An element is a substance that cannot be decomposed by electrolysis or thermal decomposition. It has all the features of the substance class plus this one further feature: under the methods that pull compounds apart, an element holds together.
There is no entity whose name is "element." The word names a class. Its members are particular substances (the iron of a nail, the copper of a wire, the gold in a ring, the oxygen recovered when water is broken down) whose feature in common is that the decomposition methods do not break them down. Apply the same electrolysis that takes water apart, apply the same thermal decomposition that breaks other compounds down, and the element does not yield. What you put in is what you get out.
An element has all the features of the substance class. Its property values are uniform throughout. Its phase_of_matter value is one. It is a single component material. From the outside there is no telling, just by looking, that an element is an element rather than a compound. The classification is not by appearance. It is by what happens, or fails to happen, under the decomposition test.
Macroscopic chemistry is bounded on the upper end of its hierarchy by the Earth System and at the lower end with chemical elements. That is where the descriptive frame ends. To say more about why the elements behave as they do, why some come apart and some do not, would be to leave the descriptive level for the explanatory. The element class is the floor of macroscopic chemistry. It is where the hierarchy of materials, built up from observation and tested by physical method, runs out of further questions to ask of itself.
Question
Macroscopic chemistry is bounded on its lower end by the chemical elements. Why does the hierarchy stop there?
Reveal Merrill's answer →Hide answer
Because the methods used to sort substances into compound and element classes (electrolysis and thermal decomposition) do not break elements down into anything simpler. The hierarchy has to stop somewhere; it stops at the level where the test for decomposition stops giving an answer. To go further would require a different kind of question and a different kind of method, the explanatory level rather than the descriptive. Macroscopic chemistry stays at the descriptive level. The element is its floor.