Final answer:
A diatomic molecule like HF is polar due to its polar bond and lack of symmetry, resulting in a dipole. However, in triatomic molecules, polar bonds do not guarantee polarity, as seen in water (polar) and carbon dioxide (nonpolar), where the molecule's shape determines if the dipole moments cancel out.
Step-by-step explanation:
Whether a molecule is polar or nonpolar depends on both the presence of polar covalent bonds and the overall geometry or symmetry of the molecule. In a diatomic molecule like HF, the presence of a polar bond between hydrogen and fluorine results in a dipole moment, making HF a polar molecule. Hydrogen fluoride is a classic example of a polar diatomic molecule with a distinct positive end and a negative end.
In contrast, triatomic molecules such as XY2 may not always be polar despite containing polar bonds. For example, molecule water (H2O) is polar because the molecule is bent, causing the dipole moments to add up and not cancel out. However, carbon dioxide (CO2) is a linear molecule with two polar bonds that are symmetrical and directly opposite each other. This symmetry causes the dipole moments to cancel out, making CO2 a nonpolar molecule.
To summarize, a molecule is polar if it contains at least one polar covalent bond and the molecule's structure does not lead to the cancellation of dipole moments. Molecules like hydrogen fluoride and water are polar because their structures result in an uneven distribution of electron density, while molecules like carbon dioxide are nonpolar due to their symmetrical structure which allows for the cancellation of dipole moments.