### 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:
• To understand it, consider a water molecule${H}_{2}O$, the 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.
• Oxygen $$O_2$$ contains two elements of an oxygen atom.
• Chlorine $$Cl_2$$ contains two elements of a chlorine atom.
Molecules of elements:
The atoms that make up an element's molecules are all of the same nature.
Many 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.

Each molecule of hydrogen $$H_2$$, oxygen $$O_2$$, and chlorine $$Cl_2$$, for example, 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 molecules.
Molecules of compounds:
Compound molecules contain atoms from two or more separate elements.

Example:
Water  $$H_2O$$, for example, has three atoms: Two hydrogen (H) atoms and one oxygen (O).

Methane  $$CH_4$$ with five atoms: One carbon (C) and four hydrogen (H) atoms.

Glucose  $$C_6H_{12}O_6$$ contains elements such as $$6$$ carbon, $$12$$ hydrogen and $$6$$ oxygen, combined to form a glucose molecule.

Sodium chloride $$NaCl$$ contains the one element of each sodium and chlorine combined to form a sodium chloride molecule.