The correct option is **B. A pure substance consisting of only one type of atom**. ### Detailed Explanation: 1. **Understanding Elements**: - In chemistry, an element is defined as a pure substance that cannot be broken down into simpler substances by chemical means. Each element is made up of atoms that are all of the same type. For example, all atoms of carbon are identical, and they are different from atoms of oxygen or hydrogen. 2. **Why Option B is Correct**: - **Pure Substance**: An element is a pure substance, meaning it has a uniform and definite composition. It consists solely of one type of atom, which is the fundamental building block of matter. - **One Type of Atom**: The defining characteristic of an element is that it contains only one type of atom. For instance, the element gold (Au) consists entirely of gold atoms, and the element oxygen (O) consists entirely of oxygen atoms. 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 a compound, not an element. Compounds are substances formed when two or more different elements chemically combine, and they can be broken down into their constituent elements through chemical reactions (e.g., water can be broken down into hydrogen and oxygen). - **Option C: A mixture of two or more substances that can be separated physically**: - This statement refers to a mixture, not an element. Mixtures consist of two or more substances that are physically combined, and they retain their individual properties. For example, a salad is a mixture of different vegetables, and the components can be separated without any chemical change. - **Option D: A compound formed by the chemical combination of two or more different elements**: - This statement also describes a compound. Compounds are formed when two or more different elements bond together chemically. For example, sodium chloride (table salt) is a compound made from sodium (Na) and chlorine (Cl) atoms. ### Summary of Key Points: - An **element** is a pure substance made up of only one type of atom. - Elements cannot be broken down into simpler substances by chemical means. - Compounds consist of two or more different elements and can be broken down chemically. - Mixtures contain two or more substances that can be separated physically and retain their individual properties. ### Revision Summary: - An element is a pure substance with only one type of atom (Option B). - Compounds can be broken down into elements (Option A is incorrect). - Mixtures can be separated physically (Option C is incorrect). - Compounds are formed from different elements (Option D is incorrect).