Q.Considering x-axis as molecular axis, which out of the following will form a sigma bond.

Step 1. With the x-axis as the molecular (internuclear) axis, a sigma bond requires the participating orbital lobes to point ALONG that x-axis, so they can overlap head-on.

Step 2. (i) 1s and 2py. The 1s orbital is spherical (no directional preference), but the 2py orbital's lobes point along the y-axis, PERPENDICULAR to the stated molecular axis x — approaching along x, the 2py orbital presents no net lobe density in that direction, so there is no effective (bonding) overlap.

Step 3. (ii) 2px and 2px. Both orbitals' lobes point directly along the x-axis, the molecular axis — this gives genuine head-on (axial) overlap, forming a σ bond.

Step 4. (iii) 2px and 2pz. One orbital (2px) lies along x, but the other (2pz) lies along z, perpendicular to x — approaching along the x-axis, the 2pz orbital again presents no lobe density in that direction, giving no effective (bonding) overlap. …

For each pairing, check whether both orbitals' lobes are oriented along the STATED molecular axis (x); only orbitals aligned along x …

• Assuming an s orbital can form a sigma bond with ANY p orbital regardless of that p orbital's own spatial direction — the s orbital is non-directional itself, but the p orbital it is trying to overlap with must still point along the actual bond axis for the overlap to be effective. …