Final answer:
The heavier elements are the ones found in smaller quantities in the universe, as hydrogen and helium make up the vast majority. Heavier elements formed inside stars and are less common due to the complex processes required for their formation.
Step-by-step explanation:
The elements found in the smallest quantities in the universe are the heavier elements. Hydrogen, by contrast, is the most abundant element, making up about 70-75% of the universe by mass. It is followed by helium, which accounts for nearly all of the remaining mass. These two elements were the first to form in the universe and remain the most plentiful. Heavier elements, which include oxygen, carbon, and iron, among others, were formed later inside stars through nuclear fusion processes. Because of this later development and their complex formation requirements, heavier elements are much less common.
Hydrogen and helium are rare in Earth's atmosphere because their low molecular mass allows them to reach escape velocity and be lost to space over billions of years. On Earth's crust, elements like oxygen and silicon are much more abundant than hydrogen due to their presence in compounds and minerals.