The nucleus of an atom is surrounded by particles called electrons. These electrons revolve around the nucleus in paths called orbitals, which vary in their complexity.
Orbitals, also known as shells, become more complicated and bigger the further they are from the nucleus. Only two electrons exist in any one shell, spinning in opposite directions. Scientists can use electron configurations to show the location of electrons in these shells, making it possible to predict various scientific properties of a given atom, including conductivity, stability and boiling point.
When making these electron configurations, the outermost shells are often the most important. For this reason, scientists often abbreviate these configurations using the noble gas that is closest in proximity up the periodic table. This implies the atom in question and the noble gas have the same electron configuration up to that point; the outermost electrons are then tacked onto the end.
All electrons surrounding an atomic nucleus in their respective shells are collectively known as the electron cloud. This cloud has a diameter 5,000 times that of the nucleus. Since electrons are extremely small and lightweight compared to protons and neutrons, the cloud that electrons take residency in is empty space for the most part.