### 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.

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:

For simple understanding, consider a water molecule${H}_{2}O$; 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.
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 ${O}_{2}$, it forms ozone${O}_{3}$, 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 ${N}_{2}$ Diatomic Hydrogen ${H}_{2}$ Diatomic Oxygen ${O}_{2}$ Diatomic Helium $$He$$ Monoatomic Chlorine ${\mathit{Cl}}_{2}$ Diatomic Ozone ${O}_{3}$ Triatomic Phosphorous ${P}_{4}$ Tetra-atomic
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.