The correct option is **B. A pure substance consisting of only one type of atom that cannot be chemically interconverted or broken down.** ### Detailed Explanation: 1. **Understanding Elements:** - An element is defined as a fundamental substance that cannot be broken down into simpler substances by chemical means. Each element is made up of atoms that have the same number of protons in their nuclei, which is known as the atomic number. For example, all carbon atoms have 6 protons, making carbon an element. 2. **Why Option B is Correct:** - Option B accurately describes an element as a pure substance made up of only one type of atom. This means that elements are the simplest forms of matter and cannot be decomposed into simpler substances through chemical reactions. For instance, gold (Au) is an element because it consists solely of gold atoms and cannot be broken down into anything simpler without losing its identity as gold. 3. **Why the Other Options are Incorrect:** - **Option A: A substance that can be broken down into simpler substances by chemical reactions.** - This statement describes compounds, not elements. Compounds are substances formed when two or more different types of atoms bond together, and they can be broken down into their constituent elements through chemical reactions. For example, water (H₂O) can be broken down into hydrogen and oxygen gases. - **Option C: A mixture of two or more substances that retains its individual properties.** - This option describes a mixture, not an element. Mixtures consist of two or more substances that are physically combined but not chemically bonded. Each component in a mixture retains its own properties. For example, a salad is a mixture of various vegetables, each maintaining its own flavor and texture. - **Option D: A compound formed from two or more different types of atoms bonded together.** - This statement also describes compounds, not elements. Compounds are formed when different types of atoms bond together chemically. For example, sodium chloride (NaCl) is a compound made from sodium (Na) and chlorine (Cl) atoms bonded together. ### Summary of Key Points: - An **element** is a pure substance made of only one type of atom and cannot be broken down chemically. - **Compounds** consist of two or more different types of atoms bonded together and can be broken down into simpler substances. - **Mixtures** are combinations of two or more substances that retain their individual properties and are not chemically bonded. - Understanding the definitions and differences between elements, compounds, and mixtures is crucial in chemistry. ### Revision Summary: - Elements are pure substances made of one type of atom. - Compounds are formed from two or more different atoms and can be broken down. - Mixtures consist of multiple substances that retain their properties. - Recognizing these distinctions is essential for mastering basic chemistry concepts.