The volume of an atom is predominantly made up of the electron cloud that surrounds the nucleus. Atoms consist of a central nucleus, which contains protons and neutrons, and this is surrounded by a cloud of electrons that are in constant motion.
While the nucleus is tiny and dense, the electrons occupy a much larger area around the nucleus. This electron cloud is defined by the areas where electrons are likely to be found, and it gives the atom its size. Essentially, even though the nucleus holds most of the atom’s mass, the sheer volume that the electron cloud occupies is significantly larger than that of the nucleus itself.
This difference in mass and volume is crucial to understanding atomic behavior. The properties of elements and their interactions with other elements are largely determined by the arrangement and energy levels of the electrons within this cloud. Thus, when we think about the size of an atom, we are really considering the space occupied by the electrons rather than the compact mass of the nucleus.