**Correct Option: B. An atom is the smallest unit of matter that retains the properties of an element.** ### Detailed Explanation: 1. **Understanding Atoms:** - Atoms are the fundamental building blocks of matter. They are the smallest units of an element that still retain the chemical properties of that element. For example, a single carbon atom has the properties of carbon, such as its ability to form four bonds and its role in organic chemistry. 2. **Why Option B is Correct:** - Option B accurately defines an atom as the smallest unit of matter that retains the properties of an element. This means that if you take a single atom of oxygen (O), it will exhibit the characteristics of oxygen, such as its reactivity and its role in combustion and respiration. If you were to break down an oxygen atom further, you would reach subatomic particles (protons, neutrons, and electrons), which do not exhibit the properties of oxygen. 3. **Why the Other Options are Incorrect:** - **Option A: An atom is a substance that can be broken down into simpler substances by chemical means.** - This statement is incorrect because it describes a compound rather than an atom. Compounds can be broken down into simpler substances (elements) through chemical reactions, but atoms themselves cannot be broken down into simpler substances while retaining their elemental properties. - **Option C: An atom is a mixture of two or more substances that can be separated by physical means.** - This option is also incorrect. A mixture consists of two or more substances that are physically combined, such as salt and sand. Atoms are not mixtures; they are individual units of elements. Mixtures can be separated by physical means (like filtration or distillation), but atoms cannot be separated in this way. - **Option D: An atom is a large particle made up of molecules.** - This statement is misleading. Atoms are not made up of molecules; rather, molecules are made up of atoms. A molecule is formed when two or more atoms bond together. For example, a water molecule (H₂O) consists of two hydrogen atoms and one oxygen atom. Therefore, describing an atom as a large particle made up of molecules is fundamentally incorrect. ### Summary of Key Points: - An atom is the smallest unit of an element that retains its properties. - Atoms cannot be broken down into simpler substances while retaining their elemental characteristics. - Mixtures consist of multiple substances and can be separated physically, unlike atoms. - Molecules are formed from atoms, not the other way around. ### Revision Summary: - Atoms are the basic units of elements. - They retain the properties of the element they represent. - Atoms cannot be broken down into simpler substances by chemical means. - Molecules are composed of atoms, not the reverse.