Which among the following is the correct arrangement of halogens in the increasing order of their oxidizing nature?
- (a)F, Cl, Br, I
- (b)Cl, Br, F, I
- (c)Br, I, Cl, F
- (d)I, Br, Cl, F
Correct — D, I, Br, Cl, F. The oxidising power of the halogens increases up the group as atomic size falls and the pull on electrons rises, so from weakest to strongest oxidiser the order is I < Br < Cl < F. Fluorine is the strongest oxidising agent and iodine the weakest, giving the increasing order I, Br, Cl, F.
- (a)F, Cl, Br, I — This is the decreasing order of oxidising power (strongest first), whereas the question asks for the increasing order.
- (b)Cl, Br, F, I — The sequence is jumbled — it wrongly puts fluorine (the strongest oxidiser) in the middle and iodine (the weakest) at the end.
- (c)Br, I, Cl, F — Iodine is a weaker oxidiser than bromine, so I cannot come after Br; the correct weak-to-strong run is I, Br, Cl, F.
The halogens form Group 17 of the periodic table. Their oxidising power, the tendency to gain an electron and oxidise other species, decreases down the group because atomic size increases and the nucleus holds incoming electrons less tightly. Fluorine, at the top, is the strongest oxidiser; iodine, at the bottom, is the weakest.
Read the trend carefully and match it to the word 'increasing'. Since oxidising power falls down the group, listing the halogens from weakest to strongest means starting with iodine and ending with fluorine.
- Oxidising power of the halogens falls down the group, F2 > Cl2 > Br2 > I2.
- So the increasing (weak to strong) order is I < Br < Cl < F.
- A halogen higher in the group displaces the ion of one below it, as chlorine displaces bromide and iodide.
- Fluorine is the most electronegative and the most reactive element.
Oxidising power rises up the group: iodine weakest, fluorine strongest.
- Reversing increasing and decreasing order.
- Forgetting that oxidising power increases up the group, not down.
NDA/UPSC ask you to rank halogens by oxidising power, reactivity or electronegativity — fix in mind that fluorine is strongest and iodine weakest.
The most reactive among the halogens is
- (a) fluorine
- (b) chlorine
- (c) bromine
- (d) iodine
Answer(a) fluorine — with the highest electronegativity and smallest atomic radius, fluorine is the most reactive and the strongest oxidising halogen.
UPSC prelims 1997 asked for the most reactive halogen — fluorine — the same trend that makes it the strongest oxidiser and places it at the top of the increasing oxidising-power order.
- practice — not a real PYQ
Which halogen is the strongest oxidising agent?
- (a)Iodine
- (b)Bromine
- (c)Chlorine
- (d)Fluorine
Answer(d) Fluorine — it is the smallest and most electronegative halogen, making it the strongest oxidiser.
- practice — not a real PYQ
When chlorine gas is passed through potassium iodide solution, iodine is liberated because
- (a)iodine is a stronger oxidiser than chlorine
- (b)chlorine is a stronger oxidiser and displaces iodine
- (c)no reaction occurs
- (d)potassium is displaced
Answer(b) chlorine is a stronger oxidiser and displaces iodine — a halogen higher in the group displaces one below it.