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In a pair of geometric isomers of an alkene, what actually differs between the two molecules?
ATheir overall molecular formula and total atom count
BThe geometric orientation of groups on either side of the C=C double bond
CThe order in which the atoms are bonded to one another in the chain
DThe total number of electrons present in the molecule
Answer & Solution
Correct answer: B. The geometric orientation of groups on either side of the C=C double bond
1. Geometric isomers share identical connectivity -- the same atoms bonded to each other in the same order.
2. The text explains: "In a set of geometric isomers, the same types of atoms are attached to each other in the same order, but the geometries of the two molecules differ. Geometric isomers of alkenes differ in the orientation of the groups on either side of a C = C" double bond.
3. So the molecular formula, bonding order, and electron count are all identical between geometric isomers -- only the spatial arrangement around the rigid double bond differs.
4. This is why geometric (cis/trans) isomers can have different physical properties despite sharing the same formula.
_Source: OpenStax Chemistry Ch 20 "Organic Chemistry", p.1104 §20.1 Hydrocarbons_
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