Question

A classmate of yours is convinced that he knows everything about electronegativity. (a) In the case of atoms X and Y having different electronegativities, he says, the diatomic molecule $X-Y$ must be polar. Is your classmate correct? (b) Your classmate says that the farther the two atoms are apart in a bond, the larger the dipole moment will be. Is your classmate correct?

Short answer

(a) Yes, your classmate is correct. When two atoms X and Y in a diatomic molecule have different electronegativities (X ≠ Y), the molecule X-Y will be polar due to the uneven electron distribution and resulting partial charges on each atom. (b) The classmate's statement is not entirely correct. While an increase in bond length can contribute to an increased dipole moment, it is not the sole determinant. The charge difference between the two atoms also plays a crucial role in determining the dipole moment.

Step-by-step solution

(a) Diatomic molecules with different electronegativities

Electronegativity is the ability of an atom to attract the electrons involved in a covalent bond towards itself. If atoms involved in a bond have different electronegativities, the electron distribution in the bond will not be uniform. The result is partial charges on each atom: the more electronegative atom has a partial negative charge, while the less electronegative one has a partial positive charge. This results in a dipole moment in the bond, meaning there is a polarity in the molecule. Thus, when two atoms X and Y in a diatomic molecule have different electronegativities (X ≠ Y), the molecule X-Y will be polar. Therefore, the classmate is correct in this case.

(b) Effect of distance between atoms on the dipole moment

A dipole moment is a product of the charge difference between the two atoms and the distance between them (bond length). The dipole moment ($\mu$) is given by: $$\mu =Q\cdot d$$ Where Q represents the charge difference between the two atoms, and d is the bond length. Atoms that are farther apart in a bond will have a larger bond length, which contributes to an increase in the dipole moment. However, an increase in bond length might also decrease the electronegativity difference between the two atoms due to the electron cloud spreading out over a larger area. It is not solely the bond length that determines the dipole moment but also the charge difference between the two atoms. So, while in some cases, the dipole moment can increase as the distance between two atoms in a bond increases, it is not always true. Therefore, the classmate's statement is not entirely correct.