In an addition reaction, what occurs between the molecules?

In an addition reaction, two molecules combine to form a single new molecule, which is characterized by the addition of atoms or groups of atoms to the existing molecular structure without eliminating any atoms. This process generally involves the breaking of π bonds in unsaturated reactants, such as alkenes or alkynes, allowing the atoms of the other reacting molecule to be added.

For example, when hydrogen gas (H₂) reacts with ethene (C₂H₄), they combine to form ethane (C₂H₆). The double bond in ethene is broken, and hydrogen atoms are added to the carbon atoms, resulting in a single product.

The other options describe processes distinct from addition reactions. Two molecules forming two new molecules would imply a different type of reaction, such as a substitution or a rearrangement, where no molar combination is occurring. One molecule splitting into two products characterizes a decomposition reaction. Lastly, the exchange of parts between molecules refers to a double displacement reaction, where groups are traded but not added to form a single new entity. Thus, the defining characteristic of addition reactions is the formation of a single product from the combination of two starting molecules.