Final answer:
The primary difference between a 3-D model of methane (CH4) and a dot diagram is that the 3-D model shows the tetrahedral geometry and bond angles, while the dot diagram focuses on electron pairs and bonding without indicating the molecule's three-dimensional shape.
Step-by-step explanation:
The 3-D model of methane (CH4) is different from the dot diagram drawing primarily in how it represents the spatial arrangement of atoms. In a 3-D model, the tetrahedral geometry of methane is clearly illustrated. The hydrogen atoms are positioned at the four corners of a tetrahedron with the carbon atom at the center. Each H-C-H bond angle is 109.5°.
This three-dimensional aspect is best visualized in a model, while a two-dimensional dot diagram, or Lewis structure, can only suggest this arrangement without a true sense of the spatial configuration.
When representing the methane molecule on paper or screen, conventions such as using solid wedged lines to indicate bonds coming out towards the viewer, and dashed wedged lines to show bonds going away from the viewer, are often employed. In contrast, dot diagrams represent electrons around atoms and are not focused on the 3-dimensional geometry, typically only indicating the connectivity and pairs of electrons around atoms without explicit portrayal of bond angles or the molecule's spatial orientation.