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Atoms and Molecules

Atom (Parmanu) is the basic unit for the chemicals or it better to say that existence of everything around us. If we look in our surrounding we see a lot of thing each and every part is made of atom of different element. We will understand how atom of an element exist all about it mass and properties and what what make them to form molecule and compound.

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Exercise 1

Exercise 2

  • Q1

    Define the atomic mass unit.

    Ans:

    Atomic mass unit: Mass unit equals to exactly 1/12 th the mass of one atom of carbon-12 is known as an atomic mass unit.

    Note:

    Carbon-12: Standard reference for measuring atomic Masses. 

    1 amu (atomic mass unit) = 1.66x10-24 g.


    Q2

    Why is it not possible to see an atom with naked eyes?

    Ans:

    The size of an atom is very very small that we do not see with our naked eyes. Atoms of mait elemente cannot exist independently.


Exercise 3

Exercise 4

  • Q1

    Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.

    Ans:

    Atomic mass includes the masses of three subatomic particles which make up an atom i.e. proton, neutron and electron.

    Unit of atomic mass is 'u'

    ‘u’ = unified atomic mass unit & 1/12 th mass of single carbon-12 atom 

    ‘u’ = 1.66 x 10-24 g

    (Just for understanding - Atomic mass approximately equal to twice of atomic no.)

    Molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of a subitance.

    Unit of molecular mass = 'u’

    Molecular mass of H2 = 2 x 1 = 2u (Atomic mass of H=1)

    Molecular mass of O2 = 16 x 2 = 32 u (Atomic mass of O=16)

    Molecular Mass of Cl2 = 35.2 x 2 = 71 u (Atomic mass of Cl = 35.5)

    Molecular Mass of CO2 = 12 x 1 + 16 x 2

                    = 12 + 32 = 44 u ( Atomic Mass of C = 12)

    Molecular Mass of CH4 = 12 x 1 + 4 x 1

                   = 12 + 4 = 16 u

    Molecular Mass of C2H6 = 12 x 2 + 6 x 1

                   = 24 + 6 = 30 u 

    Molecular Mass of C2H4 = 12 x 2 + 4 x 1

                   = 24 + 4 = 28 u

    Molecular Mass of  NH3 = 14 X 1 + 3 x 1

                   = 14 x 3 = 17 u  (Atomic Mass of N = 14)

    Molecular Mass of CH3OH = 12 x 1 + 4 x 1 + 16 x 1

                   = 12 + 4 + 16 = 32 u


    Q2

    Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn=65 u, Na=23 u, K=39 U, C=12 u and O=16 u.

    Ans:

    Formula unit mass of ZnO

    = 1 x Atomic mass of Zn + 1 x Atomic mass of ‘O’ atom 

    = 1 x 65 + 1 x 16

    = 65 + 16

    = 81 u

     

    Formula unit mass of Na2

     = 2 x Atomic mass of Na + 1 x Atomic mass of ‘O’ atom

    = 2 x 23 + 1 x 16

    = 46 + 16

    = 62 u

     

    Formula unit mass of K2CO3

    = 2 x Atomic mass of K + 1 x Atomic mass of C + 3 x Atomic mass of ‘O’

    = 2 x 39 + 1 x 12 + 3 x 16

    = 78 + 12 + 48

    = 138 u


Exercise 5

Exercise 6