**Correct Option: A. An atom is the smallest particle of an element that retains the properties of that element.** ### Detailed Explanation: 1. **Definition of an Atom**: - An atom is the fundamental building block of matter. It is the smallest unit of an element that still retains the chemical properties of that element. This means that if you take a single atom of carbon, for example, it will exhibit the characteristics of carbon, such as its ability to form four bonds and its role in organic chemistry. 2. **Why Option A is Correct**: - **Smallest Particle**: Atoms are indeed the smallest units of elements. When you break down an element into smaller parts, you reach subatomic particles (protons, neutrons, and electrons), which do not retain the properties of the element itself. For instance, a single carbon atom has 6 protons, 6 neutrons, and 6 electrons, and it behaves like carbon. If you were to separate these particles, they would not exhibit the properties of carbon. - **Retaining Properties**: The properties of an element are determined by the arrangement and number of its electrons, protons, and neutrons. Therefore, an atom, being the smallest unit, retains these properties. For example, the reactivity of an element is largely due to its electron configuration, which is preserved in the atom. 3. **Why the Other Options are Incorrect**: - **Option B**: "An atom is a combination of two or more different elements bonded together." - This statement describes a **molecule**, not an atom. Molecules are formed when two or more atoms bond together, which can be of the same element (like O₂) or different elements (like H₂O). An atom cannot be a combination of elements; it is a singular unit. - **Option C**: "An atom is a charged particle that forms when an electron is added to or removed from a neutral atom." - This statement describes an **ion**, not an atom. An ion is indeed formed when an atom gains or loses electrons, resulting in a net charge. However, a neutral atom itself is not charged. Therefore, while this statement is partially true, it does not accurately define what an atom is. - **Option D**: "An atom is a large structure composed of multiple molecules that can exist in various states of matter." - This statement is misleading. An atom is not a large structure; it is a very small unit. Atoms combine to form molecules, and those molecules can exist in different states of matter (solid, liquid, gas). However, the definition provided here does not accurately describe an atom. ### Summary of Key Points: - An atom is the smallest unit of an element that retains its properties. - Atoms consist of protons, neutrons, and electrons, and their arrangement determines the element's characteristics. - Molecules are formed from combinations of two or more atoms, while ions are charged particles derived from atoms. - Understanding the distinction between atoms, molecules, and ions is crucial in chemistry. ### Revision Summary: - An atom is the smallest particle of an element that retains its properties (Option A). - Molecules are combinations of two or more atoms, while ions are charged atoms. - Atoms consist of subatomic particles: protons, neutrons, and electrons. - The properties of elements are determined by their atomic structure.