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Atoms and Molecules Notes for class 9:

Atoms and molecules notes

Atoms and Molecules Notes: Introduction

Are you aware that you can save a lot of time while preparing for exams by reading class 9 notes? In this section we have provided you with atoms and molecules notes. It consists of Law of chemical combination (law of conservation of mass, law of constant proportions), Dalton’s atomic Theory, atoms, molecules, atomicity, to write chemical formulae, molecular mass and the most important mole concept.


  • Idea of divisibility of matter: Maharishi Kanad (ancient Indian philosopher) postulated that if we go on dividing the matter then a state will come when no further division is possible that state is called ‘parmanu’.
  • Naming of the ultimate particle: Democritus and Leucippus named this ultimate particle as atoms.
  • Chemical laws: A.L Lavoisier gave two laws of chemical combination.
  • Read further in atoms and molecules notes for detailed matter.

Law of chemical Combination:

There are two laws of chemical combination given by Lavoisier and L.Proust as follows:

Law of conservation of mass:

During a chemical reaction mass can neither be created nor destroyed.
As, A + B → C + D
Here, Mass of A and B = Mass of C and D

Law of constant proportion:

The elements in a chemical substance are always present in definite proportions by mass.
As, In the case of water (H2O), the ratio of masses of hydrogen and oxygen is always fixed i.e. 1:8.

Atomic Theories:

Many Scientists got involved in researching about the ultimate particles and tried to gave the explanation of the two chemical laws. John Dalton was the first among them who came forward to give his theory. Atoms and molecules notes considered all atomic facts. According to him all matter whether it is an element, a compound or a mixture consists of minute invisible particles called ‘atoms’.

Dalton’s Atomic Theory:

  • All matter is made of very minute particles called atoms, which takes part in chemical reactions.
  • Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
  • The atoms of a given element are identical in mass and chemical properties.
  • Atoms of different elements have different masses and chemical compositions.
  • Atoms combine in the ratio of small whole numbers to form different compounds.
  • The relative number and kinds of atoms are constant in a given compound.


  • Atoms are the building blocks of matter.
  • The smallest particle of a substance which takes part in a chemical reaction is said to be atoms.
  • The radius of hydrogen atom is 10-10m.
  • Radius of atom is measured in nanometer. (1m = 10-9 m)
  • Different substances radius is provided here in atoms and molecules notes.
Substance Radii
Atoms of Hydrogen 10-10m
Molecule of water 10-9m
Molecule of Haemoglobin 10-8m
Grain of Sand 10-4m
Ant 10-2m
Watermelon 10-1m

 Atomic Symbols:

  • Dalton was the first scientist who used Symbols for different elements.
  • Symbols of the elements should be of 1 or 2 letters from the name of that particular element, this was proposed by Berzelius.
  • Daltons symbol are:
Daltons symbol of elements
  • IUPAC (International Union for Pure and Applied Chemistry) approves the name of elements.
  • The first letter of the symbol is written in capital letters and the other in small letters.
  • Atoms and molecules notes contain all the basic elements symbol.
  • Common symbols of elements are:
Elements Latin name Symbols
Hydrogen H
Helium He
Lithium Li
Beryllium Be
Boron B
Carbon C
Nitrogen N
Oxygen O
Fluorine F
Neon Ne
Sodium Natrium Na
Magnesium Mg
Aluminum Al
Potassium Kalium K
Silver Argentum Ag
Gold Aurum Au
Mercury Hydragyrum Hg
Iron Ferrum Fe
Copper Cuprum Cu

Atomic Mass:

  • The considerable fact from Dalton’s Theory was ‘atomic mass’.
  • Every element has its own characteristics atomic mass.
  • To measure the accurate atomic mass of each element was a difficult task so they started measuring the relative atomic mass.
  • The mass of an atom of an element is called atomic mass.
  • The amount (of mass) which is equal to 1/12th of the mass of one atom of Carbon-12 is said to be Atomic Mass Unit (amu).
  • 1 amu = \(\frac{1}{12}\) x Mass of one atom of Carbon – 12. {1 u = 1.66 x 10-27 Kg)
  • Atoms and molecules notes is here with detailed explanation.

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


  • The smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance is said to be Molecules.
  • Also we say, it is a group of two or more atoms that are chemically bonded together or tightly held together by attractive forces.
  • Either atoms of the same element or of different elements can join together to form molecules.
  • The molecules of an element constitutes same type of atoms.


  • The number of atoms constituting a molecule is said to be its atomicity.
  • Atoms and molecules notes for class 9 deals with atomicity in detail.

Molecules are classified into 4 categories based on the number of atoms present in it:

  1. Monoatomic: Molecules which contains only one atom is said to be monoatomic. Example: He, Ne, Ar etc.
  2. Diatomic: Molecules which contain two atoms are said to be diatomic. Example: O2, H2, Br­2 etc.
  3. Triatomic: Molecules which contain three atoms are said to be triatomic. Example: O3, NO2, CO2 etc.
  4. Tetra atomic:Molecules which contain four atoms are said to be tetra atomic. Example: P4, SO3 etc.

