What is the difference between the concepts of a chemical element and a simple substance?

A chemical element is a collection of atoms with the same nuclear charge and, accordingly, the same number of protons in the nucleus and electrons around the nucleus.

All chemical elements can be found in the periodic table.

Simple matter is a form of existence of chemical elements with the same nuclear charge. Quite simply, a simple substance consists of atoms of one chemical element. A simple substance is endowed with physical properties (shape, density, boiling point, mass, and many others). One and the same chemical element can create several simple substances. For example, the element carbon exists in nature in the form of graphite, diamond, etc., which differ greatly in their physical properties.