Stoichiometry Important Terms Definitions

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

atom diagram

Important Stoichiometry Definitions:


Any thing that has volume as well as mass is called matter.


Quantity of matter in a body is called mass.


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


Atom is derived from 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 “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 is called ion.

Types of Ions:

Cation: Ions bearing positive charge are called cations

Anion: Ions bearing negative charge are called anions.

Formation of cations is endothermic process while formation of anions is exothermic process. Ions are of three types.

Types of Ions (In Urdu/ Hindi)

(i) simple ion: If a single atom bears +ve or –ve charge then it is called simple ion e.g. H+, O-2. (ii) compound ion: When a group of atoms bear +ve or –ve charge then it is called compound ion e.g. NH4+, SO4-2 etc.

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

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Difference between atom and ion ( in Urdu / Hindi)

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


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


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

carbon isotopes

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

Atomic Number (Z)  

“Number of protons in the nucleus of an atoms” or “Number of electrons in an atom” is called 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):       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 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 weight average of isotopic masses of an element is called 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: This 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): 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 chemical formula.

Types of Chemical formulae

Chemical formulae are of three types:

(i)         Emperical 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 actual number of their atoms is called molecular formula.

(iii)       Structural formula:       

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

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