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Class 9 revision notes for Science Chapter 3 Atoms and Molecules are created by askIITians experts for all students. These notes are a consolidated summary of the chapter including all the important points, diagrams and tables. You can use these notes to understand all the topics of the chapter, do a quick revision before the exam or memorise the main points of the chapter. These online revision notes are available for free for all students. Refer to them whenever you want and study them at your own pace.
The main topics included in the revision notes for CBSE Class 9 Chapter 3 Atoms and Molecules are the laws of chemical combinations, laws of constant proportions, the structure of atoms, symbols of different elements, atomic mass, introduction to molecules, ions, chemical formulae of different compounds, formulae of simple compounds, molecular mass, formula unit mass, and mole concept. At askIITians, you can also find NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules along with Class 6, 7, 8, 9, 10, 11, 12 notes for Science and Maths.
Democritus and Leucippus – They suggested that when we keep on dividing the matter there comes a time when no more division of particles can take place. Such particles are called atoms which means being invisible.
But all these ideas were not backed up by many experimental pieces of evidence until Antoine L. Lavoisier provided two laws of chemical combination.
1. Law of conservation of mass – mass can neither be created nor destroyed in a chemical reaction
2. Law of constant proportion/Law of definite proportion – the elements are always present in definite proportions by mass in a chemical substance
For example, Hydrogen and oxygen are present in water in a ratio of 1:8. So if we decompose 9g of water we will obtain 1g of hydrogen and 8g of oxygen.
John Dalton proposed an atomic theory that acted as an explanation of the above two laws. As per the theory, all matter whether it is an element, a compound or a mixture consists of tiny invisible particles called ‘atoms’.
The postulates of the atomic theory by John Dalton
1. The matter is made up of tiny particles called atoms that cannot be divided.
2. Atoms are never formed or destroyed during a chemical reaction.
3. Atoms of an element exhibit the same nature. They have the same size, mass, and character.
4. Atoms of different elements exhibit variant nature. They do not have the same characteristics.
5. Atoms form compounds by combining them in a ratio of whole numbers.
6. A compound contains a constant number and kinds of atoms
We can call atoms as the building blocks of matter. Just like bricks are the building blocks of a building.
Atoms are extremely small. Their size is measured in nanometers, where 1nm = 1/109 m.
Atomic radius is measured in nanometers
1/109 = 1nm
1m = 109 nm
Relative Sizes
Radii (in m)
Example
10-10
Atom of hydrogen
10-9
Molecule of water
10-8
Molecule of haemoglobin
10-4
Grain of Sand
10-2
Ant
10-1
Watermelon
Here are some examples of the symbols that are used to represent different atoms
The symbols for representing an atom are generated from the first two letters of the element’s name. The first letter is always in uppercase (capital letter) while the second letter is written in lowercase. Here are some examples –
Element
Atomic Mass
Hydrogen
1 µ
Carbon
12 µ
Nitrogen
14 µ
Oxygen
16 µ
Sodium
23 µ
Magnesium
24 µ
Sulphur
32 µ
Chlorine
35.5 µ
Calcium
40 µ
Atoms cannot survive independently. So, atoms join together and form molecules or ions.
Here a few examples of atomicity of elements –
Atomicity of some Elements
Name
Atomicity
Formula
Argon
Monoatomic
Ar
Helium
He
Diatomic
O2
H2
N2
Cl2
Phosphorous
Tetra – atomic
P4
Poly – atomic
S8
Molecules of compounds constitute atoms of different elements that combine together in a fixed proportion. For example, water comprises two atoms of hydrogen and one atom of oxygen.
We use a chemical formula to represent the composition of a compound in the form of symbols. To write a chemical formula you must know two things –
1. Symbols of elements
2. Valency
Valency – it is also known as the combining capacity of an element. In other words, valency explains how atoms of one element will mix with atoms of another element. For example, the hydrogen ion is represented as H+ which means that its valency is 1. Similarly, the oxygen ion is represented as O2- which means that its valency is 2. Here is a list of valencies of various elements.
Binary compounds – compounds that consist of two different elements
Here are a few examples of writing the chemical formula
Molecular Mass – summation of all the atomic masses in a molecule
Molecular mass is expressed in atomic mass units (amu).
For example, the molecular mass of HNO3 can be calculated as:
Atomic mass of H =1u
Atomic mass of N =14u
Atomic mass of O =16u
Molecular mass of HNO3 = 1 + 14 + (16*3) = 63u
The sum of atomic masses of all atoms in a formula unit of a compound is called its formula unit mass. The formula unit mass is used in the case of substances that constitute ions. For example, the formula unit mass of Sodium Chloride (NaCl) can be calculated as: (1*23) + (1*35.5) = 58.5u
Consider these formulae –
A quick review of how mole, Avogadro number and Mass are related to each other –
1. What are some study tips for CBSE Class 9 Science Atoms and Molecules?
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3. Why should I refer to CBSE Science revision notes online for Class 9 Chapter 3 Atoms and molecules?
4. What is Chapter 3 Atoms and Molecules of Class 9 Science?
Chapter 3 Atoms and Molecules of Class 9 Science helps you understand how atoms and molecules are made. Atoms and molecules are a part of all complex living beings and even non-living things. This gives you a better understanding of how different substances are made in our environment. This chapter also forms a basis for advanced chemistry and physics concepts in higher classes.
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