Although each atom is made up of different-sized particles, the majority of the space, or volume, of each atom consists of its electron cloud. However, among the three molecules present, the neutrons are the largest and the atom's nucleus makes up 99.9 percent of its total mass.
The basic structure of any atom includes a nucleus made of protons and neutrons and a series of tiny electrons which orbit around it in a space called the electron cloud. The only exception to this structure is the hydrogen atom, which contains no neutrons.
Atoms, like the particles they form, contain more empty space than mass since the positive charge of the protons and the negative charge of the electrons repel one another. This creates what scientists term an "electron cloud" surrounding the nucleus of each atom. In terms of volume, the electron cloud constitutes the majority of the "space" of each atom. One common illustration of this space indicates that if an atom's nucleus were the size of a marble, then the outer edge of the electron cloud would be more than 100 yards, or one football field, away from it.
However, despite the fact that majority of the space of each atom contains its electrons, most of its mass, 99.9 percent, lies in the nucleus. The mass of an atom's nucleus is split nearly evenly between the protons and neutrons, though neutrons are slightly larger. An electron's mass, by contrast, is equal to 1/1836 of a proton.