1.The idea of divisibility of matter by Indian philosopher

1.1 Maharishi Kanad: – According to Maharishi Kanad’s postulate if we keep on dividing the matter i.e. ‘padarth’ we will get smaller and smaller particles.  And soon we will achieve the smallest of particles i.e. ‘parmanu’ which may not divide further.

1.2 Pakudha Katyayama: – According to Pakudha Katyayama’s postulate there are various forms of matter because the particles of matter exist together in combinations.

2. The idea of divisibility of matter by Greek philosopher

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.

        All these ideas were based on philosophical considerations and not much experimental work.

Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

3. Laws of Chemical Combination

3.1 Law of conservation of mass: –

Mass can neither be created nor destroyed in a chemical reaction. It can only change its form from one form to another.

                     i.e.,    Total Mass of the Reactant = Total Mass of the Product

3.2 Law of constant proportions: –

This law is also known as the law of Definite proportions.

According to this Law, in a chemical substance, elements are always present in a definite proportion by mass.

                For example- water obtained from any source will have the same two elements, namely hydrogen and oxygen present in it. 2 grams of hydrogen and 16 grams of oxygen form a molecule of water. The proportion of hydrogen and oxygen is 1:8 by mass. This proportion will always remain the same, irrespective of the source of water.

                   Similarly, carbon dioxide obtained from any source will contain the same two elements, carbon and oxygen. 12 grams of carbon and 32 grams of oxygen form a molecule of carbon dioxide. Carbon dioxide obtained from any source will always have the proportion of masses of carbon and oxygen as 3:8.


                      According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

The postulates of this theory are given below-

  1. The matter is made up of tiny particles called atoms which cannot be divided.
  2. Atoms are neither formed nor destroyed during a chemical reaction. 
  3. Atoms of an element exhibit same nature (same size, mass, and character).
  4. Atoms of different elements exhibit different nature. They do not have same characteristics.
  5. Atoms form compounds by combining in a ratio of whole numbers.
  6. A compound contains a constant number and kinds of atoms.


The smallest particle of matter is an atom or we can say this is the building block. An atom is very small in size and consists of subatomic particles protons, neutrons and electron.

Atomic radius is measured in nanometers = 1/109m= 1nm, 1m= 109nm

5.1 Modern Day Symbols of Atoms of Different Elements

  • Dalton designed specific symbols for each element in a very specific sense. Each symbol was meant for a particular element also for a definite quantity of that element that is a single atom.
  • Berzelius suggested that the symbol of element can be made from one or two letters of the name of the elements. nowadays IUPAC (International Union of Pure and Applied Chemistry) is an international scientific organisation which approves names for elements symbols and units. For example: Hydrogen as H, aluminium as Al and Cobalt as Co.
  • The symbols of some elements are formed from the first letter of the name and a letter appearing later in the name. For example: Chlorine – Cl, Magnesium- Mg.
  • The symbols for some elements were derived from their Latin, German or Greek names.
English name of the element                   Latin name of the elementSymbol       
Sodium                              Natrium  Na
Potassium              Kalium                     K
Iron                                                  Ferrum                    Fe  
Copper                   Cuprum                   Cu  
Silver                      Argentum                Ag  
Gold                       Aurum                     Au  
Mercury                Hydrargyrum           Hg
Lead                        Plumbum                  Pb
Tin                         Stannum                   Sn  

 6. Modern Symbols of Elements

              The modern symbols of elements are derived from their English or Latin names, which are made up of either the first letter, the first and second letter or the first letter and a letter appearing later in the name of the element.  

6.1 Significance of Symbol of an Element

The symbol of an element signifies

(1) The name of the element.

(2) An atom of the element.

                 For example, the symbol N stands for,

                                            (1) The element nitrogen.

                                            (2) An atom of the element nitrogen.  

7. Atomic Mass

According to Dalton’s atomic theory each element has distinguishing atomic mass. This theory could also explain the law of constant proportion. Earlier, hydrogen was taken as a standard for measuring the atomic masses of elements. Later, carbon-12 isotope was chosen as a standard for measuring the atomic masses of elements.      

              1 atomic mass unit (amu) or 1 u = 1/12th mass of carbon–12 atom

             Atomic mass of Carbon is 12u (12 atomic mass units). Therefore, one atomic mass unit is the mass of 1/12 the mass of a carbon-12 atom. Here is a list of atomic masses of a few elements.

7.1 How do Atoms Exist? 

  • Atoms of a few elements exist in the free state i.e., as single atoms. For example- noble gases (helium, neon, argon and krypton etc.)
  • But chemically reactive elements, do not exist in the free state. They exist either as molecules or ions.
  • For example, an iodine crystal is a collection of many iodine molecules. These molecules are so tiny that they are not visible to the naked eye.
  • Similarly, in sodium chloride, the sodium ions and chloride ions being very tiny are not visible. But we see the compound sodium chloride as a white powder which is made up of several sodium and chloride ions.


  • A collection of various atoms that combine chemically with each other is called molecule.
  • It is the smallest particle of an element or a compound which has properties of the element or the compound and can exist in a free state.  
  • Thus, molecules are of 2types — molecules of elements and molecules of compounds.


