Question

Find hybridisation of hcl. how do i know that which will be considered the central atom in this when their are only 2 elements in the compound?

Answer 1

If you can assign the total electron geometry (geometry of all electron domains, not just bonding domains) on the central atom using VSEPR, then you can always automatically assign hybridization. Hybridization was invented to make quantum mechanical bonding theories work better with known empirical geometries. If you know one, then you always know the other.

Linear - spsp - the hybridization of one ss and one pp orbital produce two hybrid orbitals oriented 180∘180∘ apart.

Trigonal planar - sp2spX2 - the hybridization of one ss and two pp orbitals produce three hybrid orbitals oriented 120∘120∘ from each other all in the same plane.

Tetrahedral - sp3spX3 - the hybridization of one ss and three pp orbitals produce four hybrid orbitals oriented toward the points of a regular tetrahedron, 109.5∘109.5∘ apart.

Trigonal bipyramidal - dsp3dspX3 or sp3dspX3X223d - the hybridization of one ss, three pp, and one dd orbitals produce five hybrid orbitals oriented in this weird shape: three equatorial hybrid orbitals oriented 120∘120∘ from each other all in the same plane and two axial orbitals oriented 180∘180∘ apart, orthogonal to the equatorial orbitals.

Octahedral - d2sp3dX2X222spX3 or sp3d2spX3X223dX2 - the hybridization of one ss, three pp, and two dd orbitals produce six hybrid orbitals oriented toward the points of a regular octahedron 90∘90∘ apart.

I assume you haven't learned any of the geometries above steric number 6 (since they are rare), but they each correspond to a specific hybridization also.

NH3NHX3

For NH3NHX3, which category does it fit in above? Remember to count the lone pair as an electron domain for determining total electron geometry. Since the sample question says NH3NHX3 is sp3spX3, then NH3NHX3 must be tetrahedral. Make sure you can figure out how NH3NHX3 has tetrahedral electron geometry.

For H2COHX2CO

Start by drawing the Lewis structure. The least electronegative atom that is not a hydrogen goes in the center (unless you have been given structural arrangement).

Determine the number of electron domains on the central atom.

Determine the electron geometry using VSEPR. Correlate the geometry with the hybridization.

Practice until you can do this quickly.

hope this answer helpful u