The correct option is **B. A pure substance that cannot be separated into simpler substances by chemical methods.** ### Detailed Explanation: 1. **Definition of an Element**: - In chemistry, an element is defined as a pure substance that consists of only one type of atom. Elements are the fundamental building blocks of matter and cannot be broken down into simpler substances through chemical reactions. Each element is represented by a unique symbol on the periodic table (e.g., H for hydrogen, O for oxygen). 2. **Why Option B is Correct**: - Option B accurately describes an element because it emphasizes that an element is a pure substance. This means it contains only one kind of atom and cannot be decomposed into simpler substances by any chemical means. For example, if you take a sample of pure gold (an element), you cannot break it down into anything simpler without changing its chemical identity. 3. **Why the Other Options are Incorrect**: - **Option A: A substance that can be broken down into simpler substances by chemical means**: - This statement describes compounds, not elements. Compounds are substances formed when two or more different elements chemically bond together. For example, water (H₂O) is a compound made of hydrogen and oxygen, and it can be broken down into these elements through chemical reactions (like electrolysis). - **Option C: A physical mixture of two or more substances**: - This option describes a mixture, not an element. A mixture consists of two or more substances that are physically combined but not chemically bonded. For example, a salad is a mixture of various vegetables, and each component retains its own properties. Mixtures can be separated by physical means (like filtration or evaporation), which is not applicable to elements. - **Option D: A compound formed from two or more different elements chemically bonded together**: - This statement also describes compounds, not elements. Compounds are formed when two or more different elements combine in fixed ratios through chemical bonds. For instance, sodium chloride (NaCl) is a compound made from sodium and chlorine. This option does not pertain to the definition of an element. ### Summary of Key Points: - An element is a pure substance made of only one type of atom and cannot be broken down into simpler substances by chemical means. - Compounds consist of two or more different elements chemically bonded and can be separated into their constituent elements through chemical reactions. - Mixtures are physical combinations of substances that retain their individual properties and can be separated by physical methods. - Understanding the distinction between elements, compounds, and mixtures is crucial in chemistry for classifying substances correctly. ### Revision Summary: - **Elements**: Pure substances made of one type of atom; cannot be broken down chemically. - **Compounds**: Substances formed from two or more different elements; can be broken down chemically. - **Mixtures**: Physical combinations of substances; can be separated by physical means. - **Key Concept**: Elements are the simplest form of matter in chemistry.