How does hydrogen addition change an alkene into an alkane?

The conversion of an alkene into an alkane through the addition of molecular hydrogen is a process known as hydrogenation.

Alkenes are characterized by a carbon-carbon double bond, which includes a reactive pi ($\pi$) bond. Because this $\pi$ bond is a region of high electron density, it can attract electron-deficient atoms, leading to a chemical addition reaction. During hydrogenation, molecular hydrogen ($H_2$) is added across this double bond, typically in the presence of a metal catalyst such as nickel, platinum, or palladium.123

This reaction is energetically favorable because it breaks the weaker $\pi$ bond and the hydrogen-hydrogen $\sigma$ bond to form two stronger carbon-hydrogen $\sigma$ bonds. As a result, the molecule becomes "saturated"—meaning it contains the maximum number of attached groups and only $\sigma$ bonds—thereby transforming the unsaturated alkene into a saturated alkane.43