Why is it that alkanes and alkynes, unlike alkenes, have no geometric isomers?

Isomers are fundamentally like twins in chemistry—they have the same number of atoms but are unique in how they're arranged in space. There are several types of isomers, but let's focus on the stars of the show: structural isomers and stereoisomers. The former are like distant cousins with the same family features—same molecular formula but different connections between their atoms. They could be chains of different lengths or even rings.

Stereoisomers, on the other hand, are more like mirrored siblings. They're made exactly the same way but they reflect each other like a person does in a mirror, or they might just be unable to twist into each other's shape due to certain bond restrictions. This type includes geometric isomers, which are like dancers frozen mid-twist; they can't switch places because of a bond that doesn't allow a spinning movement. It's the subtle changes in these arrangements that give isomers their own physical and chemical identities even though they come from the same family of molecules.

Picture a chain where each link is a pair of hands holding on to each other: that's an alkene molecule. The hands, in this case, represent carbon atoms, and when they hold each other tightly with both hands, that's the double bond of alkenes. Because they're using both hands, they can't twist and turn easily. This rigidity results in a set spatial configuration, which can lead to the existence of geometric isomers.

Imagine two people holding hands, standing back to back. Now, if they try to turn to face each other without letting go, they can't—it's physically impossible without breaking the hold. This is similar to how the double bonds in alkenes work. They're locked into place which allows the appearance of different geometric isomers, depending on the arrangement of other groups or atoms attached to these cenrtal carbons.

In the hydrocarbon family, alkanes and alkynes are like acrobats who can rotate 360 degrees around their ‘single’ and ‘triple’ bonds respectively. Alkanes, the simplest hydrocarbons, have all single bonds, similar to a series of rings loosely connected; they swing and twist freely. This is because their single-bonded carbon atoms behave like beads on a string—they can spin around and face any direction.

Alkynes are slightly different. They do have a triple bond, and you might think that makes them rigid. However, they still have enough freedom to rotate around the remaining bonds not involved in the triple guise. Think of them as a stiff rod that can still spin end-over-end. Such freedom means that they can't have geometric isomers because no matter how you turn them, they'll look the same; there's no 'this side' or 'that side,' making their appearance uniform from every angle.

Stereochemistry is essentially the study of the 3D performance of molecules. It’s like directing a play where even the slightest movement or position of an actor—or atom, in this case—can change the entire scene, or molecular interaction. Within stereochemistry, geometric isomers are two actors standing on the same stage yet never able to mimic each other’s position exactly when a double bond ties them down. It’s this fixed spatial arrangement that gives geometric isomers their unique properties, making one form possibly nontoxic and sweet-smelling, while the other might be quite the opposite.

In understanding the diverse personalities of molecules, stereochemistry helps us to explain why certain medications work, why some molecules twist light in a particular way, and how two substances with the same atoms can be so differently experienced in the world. Truly, stereochemistry is like the grand theater of molecules where angles and arrangements make all the difference.