atom diagram

Chemistry Class 11 Stoichiometry Notes 2023

Stoichiometry is quantitative chemistry. “Calculations based on the law of conservation of mass, the law of constant composition and balanced chemical equations are known as stoichiometric calculations.” 

structure of atom

Stoichiometry Class 11th

Important Stoichiometry Definitions:


Anything that has volume, as well as mass, is called matter.


The quantity of matter in a body is called mass.


The force with which a planet attracts a body toward its centre is called weight. It depends upon the gravity of the planet e.g. gravity of the moon is one-sixth (1/6) that of earth’s gravity so the weight of a body on the moon is 1/6 of its weight on earth.


Atom is derived from the Greek word “Atomos” meaning indivisible.

“smallest particle of matter which possesses all the properties of that matter but which cannot exist independently” is known as an atom e.g. O, H, N, Cl, Br etc.

Sub-atomic particles

Fundamental sub-atomic particles of atoms are as follow :

  • Electrons
  • Protons
  • Neutrons
What is Atom Definition with Example ( in Urdu / Hindi)


It is defined as the “smallest particle of matter which possesses all the properties of that matter and which can exist independently.” e.g. O2, Cl2, C6H12O6, S8, H2O, He etc.

What is a Molecule and its Types ( in Urdu / Hindi)


Charge-carrying species are called an ion.

Types of Ions:

Ions are of three types.

(i) simple ion:

If a single atom bears +ve or –ve charge then it is called a simple ion e.g. H+, O-2.

(ii) compound ion:

When a group of atoms bear +ve or –ve charge then it is called a compound ion e.g. NH4+, SO4-2 etc.

(iii) molecular ion:

When a molecule gains or loses electrons, the resulting species is called a molecular ion e.g. CH4+, O2+, CO+, N2+ etc. Cationic molecular ions are more common than anionic molecular ions.

Difference between Cation and Anion


Ions bearing positive charge are called cations


Ions bearing negative charges are called anions.

The formation of cations is an endothermic process while the formation of anions is an exothermic process.

Types of Ions (In Urdu/ Hindi)


A substance which is made up of only one kind of atoms is called an element e.g. O2, O3, Cl2, S8, He etc. There are 92 natural elements while the total elements are more than 110.


A substance which is made up of more than one kind of atoms chemically combined together is called a compound e.g. CO2, H2SO4, C6H12O6 etc.


Atoms having the same atomic number but different atomic masses are called isotopes. e.g. Hydrogen has three isotopes protium 1H1, deuterium 1H2, and tritium 1H3, Carbon has three isotopes

6C12, 6C13, 6C14, oxygen also has three isotopes 8O16, 8O17, 8O18 etc. More than 300 isotopes occur in nature including 40 radioactive isotopes. Often, elements with even atomic numbers have more isotopes and are abundant.

Atomic Number (Z)  

“Number of protons in the nucleus of an atom” or “Number of electrons in an atom” is called the atomic number.

Number of electrons in an atom          = Z

Number of protons in an atom            = Z

Number of neutrons in an atom          = A – Z

Atomic masses:

Atomic masses are expressed in three ways:

(i) Mass Number (A):   

    The Sum of protons and neutrons in an atom is called its mass number or nucleon number.

                                    A = Protons + Neutrons

(ii)  Relative atomic mass (Ar):

      The atomic mass of an atom as compared to the mass of Carbon-12 is called relative atomic mass e.g. relative atomic mass of Hydrogen (protium i.e. 1H1) is 1.0078 amu.

(iii)   Average atomic mass (Ar)

     The weighted average of isotopic masses of an element is called the average atomic mass of the element.   

e.g. (i)                          Relative isotopic mass             Relative abundance

            Carbon-12                   12.00000 amu                         98.89%

            Carbon-13                   13.00335 amu                         1.11%

Average atomic mass =      12.00000 x 98.89   +   13.00335 x 1.11 / 100

                                                                               =  12.01 amu        



amu is a unit to measure atomic masses. It is one-twelfth of the mass of carbon-12.

  1 a.m.u = 1.66 x 10-24g            or 1.66 x 10-27kg

Relative molecular masses (Mr):

The sum of relative atomic masses of atoms present in a molecule gives relative molecular masses

Chemical Formulae:

The symbolic representation of a compound that shows elements in the compound and also the ratio of their atoms is called the chemical formula.

Types of Chemical formulae

Chemical formulae are of three types:

(i)  Empirical formula or Simple formula:            

            The formula of a compound that shows elements in the compound and their simple atomic ratio is called empirical or simple formula.

(ii)   Molecular formula:      

            The formula of a compound that shows elements in the compound and the actual number of their atoms is called the molecular formula.

(iii)  Structural formula:       

            The formula of a compound that shows elements in the compound, the actual number of their atoms and the arrangement of the atoms in the molecule is called the structural formula.

100% Original Cosmetics & Skin Care Products

Imported Products in Islamabad Pakistan
Imported Products in Pakistan (03496967013)

Check out

-> Stoichiometry Tutorials One Shot

Related Posts

10th Class Chemistry Free Downloadable pdf Notes

11th Class Chemistry Chapterwise Free pdf Notes

12th Class Chemistry Free Downloadable

Leave a Reply

%d bloggers like this: