What is the smallest particle of a compound that retains all its properties?

The smallest particle of a compound that retains all its properties is a molecule. A molecule is formed when two or more atoms chemically bond together, and it represents the smallest unit of a compound that still exhibits the characteristics and chemical behavior of that compound. For example, water (H₂O) is a compound made up of hydrogen and oxygen atoms bonded together; a single water molecule retains the properties of water, such as its ability to act as a solvent.

In contrast, while an atom is the basic unit of matter, it does not represent a complete compound on its own. A single atom may have properties of an individual element but does not encompass the properties associated with a compound, which arise from the interaction between different types of atoms. The term 'compound' itself refers to a substance made of two or more different elements that are chemically combined, but it does not specify the smallest unit that retains the properties of that substance. Lastly, the term 'particle' is a general term that could refer to atoms, molecules, or other small units, but it does not specifically identify the smallest unit of a compound. Thus, the correct answer is a molecule.