Great question Carlos! Before we can answer it though, we need to understand how we even know there is iron in the core.

First, scientists use seismic waves to calculate how Earth's layers change in density and state (e.g., solid, liquid, gas). Through seismic waves, our understanding is that Earth has a dense, solid inner core, that must be made up of mostly iron. Surrounding the inner core is a liquid outer core. The pressure is slightly less, enabling the liquid state at those very high temperatures. The outer core is also made up of mostly iron. Earth's core is about two-thirds the size of the moon, about as hot as the surface of the Sun, and at a pressure so high the iron is forced into a solid state that cannot melt.

The second line of evidence is by looking at space debris called meteorites. Meteorites are solid debris that originates from outer space, survives its passage through Earth's atmosphere, and reaches Earth's surface. Chondrites are a special meteorite thought to represent space dust from the beginning of the solar system that has never been melted or differentiated in any way. Differentiation occurs due to density, heavy materials/elements separate away from the light materials/elements, creating a dense core and relatively light exterior. You can think of how salad dressing separates in the bottle (oil below and water floating above) as how elements differentiate.

By looking at the chemistry of chondrites, scientists are able to understand what elements existed in the formation of our solar system and when it formed. Sometimes that early space material does melt and differentiate, and the only piece that survives impact to Earth is a dense iron-rich meteorite. Scientists believe that meteorites smashing into each other are one way in which planets form, thus Earth would have formed from these same materials and behaved similarly.

The final piece of evidence comes from what we know about gravity and energy waves. Scientists are able to calculate Earth's density by using Earth's size and gravitational pull. From Earth's density, scientists can estimate what elements Earth must be composed of. An all iron core best matches Earth's mass. And like the differentiation of the meteorites, Earth's interior has been so hot for so long that scientists think the process of density separation of Earth's layers has reached equilibrium, or is mostly separated. Therefore, the core is likely to only contain a single element. This does not entirely rule out other elements in the core, and likely there are tiny amounts of other elements, like nickel, in the core. But through seismic waves, meteorites, and Earth's density, scientists are confident that the core is made up of mostly iron.