Here's why:

* Elements: Mercury is a chemical element, meaning it's a fundamental substance that cannot be broken down into simpler substances by chemical means.

* Atomic Composition: All mercury atoms have the same atomic number (80), meaning they have the same number of protons. This makes mercury homogeneous, meaning it has the same composition throughout.

Therefore, mercury is classified as a pure substance, specifically an element, because it consists of only mercury atoms.