Complete the table by indicating the colour and physical state of Fluorine and Bromine at room temperature. Sketch the trend in bond dissociation energy of halogens and boiling points of hydrogen halides.

4(a)

$$\begin{array}{|c|c|c|} \hline \text{Halogen} & \text{Colour} & \text{Physical state} \\ \hline \text{Fluorine} & \text{Pale yellow} & \text{Gas} \\ \hline \text{Bromine} & \text{Reddish-brown} & \text{Liquid} \\ \hline \end{array}$$

4(b)

• Trend in bond dissociation energy of halogens ($\mathrm{X_2}$): The bond dissociation energy generally decreases down the group from chlorine to iodine. However, fluorine has an anomalously low bond dissociation energy compared to chlorine and bromine due to the small size of fluorine atoms and repulsion between lone pairs. The order is $\mathrm{Cl_2 > Br_2 > F_2 > I_2}$.

• Trend in boiling points of hydrogen halides ($\mathrm{HX}$): The boiling points of hydrogen halides generally increase down the group from $\mathrm{HCl}$ to $\mathrm{HI}$ due to increasing London dispersion forces. However, $\mathrm{HF}$ has an anomalously high boiling point compared to $\mathrm{HCl}$, $\mathrm{HBr}$, and $\mathrm{HI}$ because of strong hydrogen bonding between $\mathrm{HF}$ molecules. The order is $\mathrm{HF > HI > HBr > HCl}$.