Chemistry · Chemical Bonding · NEET
They are NOT electric charge. They are the sign (phase) of the wave function of the orbital. A p-orbital has two lobes: one lobe is + and the other is -. This is just like a wave that goes up on one side and down on the other. NCERT says this clearly: the signs show the phase of the wave function and are not related to charge. Remember this line for NEET, it is a common trap.
Positive overlap happens when the two lobes that come close have the SAME sign (same phase) and point towards each other along the line joining the two nuclei. The two waves add up (constructive), so electron density builds up between the nuclei. This holds the atoms together, so a bond forms. Greater the positive overlap, stronger the bond.
Negative overlap happens when a + lobe of one orbital meets a - lobe of the other orbital. The waves have opposite phase, so they cancel each other (destructive). Electron density drops between the nuclei. There is nothing to glue the atoms together, so no stable bond forms.
Zero overlap happens when the orbitals are oriented perpendicular (at 90 degrees) to the bond axis, so the + part of one orbital overlaps the + part of the other equally as much as the - part. The plus and minus contributions cancel exactly, giving zero net overlap. Example: an s-orbital placed sideways against a p-orbital whose lobes point up and down (perpendicular to the line joining them). No bond forms.
NCERT lists two factors: (1) the sign (phase) of the wave function, and (2) the direction of orientation of the orbitals in space. So both WHICH sign meets which sign, AND HOW the orbitals point, decide if the overlap is positive, negative or zero. Miss either factor and you get the wrong answer.
Positive overlap is the CONDITION for a bond to form (same phase, correct orientation). How STRONG the bond is depends on how MUCH the orbitals overlap. More positive overlap = stronger covalent bond. Sigma bonds overlap more than pi bonds, so sigma bonds are stronger.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Positive overlap: same-phase lobes point at each other, bond forms. Negative overlap: opposite-phase lobes meet, waves cancel, no bond. Zero overlap: orbitals are perpendicular, plus and minus contributions cancel exactly, no overlap and no bond.
No. It means the wave function phases (signs) match. The + and - signs are phase labels, not charges. This is a very common NEET trap.
The extent of overlap decides the strength of a covalent bond. Greater the positive overlap, the greater the electron density between the nuclei, and the stronger the bond.
Valence Bond Theory (VBT). It explains covalent bond formation as the overlapping of atomic orbitals of two atoms, giving positive, negative or zero overlap based on phase and orientation.
Zero overlap gives no net electron build-up between nuclei, so no bond forms from that orientation. Negative overlap also gives no bond. Only positive overlap forms a bond.
