How many different isomers can be derived from ethylene if two hydrogen atoms are replaced by a fluorine atom and a chlorine atom? Draw their structures and name them. Indicate which are structural isomers and which are geometric isomers.

The molecule ethylene, also known as ethene, is a simple but fundamental organic compound with the molecular formula \(C_2H_4\). Its structure consists of two carbon atoms double-bonded to each other, with each carbon atom also bonded to two hydrogen atoms. This configuration can be depicted as H₂C=CH₂.

The double bond is a key feature of ethylene's structure, providing the ability for the molecule to participate in chemical reactions such as addition reactions. This bond is comprised of one sigma (σ) bond and one pi (π) bond; the σ bond allows for free rotation, while the π bond restricts this movement, making the molecule planar. The pi bond's presence is vital because it offers a site for chemical reactivity, making ethylene an essential building block in the petrochemical industry.

Structural isomers are molecules that share the same molecular formula but differ in the way their atoms are connected to each other, or bonded. This variance gives rise to different properties and chemical behaviors. In our exercise, the substitution of two hydrogen atoms by a fluorine and a chlorine atom in the ethylene molecule generates two distinct structural isomers.

The first isomer has both the fluorine and chlorine substituted on the same carbon atom, forming FCH=CHCl. The other isomer shows the atoms attached to different carbon atoms, yielding H₂C(F)=CH(Cl). Despite having the same molecular formula (\(C_2H_2FCl\)), the different arrangements of atoms around the carbon double bond result in two unique compounds, each with its physical and chemical characteristics. This diversity is a testament to the rich variability of organic structures possible with a seemingly small change in the structural layout.

Unlike structural isomers, geometric isomers (also known as cis-trans isomers) occur due to the restricted rotation around a double bond or within a ring structure. This limitation leads to isomers with different spatial arrangements of the atoms or functional groups concerning a fixed element, yet maintaining the same sequence of atom connectivity.

In geometric isomerism, two forms are possible: the 'cis' isomer, where similar or the same substituent groups are on the same side of the double bond, and the 'trans' isomer, where these groups are on opposite sides. However, referring back to the exercise, geometric isomers do not arise in the given problem since the introduction of fluorine and chlorine does not produce a scenario with difference in spatial arrangement across the double bond, such as in situations with distinct substituents where both cis and trans configurations can exist. This is a critical concept in understanding the limitations and specific conditions under which geometric isomerism can take place.