What is a characteristic feature of a pure element?

A pure element is defined as a substance that is composed solely of one type of atom. This means that each atom in the sample is the same in terms of its atomic number, which characterizes the element. For instance, a pure sample of gold will contain only gold atoms, while a sample of pure oxygen will consist only of oxygen atoms.

This characteristic is fundamental because it defines the essence of what an element is within the periodic table. The uniqueness of a pure element allows it to maintain specific physical and chemical properties that are consistent and predictable, such as boiling and melting points.

The other options do not correctly describe a pure element. For instance, the ability to be separated into simpler substances refers to compounds, which are made up of two or more different types of atoms. Similarly, molecules that contain different elements signify compounds rather than pure elements. Additionally, the physical state of an element (solid, liquid, or gas) can vary depending on environmental conditions; thus, it is not accurate to assert that a pure element always exists in solid form.