Explain the three-dimensional $(3-D)$ representation of organic molecules and provide examples.

(N/A) The three-dimensional $(3-D)$ structure of organic molecules can be represented on paper using specific conventions:
$(a)$ Wedge-and-dash formula:
- Solid wedge $(\blacktriangle)$: Used to indicate a bond projecting out of the plane of the paper towards the observer.
- Dashed wedge $(\dots)$: Used to depict a bond projecting out of the plane of the paper away from the observer.
- Normal line $(-)$: Used to represent bonds lying in the plane of the paper.
Example: The $3-D$ representation of a methane $(CH_4)$ molecule is shown in the provided image.
$(b)$ Molecular Models: These are physical devices used for better visualization of the $3-D$ shapes of organic molecules.
$(i)$ Framework Model: Represents only the bonds connecting the atoms,ignoring the size of the atoms.
$(ii)$ Ball and Stick Model: Represents atoms as balls and bonds as sticks.
$(iii)$ Space Filling Model: Emphasizes the surface area of the molecule by representing atoms as spheres.