Atoms and Molecules Class 9

Atoms and Molecules Class 9 Notes

The unseen form of matter was always wondered about. First of all the  idea of divisibility had been given by the  Philosopher Maharshi Kanad. He postulated that if we keep dividing the matter (called ‘padarth’) we will get smaller and smaller particles. And finally we will achieve the smallest of particles (called ‘parmanu’) which may not be divided further.

Pakudha Katyayama had  postulated that there are various forms of matter because the particles of matter exist together in combinations.

The idea of divisibility was given by the  Greek philosophers Democritus and Leucippus. They had suggested that when we keep dividing the matter at a particular instant, no more division of particles will  take place. These particles are called atoms.

Laws of Chemical Combination

1. Law of conservation of mass 

The law of conservation of  mass states that the mass can neither be created nor destroyed in any chemical reaction

2. Law of constant proportion/Law of definite proportion 

This law states that the elements are always present in definite proportions of  mass in a chemical substance.

As an example, in water Hydrogen and oxygen are present  in a ratio of 1:8. So in the decomposition of 9g of water, we will obtain 1g of hydrogen and 8g of oxygen.

Get notes of : Matters in our surroundings

Get notes of : Is matter around us pure

The Atomic Theory

John Dalton proposed an atomic theory which acted as an explanation of the  two laws of Chemical Combination. According to this theory, all matter, whether it is an element, a compound or a mixture, consists  of tiny invisible particles called ‘atoms’.

Postulates of the atomic theory proposed John Dalton

1. The matter is generally made up of tiny particles. These particles are called atoms and these atoms cannot be divided further.

2. Atoms cannot be formed nor be destroyed during any chemical reaction.

3. Atoms of an element exhibit the same nature. These atoms have the same size, mass, and character.

4. Atoms of different elements exhibit variant nature. They do not have the same characteristics.

5. By combining in a ratio of whole numbers, the atoms form compounds.

6. A compound consists of  a constant number and kinds of atoms

Atoms :

We can call atoms the building blocks of matter. As the bricks are the building blocks of any building.

What is the size of an atom?

Atoms are extremely small. The size of the atom is measured in nanometres.

Where 1nm = 1/10^9 m.

Atomic radius is measured in nanometres

1/10^9 = 1nm

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

Symbols for Atoms

Few examples of atoms with their symbols are mentioned below :

  Element       Symbol       Element    Symbol 

Aluminium        Al               Copper         Cu 

Argon                  Ar              Fluorine        F 

Barium               Ba              Gold              Au 

Boron                  B                Hydrogen     H 

Bromine             Br               Iodine            I 

Calcium              Ca               Iron              Fe 

Carbon                C                 Lead             Pb 

Chlorine             Cl            Magnesium    Mg 

Cobalt                 Co               Neon            Ne 

The symbols for representing any atom are mainly generated with the first two letters of the  name of the element. 

The first letter is always used in capital letter, while the second letter is written in small letter ( lower case ). 

Here are some examples –

The Atomic Mass

Dalton's Atomic Theory gave information that each element has a distinguishing atomic mass. On the basis of this theory, the law of constant proportions could be explained easily.

But it is still difficult to evaluate the mass of an atom because its size is relatively small. Therefore scientists used to evaluate the mass of an atom by comparing it with other standard atom's mass. Previously 1/16 of the mass of an oxygen atom was used as a standard value for calculating the mass of other elements. But now carbon - 12 atom is considered as a standard atom for calculating the mass. Its atomic mass is 12u (12 atomic mass units). 

Hence, we say that the one atomic mass unit is the mass of 1/12 the mass of a carbon-12 atom. 

List of atomic masses of some elements : 

Elements  Atomic Mass

Hydrogen 1 u

Carbon 12 u

Nitrogen 14 u

Oxygen 16 u

Sodium 23 u

Magnesium 24 u

Sulphur 32 u

Chlorine 35.5 u

Calcium 40 u

Can atoms exist independently?

Atoms cannot survive independently. So, atoms join together with each other and form molecules or ions.

 Molecules :

 A molecule is a combination of various atoms that combine chemically with each other. These atoms are bound together with interatomic forces. Atoms of the same elements or different elements bound  together to form molecules.


Therefore, a molecule is defined as  the smallest particle of a substance that can exist independently and shows all the properties of that substance.

