our experts at Leverage Edu as they will help you have a clear picture of your upcoming future. 

Leave a Reply

Required fields are marked *