Chemistry · Chemical Bonding · NEET
s-s overlap always gives a SIGMA bond. The two s-orbitals are spherical, so they can only meet head-on along the internuclear axis (the line joining the two nuclei). Head-on (axial) overlap = sigma bond. Example: the H2 molecule forms by s-s overlap of two 1s orbitals.
In s-p overlap, an s-orbital of one atom overlaps end-to-end with a p-orbital of another atom along the axis (example: H-Cl bond, 1s of H with a 3p of Cl). In p-p overlap, two p-orbitals of two atoms meet. If they meet head-on (end-to-end) it is a sigma bond; if they lie parallel and overlap sideways it is a pi bond. So s-p is always sigma, but p-p can be either sigma or pi depending on direction.
No. This is the most common mistake. p-p overlap gives a SIGMA bond when the two p-orbitals point at each other and overlap head-on along the axis (like the F-F bond in F2). p-p overlap gives a PI bond only when the two p-orbitals are parallel and overlap sideways, above and below the axis. So the direction of overlap decides sigma vs pi, not the orbital name.
A sigma bond is always stronger than a pi bond because in sigma bonding the orbitals overlap head-on to a larger extent, so electron density between the nuclei is high. Pi bonds form by weaker sideways overlap. Among sigma bonds, more overlap = stronger bond. This is why a single bond (1 sigma) is weaker than a double bond (1 sigma + 1 pi).
No. An s-orbital is spherical, so it has no direction and cannot line up parallel for sideways overlap. It can only overlap head-on, which is sigma. So s-s and s-p overlaps are always sigma. Pi bonds need directional p (or d) orbitals that overlap sideways. This is a favourite NEET trap.
Head-on (also called axial or end-to-end) overlap happens along the internuclear axis - the orbitals point straight at each other. This gives a sigma bond. Sidewise (lateral) overlap happens when the orbital axes stay parallel and perpendicular to the internuclear axis; they overlap above and below the line. This gives a pi bond, which is weaker.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
s-s overlap, s-p overlap, and p-p (head-on) overlap. All three are end-to-end overlaps along the internuclear axis, so all three give sigma bonds.
s-s: H2 (two 1s orbitals of hydrogen). s-p: HCl or HF (1s of H with a p-orbital of the halogen). p-p: F2 (a 3... actually 2p of one F with 2p of the other F, head-on) gives a sigma bond.
Because head-on overlap (sigma) allows the orbitals to overlap to a much larger extent, concentrating electron density directly between the nuclei. Sideways overlap (pi) is smaller and less effective, so pi bonds are weaker.
A double bond = 1 sigma + 1 pi. A triple bond = 1 sigma + 2 pi. The sigma always forms first by head-on overlap; the extra bonds are pi from sideways p-p overlap.
2py and 2py will not form a sigma bond along the x-axis, because these p-orbitals are perpendicular to the axis and overlap sideways, giving a pi bond instead. 1s-1s, 1s-2px, and 1s-2s all form sigma bonds (NCERT exercise 4.29).