Molecules of Elements

The molecule of an element is formed by combinations of the same types of atoms. 

For example, 

Helium consists of only one atom while oxygen consists of two atoms.



The number of atoms present in a molecule of an element is called its atomicity. 

For example, 

Helium is monoatomic and oxygen is diatomic.



When an element is made up of only a single atom. Example – all metals


When an element is made up of  two atoms. Example – all gases



When an element made up  of three atoms



When an element made up of four atoms



When an element made up of more than two atoms

Few Examples of Atomicity of Elements –

Name Atomicity Formula

Argon Monoatomic Ar

Helium Monoatomic He

Hydrogen Diatomic H2

Nitrogen Diatomic N2

Chlorine Diatomic Cl2

Phosphorous Tetra – atomic P4

Sulphur Poly – atomic S8

Molecules of Compounds

Molecules of compounds consist of atoms of different elements that combine together in a fixed ratio. For example, water consists of two atoms of hydrogen and one atom of oxygen


Compounds contain metals as well as non-metals. These elements have charged species which are known as ions.

Thus, ions are  generally particles that have a positive or negative charge.

 Anion – negatively charged ion

 Cation – positively charged ion


There could be a single charged atom in an ion or there may be a group of charged atoms in an ion and will have a net charge on the compound. When a group of atoms carries a charge in a compound then it is known as a polyatomic ion.

Chemical Formula

We often use a chemical formula for the  representation of the composition of a compound in the form of symbols. To mention a chemical formula you must know two basic things –

1. Symbols of elements

2. Valency


Valency is defined as the combining capacity of an element. 

In other words, valency of an atom explains that upto what extent the 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. In the same way, the oxygen ion can be represented as O2- where its valency is mentioned as 2. 

Binary compounds – The compounds which consist of two different elements are known as Binary compounds.

Molecular Mass 

Molecular Mass is defined as the  summation of all the atomic masses in a molecule and it is expressed in terms of atomic mass units (amu).

For example let us calculate the molecular mass of HNO3  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

Formula Unit Mass

The sum of the atomic masses of all the atoms in a formula unit of a compound is called its formula unit mass. The formula unit mass is generally used in case of those substances which constitute ions. 

For example, 

formula unit mass of Sodium Chloride (NaCl) can be calculated as: (1*23) + (1*35.5) = 58.5u

How do we interpret a chemical equation?

Let us consider a chemical equation as shown below :

2C + O2 = 2CO2

We say that according to the equation, two molecules of carbon combine with one molecule of oxygen to give two molecules of carbon dioxide.

We may also interpret it as 24u of Carbon molecules combining with 32u of oxygen molecules to produce 56u of carbon dioxide molecules. Hence we are characterising the quantity of a substance by its mass or by its number of molecules.

A chemical equation is used to indicate the number of molecules participating in any reaction. Thus, it becomes easy for us to illustrate the number of substances in a chemical reaction as numbers of molecules or atoms.

Mole Concept

Wilhelm Ostwald introduced the concept of ‘mole’ which means a heap or a pile. Thus, we refer to a substance as a heap of atoms or molecules. It is a numerical quantity of atoms that has a mass equal to the atomic or molecular mass of a species. (atoms, molecules, ions or particles). 

Relationship of mole to mass of the particles and numbers 

1 mole of any substance = 6.022 X 10^23 number of particles (atoms, ions or molecules). This number is known as the Avogadro number or Avogadro Constant and it is represented by No.

The mass of 1 mole of a substance is equal to its atomic mass or molecular mass expressed in grams.

Gram atomic mass of a substance 

When the atomic mass of a substance is expressed in grams, it is known as its gram atomic mass.

Gram molecular mass of a substance

When the molecular mass of a substance is expressed in grams, then it is known as its gram molecular mass.

For example, the atomic mass of Sulphur is 32u and the gram atomic mass of Sulphur is 32g.

Again, 32u of Sulphur has 1 atom of Sulphur and 32g of Sulphur has 1 mole atom, i.e. 6.022 X 10^23 atoms of Sulphur.

In the same way, we  say that the gram molecular mass of Carbon Dioxide is 56g.

But we also know that in the case of a chemical equation, the mole is the measuring unit.

Therefore, 1 mole = 6.022 × 10^23 number   = Relative mass in grams

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