Concept: Electronegativity and Polarity

The nature of a chemical bond (ionic, non-polar covalent, or polar covalent) is determined by the difference in electronegativity between the bonded atoms. A larger electronegativity difference leads to a more polar bond, with the more electronegative atom acquiring a partial negative charge (${\delta }^{-}$) and the less electronegative atom acquiring a partial positive charge (${\delta }^{+}$).

Why (D) is correct:

1. Electronegativity Comparison: Chlorine (Cl) is more electronegative than Bromine (Br). On the Pauling scale, the electronegativity of Cl is approximately 3.16, and that of Br is approximately 2.96.

2. Polarity: Due to this difference in electronegativity, the shared electron pair in the Br-Cl bond will be pulled closer to the more electronegative chlorine atom.

3. Partial Charges: This unequal sharing results in the chlorine atom acquiring a partial negative charge (${\delta }^{-}$) and the bromine atom acquiring a partial positive charge (${\delta }^{+}$). Therefore, the bond is polar with the negative end on ${\mathrm{Cl}}^{-}$.

Option Analysis:

• A) ionic: The electronegativity difference between Br and Cl (0.2) is too small for an ionic bond. Ionic bonds typically form when the electronegativity difference is greater than 1.7.
• B) non-polar: A non-polar bond occurs when there is no significant electronegativity difference (typically less than 0.4), leading to equal sharing of electrons. Since Cl is more electronegative than Br, the bond is polar.
• C) polar with negative end on ${\mathrm{Br}}^{-}$: This is incorrect because chlorine is more electronegative than bromine, meaning chlorine will attract the electron density more strongly and thus bear the partial negative charge.

Correct Answer: (D)