atom diagram

Stoichiometry Definitions in Simple Words

What is Stoichiometry?

The field of chemistry known as stoichiometry analyses the quantitative relationships between the reactants and products of chemical reactions.

Quantitative chemistry is known as stoichiometry. Stoichiometric calculations are those that follow the principles of mass conservation, constant composition, and balanced chemical equations.

Important Stoichiometry Definitions:


Matter refers to anything that has both volume and mass.


The term “mass” is used to describe the amount of material present in an object.


One definition of “weight” is the force by which an object is drawn toward the centre of a planet. An object’s mass on the moon, for instance, would be equal to one-sixth of its mass on Earth because the moon has only one-sixth the gravity of 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


A molecule is the “smallest particle of matter that possesses all the qualities of that substance and which may exist independently.” such as oxygen, chlorine, hydrogen, and helium etc.

What are Ions ?

Charge-carrying species is called an ion.

Difference between atom and ion ( in Urdu / Hindi)
2 Types of Ions:

Cation: Ions bearing positive charge are called cations

Anion: Ions bearing negative charges are called anions.

 Avogadro’s Number and the Mole

In chemistry, an extremely high number of molecules, atoms, or particles is denoted by the mole, or “mol.” Avogadro’s Number, defined as the number of molecules in a fixed amount of substance, is a massive 6.02214 x 1023. These figures are crucial because they reveal information on the abundance of individual elements. In practice, one mole of a chemical might be anything from a few grams to hundreds of grams.


“Element” refers to a substance that consists of only one type of atom.s e.g. O2, O3, Cl2, S8, He etc. There are 92 natural elements while total elements are more than 110.


Compounds are substances that contain atoms of more than one type and are held together by chemical bonds. 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

carbon 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 mass:

The amount of matter in an atom of an element is measured in terms of its atomic mass.

For convenience, we’ll use the atomic mass of carbon-12 (1.992646547 1023 gram) as an example.

As expressed above, 1.660539040 x 1024 grammes is equal to 1 amu (atomic mass unit).

The English chemist John Dalton is honoured with the naming of the atomic mass unit (amu) as the dalton (Da).

 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

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.

3 Types of Chemical formulas

Chemical formulae are of three types:

(i)         Empirical 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.

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