The form an element usually has is distinct from the fact it is an element
The key for a substance to be described as an element is that it contains only one type of atom. But that leaves a variety of possible structures even for one element.
Some elements–noble gases–consist of isolated atoms, all others consist of molecules or structures containing multiple atoms of the element. Oxygen mostly appears as a diatomic gas under normal conditions but sometimes appears a a triatomic gas (ozone). Some elements are usually seen as more complex molecules. Sulfur has a variety of allotropes (that's what different structures of pure elements are called) many containing rings of 8 sulfur atoms but others are known. Carbon has an interesting range of structures. One is a ball-shaped molecule of 60 carbons, graphite is a soft compound consisting of planes of hexagons weakly held together, diamond is a network of interlinked carbon tetrahedra and is very strong.
Elements, in other words, can be found as a variety of structures and many single elements can be seen with more than one structure. Some usually appear as single atoms, some as small molecules, some as large molecules and some as network solids. Some elements can be seen in multiple forms. All those forms are still elements. Don't confuse the form with the fact the substance is an element.