**Correct Option: B. Boron** ### Explanation of Why Boron is a Metalloid 1. **Definition of Metalloids**: Metalloids are elements that have properties intermediate between metals and nonmetals. They typically exhibit a mix of metallic and non-metallic characteristics, which makes them unique. Common properties of metalloids include: - Semi-conductivity (conductivity between that of metals and nonmetals) - Brittle nature (not malleable or ductile like metals) - Ability to form alloys with metals 2. **Position on the Periodic Table**: Boron (B) is located in Group 13 (or IIIA) of the periodic table. It is situated along the zig-zag line that separates metals from nonmetals. This line is often referred to as the "stair-step" line, and elements that lie along this line are classified as metalloids. Boron is one of the most commonly recognized metalloids. 3. **Properties of Boron**: - **Electrical Conductivity**: Boron is a poor conductor of electricity at room temperature but can conduct electricity better than nonmetals when heated. This property is characteristic of metalloids. - **Physical State**: Boron is a solid at room temperature and has a high melting point, which is typical for metalloids. - **Chemical Behavior**: Boron can form covalent bonds with nonmetals and can also exhibit some metallic behavior, such as forming alloys with metals. ### Why the Other Options are Incorrect - **A. Sodium (Na)**: Sodium is classified as an alkali metal. It is located in Group 1 of the periodic table and exhibits typical metallic properties such as high electrical conductivity, malleability, and ductility. Sodium is not a metalloid because it does not have the intermediate properties characteristic of metalloids. - **C. Chlorine (Cl)**: Chlorine is a nonmetal and is found in Group 17 (or VIIA) of the periodic table. It is a gas at room temperature and is highly reactive, especially with metals to form ionic compounds. Chlorine does not exhibit the properties of metalloids, as it is not a conductor of electricity and does not have the mixed characteristics of metals and nonmetals. - **D. Calcium (Ca)**: Calcium is an alkaline earth metal located in Group 2 of the periodic table. Like sodium, it exhibits typical metallic properties, including good electrical conductivity, malleability, and ductility. Calcium does not have the characteristics of a metalloid. ### Summary of Key Points - **Boron is classified as a metalloid** due to its position on the periodic table and its intermediate properties between metals and nonmetals. - **Metalloids** like boron exhibit semi-conductivity, brittleness, and the ability to form covalent bonds. - **Sodium, Chlorine, and Calcium** are not metalloids; they are classified as metals and nonmetals based on their distinct properties. - Understanding the classification of elements on the periodic table helps in predicting their behavior and properties in chemical reactions. ### Revision Summary - Metalloids have properties between metals and nonmetals. - Boron is a metalloid located in Group 13 of the periodic table. - Sodium (metal), Chlorine (nonmetal), and Calcium (metal) do not exhibit metalloid characteristics. - The periodic table's zig-zag line helps identify metalloids.