Final answer:
A central atom surrounded by just two regions of valence electron density exhibits sp hybridization, resulting in a linear molecular arrangement. Examples include HgCl₂ and CO2. Atoms with different numbers of electron density regions show sp² or sp³ hybridization and corresponding geometric arrangements.
Step-by-step explanation:
Any central atom that is surrounded by just two regions of valence electron density in a molecule will exhibit sp hybridization. For example, the mercury atom in a linear HgCl₂ molecule, the zinc atom in Zn(CH₃)₂ with a linear C-Zn-C arrangement, and the carbon atoms in diatomic molecules like HCCH and CO2 all show sp hybridization. The sp hybridization occurs when one s orbital and one p orbital combine to create two degenerate hybrid orbitals that are oriented linearly around the central atom.
When a molecule's central atom is surrounded by three regions of electron density, such as a lone pair or different bond types (single, double), the atom is sp² hybridized. In this case, it has three sp² hybrid orbitals in a trigonal planar arrangement and one unhybridized p orbital. Examples of sp² hybridization include molecules with a lone pair on the central atom like CINO, as well as molecules like formaldehyde (CH₂O) and ethene (H₂CCH2), which have two single bonds and a double bond connected to the central atom.
Finally, atoms surrounded by four regions of valence electron density, such as in methane (CH₄), will exhibit sp³ hybridization, which involves one s orbital and all three p orbitals mixing to produce four identical sp³ hybrid orbitals. These are oriented toward the corners of a tetrahedron, allowing for the tetrahedral geometry of molecule's with four regions of electron density.