Question

Which of the following statements is not true for halogens?

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> NEET 2018

• A. All are oxidizing agents
• B. All but fluorine show positive oxidation states
• C. Chlorine has the highest electron-gain enthalpy
• D. All form monobasic oxyacids

Answer 1

The correct option is B All but fluorine show positive oxidation states

All Halogens due to very high -ve reduction potential values show oxidising characteristics.

All halogen atoms can form monobasic oxyacids.

Down the group, the electron gain enthalpy value becomes less negative. Fluorine is an exception as it has lower electron gain enthalpy than chlorine because of its small structure and high $e^{-}$ density due to which incoming $e^{-}$ faces repulsion.
Hence order for electron gain enthalpies will be : $C{l}_2>F_2>B{r}_2>I_2$

Due to fluorine's high electronegativity and small size, it forms only one oxoacid, $HOF$. The oxidation number of $F$ is always -1.
Hence option (d) is not true.