Which of the following arrangements shows increasing order of reactivity of the halogens?

Analyze the question: You are asked for the arrangement that shows increasing reactivity of the halogens (Group 17), so the least reactive comes first and the most reactive comes last.

Concept: Halogens react by gaining one electron to form X⁻. Going down Group 17 (F → Cl → Br → I):

• Atomic size increases.
• The outer shell is farther from the nucleus and is more shielded by inner electrons.
• The nucleus attracts an incoming electron less strongly.
• So the tendency to gain an electron decreases, and reactivity decreases down the group.

The order of reactivity is therefore: F₂ > Cl₂ > Br₂ > I₂.

You can see this in displacement reactions: Cl₂ displaces Br⁻ and I⁻ from solution, and Br₂ displaces I⁻, but I₂ cannot displace Cl⁻ or Br⁻.

Checking the options:

• A. F₂ > Cl₂ > Br₂ > I₂ is the correct order of reactivity, but it shows decreasing order, not increasing.
• B. I₂ < Br₂ < Cl₂ < F₂ starts with the least reactive (I₂) and ends with the most reactive (F₂). This is increasing reactivity. ✔
• C. F₂ < Cl₂ < Br₂ < I₂ is the wrong way round.
• D. I₂ > Br₂ > Cl₂ > F₂ is also wrong.

Answer: B. Option A has the right ranking but states it as decreasing, so it does not fit the question. Option B states the same ranking in increasing form.

Tip: Read the wording carefully. "Increasing" means smallest to largest, and the symbols < or > must match that. Remember: for halogens, reactivity goes down as you go down the group, while for Group 1 metals it goes up as you go down.