Atoms and Molecules NCERT Notes for CBSE Class 9th

Atoms and Molecules NCERT Notes for CBSE Class 9th

Atomic Theories

Democritus (470-380 B.C.) [Father of Atomic Theory (early)]

  • He proposed that matter could NOT be divided into smaller pieces forever.
  • He claimed that matter was made of small, hard particles that he called “atomos”

Antoine Lavoisier (father of modern chemistry)

Laws of Chemical Combination

  1. Law of Conservation of Mass: The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reactions, i.e., the total mass of the products is same as the total mass of the reactants.
  2. Law of Constant Proportions – (Stated by Joseph Proust): In a chemical substance the elements are always present in definite proportions by mass. e.g., H20 prepared from any source contain H and O in the ratio of 1:8 by mass.

John Dalton – 1904 [Father of Atomic Theory (modern)]

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

E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations.

J. J. Thomson – 1897 (Raisin bun model) (Plum pudding model)

  • An atom consists of a positively charged sphere and the electrons are embedded in it [Drawback].
  • The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

JJ Thomson (1856-1940) Manchester, Uk was awarded Nobel Prize in Physics (1906) for his work on the discovery of electrons.

Ernest Rutherford – 1911 (Father of Nuclear Physics)

Experiment Setup

  • Fast moving alpha (α or He2+ or α2+) particles were made to fall on a thin gold foil.
  • Used gold foil (about 1000 atoms thick) for the experiment because he wanted the layer to be as thin as possible.

Experiment Result

  • Most of the α-particles passed straight through the gold foil.
  • Some of the α-particles were deflected by the foil by small angles.
  • 1 out of every 12000 particles appeared to rebound.

The Nobel Prize in Chemistry (1908) was awarded to Ernest Rutherford “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances”.

Rutherford’s Nuclear Model of the atom

  • There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
  • The electrons revolve around the nucleus in well-defined orbits
  • The size of the nucleus is very small as compared to the size of the atom.

Neils Bohr – 1913

  • Only certain special orbits known as discrete orbits of electron, are allowed inside the atom.
  • While revolving in discrete orbits the electrons do not radiate energy.
  • These orbits or shells are called energy levels and are represented by the letters K, L, M, N,… or the numbers n = 1,2,3,4,…

Neils Bohr (1885-1962), Copenhagen, Denmark received Nobel in Physics (1922) for his services in the “investigation of the structure of atoms and of the radiation emanating from them”.

Discovery Table

Discovery Discoverer Mass
Protons (p, p+ or N+) Observed as H+ by Eugen Goldstein (1886), Identified & named in other nuclei by Ernest Rutherford (1917–1920). 1.673 × 10-27
Electrons (e or β) JJ Thomson (1897) 9.1 x 10-31
Neutrons (n, n0) James Chadwick (1932) 1.675 × 10-27 ≃ mass of p


An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element.

Atomic radius is measured in nano-meter.

1 nm = 109 nm

Atomic Mass

Atomic mass of an element can be defined as the number which indicates how many times the mass of one atom of the element is heavier in comparison to 1/12th part of the mass of one atom of carbon.

   Atomic Mass = Mass of Protons + Mass of Neutrons
                 Mass of 1 atom / (1/12 part of mass of one carbon atom)


A molecule is in general a group of two or more atoms that are chemically bonded together.


Compounds composed of metals and non-metals contain charged species. The charged species are known as ions.

  • A negatively charged ion is called an anion and positively charged ion is called a cation.

Chemical Formula

The chemical formula of a compound is a symbolic representation of its composition.

Rules for writing a chemical formula as follows:

  • The valencies or charges on the ion must be balance.
  • When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. eg., Cao, FeS.
  • In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.


The number of atoms constituting a molecule is known as its atomicity.

Molecular Mass / Relative Molecular Mass / Formula Unit Mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.

   Molecular Mass of water (H2O)
   Atomic mass of H = 1 u, Atomic mass of O = 16 u
   ∴ molecular mass of H2O = 2 x 1 + 1 x 16 = 18 u

   Molecular Mass of HNO3
   Atomic mass of H = 1 u, Atomic mass of N = 14 u, Atomic mass of O = 16 u
   ∴ molecular mass of HNO3 = 1 x 1 + 1 x 14 + 3 x 16 = 63 u

Mole Concept

  • Mole is a link between the mass of atoms (or molecules) and the number of atoms (or molecules).
  • Avogadro Constant (6.022 x 1023): The number of particles (atoms, molecules or ions) present in 1 mole of any substance.
   Calculate the no. of moles in 52 g of He
   n = m / M = 52 / 4 = 13

   Calculate the no. of moles in 12.044 x 1023
   n = N / N0 = 12.044 x 1023 / 6.022 x 1023 = 2

Structure of Atom

  • Atoms are made up of three subatomic particles electron (negatively charged), proton(positively charged) and neutron(neutral).
  • Protons and neutrons are present in the nucleus at the centre of the atom and the electrons revolves around the nucleus in fixed orbits.
  • The nucleus is always positive because it has positively charged protons in it.
  • Atoms are electrically neutral because the number of protons is equal to the number of electrons present in an atom.


Valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules.

  • Valence Electrons: The electrons present in the outermost shell of an atom.
  • There are various energy levels or shells (k, l, m, n) around the nucleus in which electrons revolves. Remember this general formula 2)8)18)32
  • Note: Max electrons in outer orbit = 8, Inner shells must be filled before filling outer shells.

Valency of a few atoms

   Hydrogen [Atomic Number = 1] i.e., 1)
   i.e., k = 1, max electrons k shell can have = 2, 
   thus valency of H = 2 - 1 = 1

   Neon [Atomic Number = 10] i.e., 2)8)
   i.e., k = 2, l = 8, max electrons l shell can have = 8, 
   thus valency of Ne = 8 - 8 = 0

   Phosphorous [Atomic Number = 15] i.e., 2)8)5
   i.e., k = 2, l = 8, m = 5, max electrons m shell can have = 8, 
   thus valency of P = 8 - 5 = 3

   (Why not 8 - 3 ? 
   Which one do you find easy losing 5 or gaining 3? 
   Definitely gaining 3 is easier.)

   Chlorine [Atomic Number = 17] i.e., 2)8)7
   i.e., k = 2, l = 8, m = 7, max electrons m shell can have = 8, 
   thus valency of Cl = 8 - 7 = 1

Note: All Noble gases(He, Ne, Ar, Kr, Xe, Rn) has a valency of 0, because their outermost shell is completely filled.

Calculation of Atomic Number (Z) and Mass Number

The number of protons of an atom is known as its atomic number and the sum of the number of the protons and the neutrons of an atom is known as mass number.


  1. Atomic Number of H (p = 1, e = 1, n = 0 ) is 1 and Mass Number (np + nn) = 1 u + 0 u = 1 u
  2. Atomic Number of C (p = 6, e = 6, n = 6 ) is 6 and Mass Number is 6 u + 6 u = 12 u
  3. Atomic Number of Al (p = 13, e = 13, n = 14 ) is 13 and Mass Number is 13 u + 14 u = 27 u

Atomic Notation

Mass NumberSymbol of the elementAtomic Number


Atoms of the same element, having same atomic number but different mass numbers.

  • Hydrogen has 3 isotopes: protium (1H1), deuterium (2H1), tritium (3H1)
  • Chlorine has 2 isotopes: 35H17 and 37H17
  • Many elements consist of a mixture of isotopes. The chemical properties of Isotopes are similar but their physical properties are different.

Calculation of Atomic Mass

The mass of an atom of any natural element is taken as the average mass of all the naturally occurring atoms of that element. If an element has no isotopes, then the mass of its atom would be the same as the mass number.

Chlorine occurs in nature in two isotopic forms, with masses 35 u and 37 u in the ratio of 3:1. Hence

The average atomic mass of Cl atom is = 35 x 75 % + 37 x 25 %
                                      = 35 x 3/4 + 37 x 1/4
                                      = 142 / 4
                                      = 35.5 u


Atoms of different elements having same mass number but different atomic numbers.

  • Ca (atomic number: 20), Ar (atomic number: 18) both have same atomic number i.e., 40


The property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements.

  • Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner.
  • The allotropes of carbon include diamond (the carbon atoms are bonded together in a tetrahedral lattice arrangement), graphite (the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations).
  • Two allotropes of oxygen (dioxygen, O2, and ozone, O3) can both exist in the solid, liquid and gaseous states.
  • Phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.

Dalton was the first scientist to use the symbols for elements in a very specific sense, While JJ 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)

Established in 1919, handles all of the nomenclature and discovery of elements.

  • President: Zhou Qifeng (China)
  • Many of the symbols are the first one or two letters of the element’s name (eg., He, Li) while some are formed from the first letter of the name and a letter appearing later in the name (eg., Cl, Zn) in English
  • The first letter of the symbol is always written in uppercase and the second letter in lowercase.
  • Examples: Hydrogen (H), Aluminium (Al), Cobalt (Co)

Atoms of Important Elements

Symbol A. No. Valency A. Mass
H 1 1 1
He 2 0 4
Li 3 1 7
Be 4 2 9
B 5 3 11
C 6 4 12
N 7 3 14
O 8 2 16
F 9 1 19
Ne 10 0 20
Na 11 1 23
Mg 12 2 24
Al 13 3 27
Si 14 4 28
P 15 3,5 12
S 16 2 32
Cl 17 1 35.5
Ar 18 0 40
K 19 1 39
Ca 20 2 40

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