The correct option is **B. An element is a pure substance that consists entirely of one type of atom.** ### Detailed Explanation: 1. **Definition of an Element**: - In chemistry, an element is defined as a fundamental substance that cannot be broken down into simpler substances by chemical reactions. Each element is made up of atoms that all 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 composed entirely of one type of atom. This means that if you have a sample of an element, every atom in that sample is identical in terms of its atomic structure. This is a key characteristic of elements, distinguishing them from compounds and mixtures. 3. **Why the Other Options are Incorrect**: - **Option A**: "An element is a substance that can be broken down into simpler substances by chemical means." - This statement is incorrect because it describes a compound, not an element. Compounds are substances formed when two or more different types of atoms bond together, and they can be broken down into simpler substances through chemical reactions. Elements, on the other hand, cannot be broken down into simpler substances. - **Option C**: "An element is a mixture of two or more substances that can be separated by physical means." - This statement is also incorrect. A mixture consists of two or more substances that are physically combined, and they can be separated by physical methods (like filtration or distillation). An element is not a mixture; it is a pure substance made up of only one type of atom. - **Option D**: "An element is a compound formed by the chemical combination of two or more different atoms." - This statement is incorrect because it defines a compound, not an element. Compounds are formed when two or more different types of atoms bond together chemically. An element, by definition, cannot be formed from a combination of different atoms; it consists solely of one type of atom. ### Summary of Key Points: - An element is a pure substance made entirely of one type of atom. - Elements cannot be broken down into simpler substances by chemical means. - Compounds consist of two or more different types of atoms and can be broken down chemically. - Mixtures contain two or more substances that can be separated by physical means. ### Revision Summary: - **Elements**: Pure substances made of one type of atom. - **Compounds**: Formed from two or more different atoms; can be broken down chemically. - **Mixtures**: Combinations of substances that can be separated physically. - **Atomic Structure**: Elements are defined by their atomic number (number of protons).