NCERT Class 9 Science Chapter 3 Notes Atoms And Molecules- Download PDF Notes

Atoms and molecules is a very important chapter from the NCERT Book of the CBSE Board. The NCERT Class 9 Science Chapter 3 notes provide basic and detailed knowledge about atoms and molecules. The main topics covered in NCERT Class 11 Biology notes are definition, laws of chemical combination, such as the law of conservation of mass, the law of constant proportions, what is an atom, atomic mass, how do atoms exist, what is a molecule, what is an ion, writing chemical formulae, molecular mass, and mole concept. Download the CBSE Notes for Class 9 Science, Chapter 3, PDF to use offline anywhere. Students must go through each concept, including formulae and examples of atoms and molecules Class 9 Notes Science in the easiest and most effective way possible with the help of NCERT Notes for Class 9.

Class 9 Science chapter 3 notes also cover all the important concepts related to this chapter, which are the foundation for classes 11 and 12. Atoms and molecules NCERT Notes for Class 9 Science help you revise these major concepts given in the NCERT Book in no time during CBSE exam preparation. CBSE Class 9 Science Chapter 3 notes will help you with quick revision. The chapter Atoms and Molecules covers all headings of NCERT. CBSE Class 9 Science chapter 3 notes also contain important examples that have been frequently asked. Having revision notes and NCERT Solutions for Class 9 Science Chapter 3 handy is beneficial to save you time. The NCERT Class 9 notes PDF can be downloaded through the link given below.

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• NCERT solutions for class 9 chemistry chapter 3 Atoms and Molecules
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NCERT Class 9 Chapter 3 Class Notes

Atoms and Molecules

Definition: Democritus explained that if we keep on dividing a matter, we will end up with its smallest unit, which won’t be divisible further, and he named it an atom.

Laws of Chemical Combination

Antoine L. Lavoisier is called the father of chemistry, for he gave two important laws of chemical combination which laid the foundation of chemical sciences, and the laws are as follows:

Law of Conservation of Mass

It states that “Mass is not created and not destroyed, in a chemical reaction”.

Law of Constant Proportions

“The elements are present in definite proportions by mass, in a chemical substance”.

Explanation of these laws as given by Dalton (Dalton’s Atomic Theory).

The postulates of the theory are given below:

1. “Matter is made of very small particles called atoms”.

2. “Atoms are not divisible particles, which are not created and not destroyed in a chemical reaction”.

3. “Atoms of the same element have identical mass and chemical properties”.

4. “The chemical properties of the atoms are different for different elements.”.

5. “Compounds are formed by combining atoms of different elements in ratios of small whole numbers”.

How big are atoms?

Relative size of different matters:

What are the modern day symbols of atoms of different elements?

Some elements and their symbols:

Atomic Number

It is the number of protons present in an element.

Atomic Mass

It is the number of protons and neutrons present in an element.

Carbon-12 isotope is the standard reference for calculating atomic masses of other elements. One atomic mass unit is a mass unit equal to one-twelfth (1/12th) the mass of one atom of carbon-12.

What is a Molecule?

It is a group of two or more elements held together in a chemical bond by attractive forces.

Atomicity

The number of atoms in a molecule is called the atomicity of the molecule.

• For example:, argon and helium are monoatomic,
• Oxygen and chlorine are diatomic.
• Phosphorus is tetra-atomic and
• Sulphur is polyatomic.

Ion

An ion is a charged particle; it can be positively charged or negatively charged. Positively charged ions is called ‘cation’ and negatively charged ion is called ‘anions’.

How to Write Chemical Formulae

It is a symbolic representation of a compounds and its composition.

Given below are the rules for writing a chemical formula:

• The charges on the ions have to balance each other.

• If the compound contains a metal and a nonmetal, the metal is written first.

For example: sodium chloride (NaCl), where chlorine is a non-metal, so written on the right, and sodium is a metal, so written on the left.

• In compounds with polyatomic ions, the ion is enclosed in a bracket and then the number of the ion is indicated. If number of polyatomic ion is one, the bracket is not required.

For example: Formula for aluminium oxide is given by , as Al has 3+ charge and O has 2- charge.

Molecular Mass and Mole Concept

The molecular mass of a substance is equal to the sum of the atomic masses of all the atoms in that molecule.

If the substance contains ions as its constituents, then molecular mass is called Formula Unit Mass.

The number of particles present in 1 mole of any substance is given by Avogadro Number, which is 6.022 × 10²³.

Mole Concepts

Any quantity in number having a mass equal to its atomic or molecular mass in grammes is referred to as one mole of any species (atoms, molecules, ions, or particles). A single mole of any given substance comprises 6.022 × 1023 particles, which can be atoms, molecules, or ions. This is an experimentally obtained value. Named in honour of the Italian scientist Amedeo Avogadro, this number is also known as the Avogadro Constant or Avogadro Number (represented by N0).

1 mole (of anything) = 6.022 × 1023 in number

Chapter-Wise NCERT Class 9 Notes Science

Significance of NCERT Class 9 Science Chapter 3 Notes

Atoms and Molecules class 9th notes are helpful to revise the chapter and get an idea about the main topics covered in the chapter. Also, this NCERT class 9 science chapter 3 notes are useful to set base for competitive exams like VITEEE (Link), BITSAT (Link), JEE Main (Link), NEET (Link), etc. These Notes for Class 9 Science Chapter 3 are also not complicated, and they thoroughly explain step-by-step approaches to guarantee that students understand the concepts of this chapter, which is part of the CBSE Science syllabus for class 9. Class 9 science chapter 3 notes pdf download can be very helpful in reading through paper.

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