The correct option is **C. their reactivity decreases down the group**. ### Detailed Explanation: Group VII elements, also known as the halogens, include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are known for their high reactivity, particularly with alkali and alkaline earth metals, forming salts. #### Why Option C is Correct: 1. **Reactivity Trend**: As you move down Group VII from fluorine to astatine, the reactivity of the halogens decreases. This trend can be explained by considering the atomic structure and the ability of these elements to gain an electron to achieve a stable electron configuration (octet). 2. **Atomic Size**: As you go down the group, the atomic size increases due to the addition of electron shells. This increase in size means that the outermost electrons are further from the nucleus, which reduces the effective nuclear charge experienced by the outer electrons. Consequently, it becomes more difficult for these larger atoms to attract an additional electron. 3. **Electronegativity**: Electronegativity, which is the ability of an atom to attract electrons, also decreases down the group. Fluorine is the most electronegative element, while iodine is less so. This decrease in electronegativity correlates with a decrease in reactivity. 4. **Bonding**: The halogens typically form diatomic molecules (e.g., F2, Cl2, Br2, I2) in their elemental form. The bond strength of these diatomic molecules also decreases down the group, which contributes to the decrease in reactivity. #### Why the Other Options are Incorrect: - **Option A: They are present in the same physical state** This statement is incorrect because the physical states of the halogens vary at room temperature. Fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Therefore, they do not all exist in the same physical state. - **Option B: They are strong reducing agents** This statement is misleading. Halogens are actually strong oxidizing agents, not reducing agents. They tend to gain electrons during chemical reactions, which is characteristic of oxidizing agents. A reducing agent, on the other hand, donates electrons and is oxidized in the process. Thus, this option is incorrect. - **Option D: They exist as monoatomic molecules** This statement is incorrect because halogens exist as diatomic molecules (e.g., F2, Cl2, Br2, I2) in their elemental form, not as monoatomic species. Monoatomic gases are typically noble gases (Group 0), which are chemically inert. ### Summary: - The reactivity of Group VII elements decreases down the group due to increasing atomic size and decreasing electronegativity. - Fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid at room temperature, indicating they do not share the same physical state. - Halogens are strong oxidizing agents, not reducing agents. - Halogens exist as diatomic molecules, not monoatomic. ### Revision Summary: - Group VII elements (halogens) show decreasing reactivity down the group. - Physical states vary: F and Cl are gases, Br is a liquid, I is a solid. - Halogens are strong oxidizing agents, not reducing agents. - They exist as diatomic molecules (e.g., F2, Cl2).