The 12 stands for the number of electrons that fill the f orbital
Further explanation
In an atom there are levels of energy in the shell and sub shell
This energy level is expressed in the form of electron configurations.
Writing electron configurations starts from the lowest to the highest sub-shell energy level. There are 4 sub-shells in the shell of an atom, namely s, p, d and f. The maximum number of electrons for each sub shell is
• s: 2 electrons
• p: 6 electrons
• d: 10 electrons and
• f: 14 electrons
4f¹² :
4 : number of shell
12 : the number of electrons that fill the f orbital
