Final answer:
The two most abundant elements in the Earth's core are iron and nickel. The crust, rich in oxygen and silicon, contrasts with the denser core, which is around 85% iron, with most of the rest being nickel, based on density calculations and meteorite analysis.
Step-by-step explanation:
The two most abundant elements in the Earth's core are iron and nickel. This is due to the metallic nature of the core and the findings from studies of Earth's density and metallic meteorites.
The solid matter of the Earth's crust is composed of different elements than the core. While oxygen accounts for 46.1% of the Earth's crust and silicon for 28.2%, the core, which is the planet's center, is much denser, consisting mainly of metals. Scientific evidence suggests that the core is about 85% iron with nickel comprising much of the remaining 15%. This information comes from the understanding of the planet's overall density, based on its rotation, as well as the study of metallic meteorites, which are thought to represent the core's composition.
It's also worth noting that while hydrogen is the most abundant element in the universe, and significant in the composition of stars and substances on Earth, such as water, it does not play a major role in the composition of the Earth's core.