The joining of two or more uncharged atoms by a linkage known as a covalent bond creates molecules such that changing the connection, or the arrangement will change the molecule and its properties.
For example, joining two Cl atoms give a Cl-Cl molecule. Changing the linkage to form H-Cl gives a new molecule with different properties altogether.
A molecule can have a small or large chain depending on the number of atoms linked.
An isolated atom like He or O is not a molecule.
Many molecules, when united, make a substance. Therefore, a molecule is the smallest part of a substance that has all its properties. For example, a glass of water is an extensive collection of invisible H2O molecules. The property of the H2O molecule reflects the property of water. The table sugar is made of many C6H12O6 molecules, a source of its sweetness.
The molecules are classified into two types based on the type and the nature of the atoms involved.
If the atoms involved are of the same type, a homoatomic molecule is formed—for example, Hydrogen (H2) molecule. If the atoms that combine to create a molecule are different, a heteroatomic molecule is formed—for example, a HCl molecule.
Depending on the nature of the atoms involved, a molecule can also be classified as polar or nonpolar. A polar molecule has one positive end and another negative end. A nonpolar molecule has no such separation of charges.