Atoms and Molecules Concepts For Class 9th Chemistry

  • Sugar,Salt,Rice,Table,Chair all are made up of different kind of Matters.
  • We call matter as Padarth in Hindi
  • An Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter (padarth), we shall get smaller and smaller particles.     
  • Ultimately, a time will come when we shall come across the smallest particles beyond which further division will not be possible.
  • He named these particles Parmanu.
  • Another Indian philosopher, Pakudha Katyayama said that these particles normally exist in a combined form which gives us various forms of matter.
  • Ancient Greek philosophers – Democritus and Leucippus suggested that if we go on dividing matter, a
    stage will come when particles obtained cannot be divided further.
  • Democritus called these indivisible particles atoms (meaning indivisible).
  • All this was based on philosophical considerations and not much experimental work to validate these ideas
    could be done till the eighteenth century.
  • Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination

Laws of Chemical Combination

  • Lavoisier and Joseph L. Proust established two laws of chemical combination


  • Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
  • For Example if we take 20 gm of Sodium chloride and Seperate Sodium And chlorine chemically then the sum of mass of Sodium and Chlorine will be same .


  • This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.
  • Let us understand this with an example.
    in water the mass of hydrogen to the mass of oxygen is always 1:8. Thus if  we break 9 gm of water we will get 1 gm of hydrogen and oxygen. and if we break 27 gm of water which is  3 times the taken amount it becomes 3 gm of hydrogen and 24 gm of oxygen.

John Dalton tried providing explanation for the law of conservation of mass and the law of definite proportions.

A sample Problem from NCERT Based on Law of conservation of Mass

Q1. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass carbonate.


Q2. Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Ans. The ratio of H : O by mass in water is:
Hydrogen : Oxygen —> H2O
∴ 1 : 8 = 3 : x
x = 8 x 3
x = 24 g
∴ 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

                                               John Dalton

Dalton’s atomic theory

(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.

Size of an atom

  • Atoms are very small, they are smaller than anything that we can imagine or compare with.
  • More than millions of atoms when stacked would make a layer barely as thick as this sheet of paper.
Atoms in Solid seen in scanning tunneling microscope

Symbols of atoms of different elements

  • Dalton was the first scientist to use the symbols for elements in a very specific sense
  • Berzelius suggested that the symbols of elements be made from one or two letters of the name of the element.








IUPAC (International Union of Pure and Applied Chemistry) approves names of elements

  • Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Examples are: (i) chlorine, Cl, (ii) zinc, Zn etc.
  • Other symbols have been taken from the names of elements in Latin, German or Greek.
  • For example, the symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium.
  • Therefore, each element has a name and a unique chemical symbol.
  • in 1961 for a universally accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses.

One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.

  • Mathematically Atomic mass unit = 1/12 the mass of carbon-12 atom
  • or 1 u or 1 atomic mass unit (amu)=1/12 the mass of a carbon-12 atom
  • we take the atomic mass of carbon-12 is 12 atomic mass unit.
  • From experiments it has been found that the mass of carbon 12 atom is 1.9926*10-23 gram
  • so 12 amu =1.9926*10-23 gram

The relative atomic masses of all elements have been found with respect to an atom of carbon-12.










  • Atoms of most elements are not able to exist independently.
  • Atoms form molecules and ions.
  • These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.

Click here to visit and download Free NCERT Solutions on Atoms and Molecules

Here is Few Short Type questions you should solve and upload here.for better understanding.

  1. Name the element used as a standard atomic mass scale
  2. The radius of an oxygen atom is 0.073nm. what does the symbol nm represent?
  3. What are the building block of matter
  4. Name the scientist who gave the atomic theory of matter
  5. State the law of conservation of mass ?give an example to explain it .
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