The phrase "lighter elements" refers to the chemical building blocks that are less heavy or have a smaller atomic weight compared to other elements. Full definition
The inner core, although composed almost entirely of pure iron, is thought to contain traces of lighter elements, primarily oxygen and silicon. (discovermagazine.com)
The new F - 150 uses LED lighting elements in three key areas: for the headlights, in the bed, and in the side mirrors. (automobilemag.com)
But the ancient star shows signs of pollution with lighter elements such as carbon and magnesium, and no sign of pollution with iron. (sciencedaily.com)