Molecules are further divided into two types:

  1. Homo-atomic Molecule: If the molecule is constituted with only single type of atoms then it is said to be Homo-atomic molecules. Learn and revise Atoms and Molecules Notes to score better.
Name Atomicity
Argon Mono-atomic
Helium Mono-atomic
Oxygen Di-atomic
Hydrogen Di-atomic
Nitrogen Di-atomic
Chlorine Di-atomic
Phosphorous Tetra-atomic
Sulphur Poly-atomic
  • Hetero-atomic Molecule: When different type of atoms combine to form a molecule then these molecules are said to be hetero-atomic molecules. We have provided you with examples in atoms and molecules notes.
Compound Combining Elements Atomicity
Water Hydrogen, Oxygen Tri-atomic
Ammonia Nitrogen, Hydrogen Poly-atomic
Carbon dioxide Carbon, Oxygen Tri-atomic


  • Charged Particles are called ions.
  • They can have positive or negative charge on it.
  • Negatively charged ion are known as anion. Example: Cl, O2- etc.
  • Positively charged ion are called cation. Example: Na+, K+ etc.


  • The combining capacity of an element is known as its valency.
  • The main use of valancy is to find out how atom of an element will combine with the atom of another element to form a chemical compound.
  • To gain stability, atoms either gains or loose or shares its electron. Read atoms and molecules notes below for more.
  • If there is 1, 2, 3 electrons are their in valance shell then its valency is 1, 2, 3 respectively as it can lose its electron.
  • If there is 5, 6, 7 electrons in their valance shell then its valency will be 3, 2, 1 respectively as they will gain the electrons to become stable.
  • If an atom has 4 electrons in the valance shell than it will share its electron and hence its valency will be 4.
  • The valency of an element is 0 if an atom has 8 electrons in the outermost shell.

Remember: “All polyatomic ions with names starting with ‘S’ has valency 2 and CO32- has 2 and PO42- has valency of 2 otherwise each element has valency of 1.”

Revise atoms and molecules notes regularly.

Chemical Formulae:

Rules for writing Chemical formulae:

  • First of all, the charges or valency of cation and anion must be balanced.
  • If the compound contains both metals as well as non-metals then, write their symbols first.
    (As, H + Cl → HCl)
  • When ions are polyatomic then we indicate its number along with charge by separating bracket.
    [As, (SO4)2-, Mg(OH)2]
  • Learn and revise atoms and molecules notes wisely.

Chemical Formula of some simple compounds:

  • While writing the chemical formulae for compounds, we crossover the valencies of the combining elements. Read carefully atoms and molecules notes.
  • Formula of hydrogen chloride or Hydrochloric acid:
  • Formula of hydrogen sulphide:
  • Formula for aluminium oxide
  • Formula of calcium hydroxide:

Molecular Mass:

  • The sum of the atomic masses of all the atoms in a molecule of the substanceis said to be molecular mass.
  • It is also expressed in atomic mass unit (amu).
  • The other subsection of atoms and molecules notes is here.

Formula Unit Mass:

  • The sum of the atomic masses of all atoms in a formula unit of a compound is said to be formula unit mass of a substance.
  • It is similar as the molecular mass but the only difference is that in this types of compounds constituent particles are ions rather than atoms.
  • Example: In NaCl, 1 x 23 + 1 x 35.5 = 58.5 u
  • Further we will read in higher section of atoms and molecules notes.

Mole Concept:

  • We describe quantity of substance with the help of mole.
  • Mole: The amount of substance which contains 6.023 x 1023 units is said to be 1 mole.
  • The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.
  • 1 mole = 6.022 x 1023 units = 1 NA (NA is Avogadro number).
  • Example: 1 mole of Oxygen = 6.023 x 1023 atoms of oxygen.
  • Atoms and molecules notes contains a part from mole concept from NCERT.

Q/A on atoms and molecules notes:

To understand it better here are some examples from NCERT class 9 chapter 3 notes. Let’s revise them as well:

Calculate the relative molecular mass of water (H2O).

As we Know that,
Atomic mass of hydrogen (H) = 1u, oxygen(O) = 16 u
So the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is = 2 × 1+ 1×16 = 18 u

Calculate the formula unit mass of CaCl2.

Formula unit mass of CaCl2 =
Atomic mass of Ca + (2 × atomic mass of Cl)
= 40 + 2 × 35.5 = 40 + 71 = 111 u

Calculate the mass of 0.5 mole of O2 gas.]

Mass = molar mass × number of moles

⇒ 32 x ½ = 16 g

Calculate the mass of 6.022 × 1023 number of O2 molecules.

As we know that

n = \(\frac{ given\:umber\: of\: particles}{Avogadro\: number}\) = \(\frac{N}{N_{A}}\)
n = \(\frac{6.022 × {10}^{23}}{6.022 × {10}^{23}}\)=1

Atoms and molecule notes for class 9 thus provides you with the actual curriculum what CBSE and other boards follow. Read, learn and revise it regularly.

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