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FSc Notes Chemistry Basic Concepts of Chemistry

                     Basic Concepts of Chemistry:

Chemistry: The branch of science which deals with the study of structure of matter/, composition of matter, properties of matter, changes in matter and laws and principles which govern theses changes is called chemistry.

There are many branches of chemistry:
  • Physical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry
  • Biochemistry
  • Analytical Chemistry
  • Industrial Chemistry
Atom: The smallest particle of an element which may or may not exist independently and can take part in chemical reaction is called atom. For exp: He, Ne, O, N, Fe, Al.


A pure substance which is formed by the chemical composition of two of more than two different elements in a fixed ratio by mass is called compound. Compound is always homogeneous. For exp: NH3, H2O, NaOH. There are two types of compounds.

  • Ionic Compounds
  • Molecular Compounds


The smallest particle of an element or compound which exist independently is called a molecule. Molecules are classified in different ways: on basis of size, number of atoms and nature of atoms:

On the basis size:
Macromolecules: Molecules which have large number of atoms of different elements in it. For exp: haemoglobin (10000 atoms), proteins, lipids.

Micromolecule: Molecules of smaller size and lower molecular mass for exp: H2SO4. H2O.

On basis of nature:
Homoatomic molecules: Those molecules which have same types of atoms are called homoatomic molecules. for exp: H2, O2, Cl2.
Heteroatomic molecules: Those molecules which consist of atoms of different elements are called heteroatomic molecules for exp: CH4, NH3, SO2.

On the basis of number of atoms:
Monoatomic molecules: The molecule which consist of a single atom is called monoatomic molecule for exp: He, Ne, Ar.
Diatomic molecule: The molecule consisting of two atoms is called diatomic molecule for exp: H2, O2, CO.
Polyatomic molecule: Molecule consisting of more than two atoms is called polyatomic molecule for exp: H2O, NH3, CCl4.

Atomicity: The number of atoms present in a molecule is called atomicity. Atomicity of H2SO4 is 7.


The species which carry either positive or negative charge is called ions. There are two types of ions on the basis of charge present on them i.e cations and anions.

On basis of Charge:
Cation: When an atom loses electron. it forms cation. Some energy is required to ionize and atom. It is an endothermic process.
A ------------>  A+  +  e-
A+ is called cation. Charge is based upon the number of electrons which and atom loses. If an atom loses one electron its charge will be +1 if it loses two electrons its charge will be +2. for exp: Na+, Sr 2+.
Anions: When an atom gains one or more electrons anions is formed . Some amount of energy is released when an electron is added to an atom. Foundation of anions is an exothermic process.
B  +  e-  ---------------> B-
For exp atoms gaining one electron have -1 charge on them F-, Cl-. Atom gaining two electron have -2 charge.

On basis of number:
  • Simple ions
  • Complex ions
Simple ion: If only a single atom changes to charge species it is called simple ion for exp: Na+, K+.
Complex ion: If group of atoms have charge on them they are called complex ions. for exp NH4+, CH3+.
Molecular ion: If an electron is removed from a molecule of a substance it is called molecular ion for exp CH4+, CO+. If we remove electron from a molecule it is called molecular ion.


The atoms of same element having same atomic number but different atomic masses are called isotopes. Number of protons present in the nucleus of an atom is called atomic number or proton number, The number of protons and neutron present in the nucleus of an atom is called mass number. Mass number is always a whole number. Isotopes of an element have same chemical properties and same position in the periodic table.

Written By: Asad Hussain