A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance. Atoms of the same element or of different elements can join together to form molecules.

The smallest particle of a substance that is capable of independent existence is a molecule.

Explanation:
- A molecule is a group of two or more atoms bonded together. It represents the smallest particle of a substance that retains the chemical properties of that substance and is capable of independent existence.
- Atoms are the basic building blocks of matter, but they cannot exist independently in most cases. They are constantly bonding with other atoms to form molecules.
- A molecule can consist of atoms of the same element or different elements. For example, a molecule of oxygen gas (O2) consists of two oxygen atoms bonded together, while a molecule of water (H2O) consists of two hydrogen atoms bonded to one oxygen atom.
- Molecules can be as small as two atoms or they can be much larger and more complex. For instance, a molecule of glucose, a type of sugar, contains 24 atoms bonded together.
- Molecules can exist in different states of matter, such as solid, liquid, or gas, depending on the nature of the substance and the intermolecular forces between the atoms within the molecule.
- Molecules are constantly in motion and can interact with other molecules through various chemical reactions.
- The concept of a molecule is crucial in understanding the behavior and properties of substances. It helps us explain why different substances have different physical and chemical properties.
- The study of molecules and their interactions is an important branch of chemistry called molecular chemistry.
- It is worth noting that there are other particles smaller than molecules, such as atoms and subatomic particles like electrons and protons. However, these particles do not possess independent existence and are not considered the smallest particles capable of independent existence.

In conclusion, a molecule is the smallest particle of a substance that can exist independently and retains the chemical properties of that substance.