Here's why:

* Elements are pure substances that cannot be broken down into simpler substances by chemical means. Iron (Fe) is a single type of atom, and it's found on the periodic table.

* Compounds are formed when two or more different elements chemically combine in a fixed ratio.

* Mixtures are combinations of two or more substances where each substance retains its own properties.

Since a block of iron is made up of only iron atoms, it's an element.