**Correct Option: B. An atom is a particle that can exist independently and is composed of protons, neutrons, and electrons.** ### Detailed Explanation: 1. **Understanding Atoms**: - An atom is the fundamental building block of matter. It is the smallest unit of an element that retains the chemical properties of that element. Atoms can exist independently, meaning they do not need to be bonded to other atoms to exist. 2. **Composition of Atoms**: - Atoms are made up of three primary subatomic particles: - **Protons**: Positively charged particles found in the nucleus of the atom. - **Neutrons**: Neutral particles (no charge) also located in the nucleus. - **Electrons**: Negatively charged particles that orbit the nucleus in various energy levels or shells. - The number of protons in an atom defines the element (e.g., hydrogen has 1 proton, carbon has 6 protons). 3. **Independence of Atoms**: - Atoms can exist on their own (like noble gases such as helium and neon) or can bond with other atoms to form molecules (like H₂O, which is water made of hydrogen and oxygen atoms). ### Why Other Options Are Incorrect: - **Option A**: "An atom is the smallest unit of a compound that retains the properties of that compound." - **Why it's wrong**: This statement is misleading because it describes a molecule rather than an atom. A compound is made up of two or more different types of atoms bonded together. The smallest unit of a compound that retains its properties is a molecule, not an atom. - **Option C**: "An atom is a collection of molecules that form a solid structure." - **Why it's wrong**: This statement is incorrect because it confuses the definitions of atoms and molecules. Atoms are the basic units that make up molecules, not the other way around. A solid structure is formed by the arrangement of many atoms or molecules, but an atom itself is not a collection of molecules. - **Option D**: "An atom is a type of chemical bond formed between two elements." - **Why it's wrong**: This statement is fundamentally incorrect as it describes a chemical bond, not an atom. A chemical bond is the force that holds atoms together in a molecule or compound, while an atom is the individual unit that can participate in forming these bonds. ### Summary of Key Points: - An atom is the smallest unit of an element that retains its chemical properties. - Atoms consist of protons, neutrons, and electrons, and can exist independently. - The smallest unit of a compound is a molecule, not an atom. - Chemical bonds are interactions between atoms, not a description of what an atom is. This understanding of atoms is crucial for grasping more complex concepts in chemistry, such as chemical reactions, bonding, and the behavior of matter.