The reactivity of fluorine is high because of

Step 1: Analyze the question We need the main reason fluorine is so reactive. Fluorine is the most reactive non-metal, so we look for the property that best explains this.

Step 2: Explain the concept Fluorine is in Group 17 (halogens). Halogens react by gaining one electron to complete their octet. Reactivity here depends on how strongly an atom attracts electrons (electronegativity) and how easily the F-F bond breaks.

Step 3: Evaluate each option

• A. High electronegativity: Fluorine has the highest electronegativity of all elements (4.0 on the Pauling scale). It attracts electrons very strongly, so it readily forms fluoride ions or polar bonds. This is the main reason for its reactivity. ✔
• B. Small atomic size: This contributes to fluorine's high electronegativity, since the nucleus is close to the incoming electron. However, it is the underlying cause, not the direct explanation. It is a weaker choice than A.
• C. Availability of d-orbitals: Fluorine is in Period 2 and has no d-orbitals. ✘
• D. Strong F-F bond: The F-F bond is actually weak (about 159 kJ/mol), because lone pair repulsion between the small atoms destabilises it. A weak bond makes fluorine reactive, so "strong" is incorrect. ✘

Step 4: Answer The best answer is A, its high electronegativity.

Note that a weak F-F bond also helps fluorine's reactivity, but that is not what option D says, and the examiners' focus is electronegativity.

Tip: For halogen reactivity questions, remember: "Up the group, electronegativity and oxidising power go up." Fluorine is at the top, so it is the strongest oxidising agent. Also, never link fluorine with d-orbitals, because Period 2 elements have none.