Theory:

A molecule is the smallest particle of an element or compound capable of an independent life and exhibiting all of the substance's properties.
 
shutterstock_1169732683.jpg
In general, a molecule is a set of two or more atoms that are chemically bound or held together by attractive forces.
Atoms from the same or different elements may combine to form a molecule.
Example:
shutterstock_1064327462.jpg
 
For simple understanding, consider a water moleculeH2O; two elements, hydrogen and oxygen atoms combined to form a water molecule.

Calcium oxide  \(CaO\) contains two elements, calcium and oxygen atoms combined to form a calcium oxide molecule.

Glucose  \(C_6H_{12}O_6\) contains three elements, carbon, hydrogen and oxygen atoms combined to form a glucose molecule.

Sodium chloride  \(NaCl\) contains two elements; hydrogen and oxygen atoms combined to form a sodium chloride molecule.
 
Chlorine  \(Cl_2\) contains two elements of a chlorine atom.
 
Oxygen \(O_2\) contains two elements of the oxygen atom.
Atomicity:
We can distinguish the molecules respective to the number of elements they combined to form a molecule. We call this atomicity .i.e. the number of atoms constituting a molecule.
The number of atoms present in a single molecule is termed as its atomicity.
shutterstock_1865444311.jpg
Molecules of elements:
The atoms that make up an element's molecules are all of the same nature.
Most of the elements' molecules are made up of just one electron, such as argon \(Ar\), helium \(He\), and so on.
  • Monoatomic Elements: In their molecular form, certain elements are monatomic, meaning they are made up of just one atom (mono-atomic). Helium \(He\) is an example of a monoatomic element.
  • Diatomic Elements: If the molecule constitutes two atoms, we can classify it as a diatomic molecule. In their molecular shape, other elements have two or more atoms. For example, each molecule of hydrogen \(H_2\), oxygen \(O_2\), and chlorine \(Cl_2\), has two atoms.
  • Triatomic Elements: If another atom is combined with O2, it forms ozoneO3, which is a triatomic molecule.
Note:
Monoatomic and diatomic molecules are more stable than triatomic molecules.
Most non-metal elements are diatomic.
 
Classification of elements
Name of the elements
Symbol
Atomicity
Metals
Copper
\(Cu\)
Monoatomic
 
Aluminum
\(Al\)
Monoatomic
 
Sodium
\(Na\)
Monoatomic
 
Iron
\(Fe\)
Monoatomic
Non-metals
Nitrogen
N2
Diatomic
 
Hydrogen
H2
Diatomic
 
Oxygen
O2
Diatomic
 
Helium
\(He\)
Monoatomic
 
Chlorine
Cl2
Diatomic
 
Ozone
O3
Triatomic
 
Phosphorous
 P4
Tetra-atomic
shutterstock_1862776762.jpg
Molecules of compounds:
Compound molecules contain atoms from two or more separate elements.
Example:
Water \(H_2O\),  has three atoms: two hydrogens \(H\) atoms and one oxygen \(O\).
Methane \(CH_4\), has with five atoms: one carbon \(C\) and four hydrogen \(H\) atoms.