Orbital Hybridization question, PLEASE help me on this one?

[sam12103]

1.   What are the electron-pair and molecular geometries? What orbitals on C, H, and Cl overlap to form bonds involving these elements?


Answer in the back of the book

Electron pair geometry= tetrahedral
Molecular geometry=tetrahedral

The H-C bonds are a result of the overlap of the hydrogen s orbital with sp^3 hybrid orbitals on carbon.
The Cl-C bonds are formed by the overlap of the sp^3 hybrid orbitals on carbon with the p orbitals on chlorine
(Lone pairs on the chlorine atoms have been omitted for the sake of clarity).


Can someone explain to me how the book came up with the answers for the second portion, I know how to find the Electron pair geometry and Molecular geometry, but I don’t understand in general how to find the orbitals that overlap to form bonds involving the elements.

Can you please explain this to me?

Thank you

[kingchemist]

If carbon is joining with 4 atoms of other elements, it needs to do something with its outer electrons (1s2) 2s2 2p1 2p1.
So the 4 outer electrons are promoted to make four bonding orbitals of equal energy. As the bonding orbitals (or hydbrid orbitals) involve an s orbital (the 2s) and the p orbitals, we call each orbital an sp3 hybrid orbital. This is because the bonding orbitals are made from the equivalent of a s orbital and three p orbitals.

In the case of a C atom, the 4 hybrid orbitals each consist of a single electron. These repel each other and move as far from each other as possible to give a tetrahedral geometry with angles of 109.5 degrees. In methane, CH4, the four sp3 hybrid orbitals share with the one electron from each hydrogen atom to make a tetrahedral molecule, via sigma bonds (electrons that are shared lie in the plane between the atoms.

In ethene, C2H4, the hybridisation is different. Three hybrid orbitals are produced from the equivalent of the 2s orbital and 2 of the 2p orbitals. So the hybrid orbitals are called sp2 hybrid orbitals. These three orbitals each contain 1 electron and repulsion causes them to form a planar arrangement with angle of 120 degrees between them. The sp2 orbitals form sigma bonds with 2 atoms of H and with the other atom of C. Additionally each carbon atom still has 1 electron in a p orbital. This is a two-lobed structure. The two lobes on one C atom overlap with the two on the other atom to produce two regions of electron density above and below the plane of the sigma bond between the two carbon atoms. Itis known as a pi bond. The final structure of the molecule then has all 6 atoms in a square planar molecular arrangement.

In ethyne, C2H2, the hybridisation of carbon is sp hydridisation (involving the s and one p orbitals. Repulsion causes them to take up a linear arrangement with 180o between them. Each C atom form an sigma bond with an atom of H and with the other atom of Carbon. Each carbon atom, though has 2 p orbitals which overlap above/below and in front/behind the sigma bond so that it appears there is a triple bond between the C atoms (however, it is a sigma bond and two pi bonds). The molecule is linear with all 4 atoms in a line.