Final answer:
Molecules with four electron pairs around the central atom exhibit specific electron-pair geometry. If all four pairs are bonding pairs, as in methane, the molecular geometry is tetrahedral. If there are nonbonding pairs involved, as with water, the molecular shape can deviate from tetrahedral.
Step-by-step explanation:
When a central atom has four pairs of electrons surrounding it, which can include bonding electron pairs (as in a covalent bond) or nonbonding electron pairs (lone pairs), this relates to the atom's electron-pair geometry.
A molecule with exactly four bonding electron groups around a central atom will adopt a tetrahedral geometry. An example of this is methane (CH4), where the central carbon atom forms four single covalent bonds with four hydrogen atoms, and the bond angles are approximately 109.5°.
However, if the central atom has not just bonding pairs but also lone pairs, the molecular geometry can vary. For instance, in water (H2O), the oxygen has two bonding pairs and two lone pairs, resulting in a bent molecular shape due to the lone pairs' repulsion.
Molecules with 4 electron pairs around the central atom are said to have a tetrahedral electron-pair geometry.
If there are four atoms attached to these electron groups, then the molecular shape is also tetrahedral. Methane (CH4) is an example of such a molecule.
If there are only two atoms attached to the electron groups, then the molecular shape may be different. For example, H2O has four electron groups around the central atom, but only two of them are bonded to surrounding atoms, resulting in a bent or V-shaped molecular structure.