Draw the expanded structural formula for each of the following organic compounds: a) Propene, b) 2-Methylbutane, c) But-1-yne

Geometric isomerism, also known as cis-trans isomerism or E/Z isomerism, is a type of stereoisomerism where molecules have the same molecular formula and connectivity but differ in the spatial arrangement of atoms due to restricted rotation around a bond. This typically occurs around a carbon-carbon double bond, as seen in your image. For geometric isomers to exist, each carbon atom of the double bond must be attached to two different groups.

If the two identical or similar groups are on the same side of the double bond, it's a cis isomer; if they are on opposite sides, it's a trans isomer. For example, in 2-butene, the two methyl groups can be arranged as follows:

cis-2-butene:

$$\begin{array}{cc} \mathrm{CH}_{3} & \mathrm{CH}_{3} \\ \backslash & / \\ \mathrm{C} & = \\ / & \backslash \\ \mathrm{H} & \mathrm{H} \end{array}$$

trans-2-butene:

$$\begin{array}{cc} \mathrm{CH}_{3} & \mathrm{H} \\ \backslash & / \\ \mathrm{C} & = \\ / & \backslash \\ \mathrm{H} & \mathrm{CH}_{3} \end{array}$$

The visible part of your molecule shows a carbon in a double bond with an H and a $\mathrm{CH}_{3}$ group, which are different, fulfilling one of the conditions for geometric isomerism.