What is the definition of an atom?

• A. The smallest unit of life
• B. The smallest unit of matter
• C. The smallest unit of energy
• D. A type of molecule

Answer: (B)

An atom is defined as the smallest unit of matter that retains the properties of an element. This means that an atom can combine with other atoms to form molecules, but it cannot be broken down into smaller components without losing those elemental properties. Atoms consist of protons, neutrons, and electrons, and they are the foundational building blocks of all substances in the universe.

The other choices do not accurately describe the nature of an atom. For instance, the smallest unit of life refers to a cell, which encompasses many atoms and molecules. Energy does not have a physical structure and cannot be categorized in the same way as matter. Lastly, while molecules are formed from atoms bonded together, defining an atom simply as a type of molecule misrepresents its significance as the fundamental unit of matter.