Chemistry · Chemical Bonding · NEET
The Debye is just the unit we use to measure dipole moment, the same way metre is the unit for length. It is named after the scientist Peter Debye. One Debye equals 3.33564 × 10⁻³⁰ coulomb·metre (C·m). We use it because molecular dipole moments are tiny numbers in C·m, so the Debye gives us clean values like 1.85 D for water instead of a long power of ten.
Use the formula μ = Q × r. Here Q is the amount of charge at each end (in coulombs) and r is the distance between the centre of positive charge and the centre of negative charge (in metres). So dipole moment = charge × distance. NCERT states this exact formula. A larger charge or a larger separation both make μ bigger.
You can, but the number is very small and awkward. For example, one electronic charge separated by 1 Å gives about 4.8 D, which in C·m is around 1.6 × 10⁻²⁹ C·m. The Debye lets you write and compare molecular values easily. For NEET, remember the conversion: 1 D = 3.33564 × 10⁻³⁰ C·m.
It is a vector. It has both size (magnitude) and direction. The direction points from one charge centre to the other. This matters a lot: when a molecule has more than one polar bond, you must add the bond dipoles as vectors, not just add their numbers. That is why symmetric molecules like CO₂ can have zero net dipole even though each bond is polar.
In physics the arrow points from the negative centre to the positive centre. But in chemistry (and NCERT) we draw a crossed arrow where the cross (plus sign) sits on the positive end and the arrow head points to the negative end. For HF, the crossed arrow points toward F because F is more electronegative and pulls the electrons, making F the negative end.
Zero. If a molecule is nonpolar, its dipole moment μ = 0 D. This happens either because the bonds are nonpolar (like H₂) or because polar bonds cancel out by symmetry (like CO₂, BF₃, CCl₄). A zero dipole moment is a strong clue that the molecule is symmetric.
The correct order of dipole moments for molecules NH₃, H₂S, CH₄ and HF, is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
μ = Q × r, where Q is the magnitude of charge and r is the distance between the positive and negative charge centres. It is measured in Debye units (D).
1 D = 3.33564 × 10⁻³⁰ coulomb·metre (C·m). This is the conversion factor given in NCERT.
It is named after Peter Debye, a scientist who studied molecular structure and dipole moments.
Because it has both magnitude and direction. When a molecule has several polar bonds, the bond dipoles must be added as vectors, which can give a large net dipole or cancel to zero.
It means the molecule is nonpolar. Either its bonds are nonpolar, or the polar bonds cancel out because the molecule is symmetric, like in CO₂, BF₃ and CCl₄.