Two or more similar atoms combined together to form a molecule of element. They are of different types such as diatomic, triatomic, tetra-atomic and poly-atomic molecules, depending on the number of atoms present in them. The number of atoms constituting a molecule is known as its atomicity.

Hydrogen H2Diatomic (2)
Helium HeMonoatomic (1)
Nitrogen N2Diatomic (2)
Sulphure S8Polyatomic (8)
Ozone O3Triatomic (3)
Phosphorus P4Tetra-atomic (4)

 8.2 Molecules of compounds

Atoms of different elements join together in definite proportions to form molecules of compounds.

CompoundCombining atomsSimplest ratio
Water (H2O)Hydrogen, Oxygen18:1
Ammonia (NH3)Nitrogen, Hydrogen14:3
Carbon dioxide (CO2)Carbon, Oxygen3:8

9. Ions & radicals

  • Compounds composed of both metal and non-metals. These elements include charged species known as ions.
  • These are formed by loss or gain of electron. So, ion is a particle having positive or negative charge. The positively charged ions are called as cation and the negatively charged ions are called as anions.
  • They are called radicals or most commonly known as ions.

9.1 types of ions or radicals

 Ions are of 2 types based on charge.

  1. Cations

example- sodium ion (Na+)

2. Anions

Example- chloride ion (Cl)

10. writing chemical formula

The chemical formula is a symbolic representation of the composition of the compound in the form of symbol. To write a chemical formula you must know two things – 1. Symbols of elements 2. Valency

Valency – The combining power (or capacity) of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.

                   For example, the oxygen ion is represented as O2- which means that its valency is 2. Similarly Sulphate ion is written as SO42- , where the 2- charge indicates that its valency is -2.

Sometimes, the same element may exhibit one valency in one compound and another valency in some other compound. This property is called variable valency. For example – Copper has 2 valencies – Cu+, Cu2+. Iron also has 2 valencies – Fe2+, Fe3+.

10.1 Steps of Writing Chemical Formulae

Step 1: Write the symbol of a basic radical (element with a positive valency) on the left-hand side and that of the acidic radical (element with a negative valency) on the right-hand side.

Step 2: Write the valency number/charge of each of the respective ions at the bottom of its symbol.

Step 3: Interchange the valency number. Ignore the (+) and (-) sign.

Step 4: Write the interchanged number.

Step 5: Write the compound’s formula.

Step 6: Cross the reduced valencies. If 1 appears, ignore it. And if a group of atoms receives a valency number more than 1, enclose it within brackets.

Examples –

11. Molecular Mass and Mole Concept

11.1 Molecular Mass

Molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. The relative mass of a molecule is expressed as atomic mass unit (u).

For Example: Let us determine the molecular mass of HNO3

Atomic mass of H =1u

Atomic mass of N =14u

Atomic mass of O =16u

Molecular mass of HNO3 = 1 + 14 + (16×3) = 63u

11.1.1Formula Unit Mass
• It is the sum of the atomic masses of all the atoms in a formula unit of a compound.
• We do not use term molecular mass for ionic compounds. Thus, we use term formula unit for those substances whose constituent particles are ions.
For example – calculate formula unit mass of K2CO3
= 2 × atomic mass of K + 1 × atomic mass of C + 3 × atomic mass of O
= 2 ×39u + 1× 12u + 3 × 16
= (78 + 12 + 48) u
= 138u

11.2 Mole Concept

Mole is a numerical quantity which is used to describe a collection of particles i.e. atoms, molecules or ions.

1 mole =6.022 × 1023 in numbers

Avogadro experimentally found that one mole of any substance always contained 6.022 × 1023 particles. This number is called the Avogadro’s number, denoted by NO.

The mass of 1 mole of a substance is the same as that its atomic mass or molecular mass expressed in grams.

i. Gram atomic mass of a substance – the atomic mass of a substance when expressed in grams is known as its gram atomic mass.

ii. Gram molecular mass of a substance – the molecular mass of a substance when expressed in grams is known as its gram molecular mass.

Example – One mole of carbon atom is equals to 12 grams of carbon atom equals to 6.023×1023 atoms of carbon. One mole of hydrogen atom is equals to 1 gram of hydrogen atom which is equals to 6.022×1023 atoms of hydrogen.

No. of moles = n

Given mass = m

Molar mass = M

Given number of particles = N

Avogadro number of particles = N0

1. The number of moles(n) = given mass/ molar mass= m/M

2. (For the problems based on Avogadro number)

The number of moles = Given number of particle /Avogadro number =N/N0                                                                                                        

3. To find mass Mass (m) = Molar mass(M) × Number of moles (n) 

4. To find the number of atoms when Avogadro number is given in the question,

The number of atoms = (Given mass × Avogadro number) /Molar mass

                                                          N= (m×N0)/M

5. The number of particles (atoms) = Number of moles of particles × Avogadro number

                                                     N= n × N0

Categories: General

1 Comment

RITU PRIYA · December 13, 2020 at 6:09 am

Well explained!it was helpful

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