Final answer:
A molecule with four atoms around a central atom and a bond angle of 109.5° is described as having a tetrahedral molecular shape. Methane (CH4) is a common example with this geometry. The presence of lone pairs can alter the bond angles slightly, resulting in a bent shape.
Step-by-step explanation:
A molecule with four atoms around a central atom and a bond angle of 109.5° typically exhibits a tetrahedral geometry. This is because four electron groups about the central atom will orient themselves to minimize repulsion, resulting in a tetrahedral placement. This shape is characterized by atoms being placed around a central atom such that a three-dimensional shape is generated with four corners and 109.5° angles between each pair and the central atom. An example of this is methane (CH4), where the molecular shape is also tetrahedral. The bond angles in a perfect tetrahedron are exactly 109.5°, but when lone pair interactions are present, as seen in water (H2O) for example, the bond angle can be slightly less due to the increased repulsion from lone pairs. In this case, the molecular shape becomes bent or V-shaped, with angles slightly less than 109.5°.
In the context of Lewis structures and molecular geometry, the presence of four electron groups suggests a tetrahedral electron-pair geometry, as shown in different figures provided in textbooks. When two of the four groups are lone pairs of electrons, the molecular shape adjusts to accommodate the repulsion and is described as bent, similar to a tetrahedron with two missing vertices.