A superscript refers to the number of electrons within a subshell. So, a configuration of 1s2 means there are two electrons in the s subshell of the first major energy level. Iron atoms have an electron configuration of 1s22s22p63s23p63d64s2.
In magnetite, Fe3O4, some of the electrons from the iron have been donated to oxygen to form oxide, O2-. To balance the eight electrons received by the four oxygens in Fe3O4, one of the iron atoms has given up two electrons, to form Fe2+ or Fe(II), while the other two iron atoms have each given up three electrons to form Fe3+ or Fe(III). Fe(II) loses its 4s electrons and has a configuration of 1s22s22p63s23p63d6, while Fe(III) loses an additional 3d electron and has a configuration of 1s22s22p63s23p63d5.
Each subshell is further divided into orbitals, which are regions of space in which electrons are most likely to be found. Any orbital can hold a maximum of two electrons. The d subshell contains five orbitals and can hold a maximum of 10 electrons.
Within an orbital, electrons create tiny magnetic fields as if they’re spinning. One electron creates a magnetic field oriented in one direction, and the other electron’s magnetic field is oriented in the opposite direction. The two opposite directions, called spins, are denoted by an upward or downward arrow.
According to Hund’s rule, each orbital within a subshell must be filled by one electron before a second electron can be added. Since electrons are negatively charged, they repel one another, so it is more energetically favorable for an electron to occupy its own space than for two electrons to crowd into a single orbital.
An orbital-filling diagram depicts the arrangement of electrons within their orbitals.
For example, the diagram below shows electrons’ orientations and distribution in iron’s d-subshell in an Fe(II) ion: