Question

Which does not form polyhalide ions like$X_{3}^{-}$ ?
(A) Fluorine
(B) Chlorine
(C) Bromine
(D) Iodine

Answer 1

Hint: Think about what polyhalide ions are. Think about interhalogen compounds. Write the electronic configuration of each of the given options and then think which element will not form a polyhalide. All the four options are halogens so think about their valence shells and empty shells.

Complete step by step solution:
- Polyhalide ions are characteristic properties of halogens to form triplet structure. When halide ions combine with interhalogen compounds or halogen molecules, monovalent ions are obtained as polyhalide ions.
- Interhalogen compounds are the compounds formed by one halogen with different halogen atoms like chlorine trifluoride, iodine trifluoride, bromine trichloride, etc.
- All the halogens have last shell electronic configuration as $n{{s}^{2}}n{{p}^{5}}$ and so they are highly electronegative in nature because they need only one electron to complete their octet. In nature, halogens exist in a diatomic state.
- As we move down the group, the atomic size increases as more numbers of shells are added.
- Let’s see electronic configuration of fluorine and chlorine.
\[^{9}F=1{{s}^{2}}2{{s}^{2}}2{{p}^{5}}\]
\[^{17}Cl=1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{5}}3{{d}^{0}}\]
- So, fluorine has one unpaired electron which it can share with another halogen atom like F, Cl, Br or I and form interhalogen compounds like ClF, BrF, IF. Similarly chlorine can also share its unpaired electron in 3p orbital and form diatomic compounds like ClF, ClBr, etc.
- But chlorine also has the presence of an empty 3d orbital. So, it can accept lone pairs from other halogen atoms to form polyhalides but the same is not true with fluorine. Fluorine can only form one covalent bond or an ionic bond by gaining one electron or donate its lone pair to form dative bonds in coordination compounds.
- Therefore, due to the absence of d-orbitals, fluorine doesn’t form polyhalide ions.

- Therefore, the answer is option (A).

Note: Remember in order to form polyhalide ions, the atoms should contain empty orbitals. All the halogens except, fluorine have vacant orbitals in them to gain lone pair of electrons and become monovalent ions but fluorine lacks the presence of d-orbitals and therefore has a small size and high electronegativity as it needs one electron to complete its octet and attain noble gas configuration.