The electronegativity difference between atoms in a molecule is a useful indicator for determining the polarity of the molecule. A larger electronegativity difference between atoms generally indicates a more polar bond. However, in the case of CCl3 (trichloromethane or chloroform), although the electronegativity difference between carbon (C) and chlorine (Cl) is approximately 0.5, the molecule is still considered nonpolar. This can be explained by considering the molecular geometry and the overall symmetry of the molecule.
In CCl3, carbon is bonded to three chlorine atoms. The molecule has a trigonal planar geometry, with the carbon atom at the center and the three chlorine atoms surrounding it in a flat, triangular arrangement. The chlorine atoms are positioned symmetrically around the carbon atom, with the same electronegativity. Due to this symmetric arrangement, the dipole moments of the C-Cl bonds cancel each other out, resulting in a nonpolar molecule.
Each C-Cl bond has a polar nature, with the chlorine atom being more electronegative and having a partial negative charge, while the carbon atom has a partial positive charge. However, since the bond dipoles point in opposite directions and have equal magnitude, their effects cancel out, resulting in a net dipole moment of zero for the molecule.
It's important to note that while the individual C-Cl bonds are polar, the overall molecular geometry and symmetry determine the molecule's polarity. In the case of CCl3, the symmetric arrangement of the bonds leads to a cancellation of dipole moments, making the molecule nonpolar despite the small electronegativity difference between carbon and chlorine.