Here's a breakdown:

* Outer core: This liquid layer is mostly iron (88%) with a smaller percentage of nickel (5.5%). It also contains trace amounts of other elements like sulfur, silicon, and oxygen.

* Inner core: This solid sphere is mostly iron (around 90%), with nickel comprising the remaining percentage. While there are trace amounts of other elements, the exact composition is still a subject of scientific study.

It's important to note that:

* The exact composition of the core is still being investigated.

* The pressure and temperature conditions within the core are extreme, unlike any we can recreate on Earth. This makes directly studying the core difficult.

* Scientists use seismic waves, magnetic field data, and laboratory experiments to infer the composition and properties of the Earth's core.