An element is is something, a substance, that has one and only one type of atom. All substances have atoms but many have a variety of types of at least two types. An element is unique in that it has only one kind.
oxygen is an element. It has only one type of atom. Atoms are the smallest part of an element. it takes millions of atoms to make up even a very small element of half a millimeter in size. Elements are part of the composition of a substance and they contain protons, electrons, and neutrons within themselves.
There ae some elements that have atoms that refuse to join with one another...such as helium. Different characteristics are defined when elements possess atoms that join or don't join.