2n² rule:
This rule is used to determine number of electrons in particular shell.
n=1 means K shell
n=2 means L shell
n=3 means M shell
n=4means N shell
The first K shell can hold upto 2 electrons, L shell can hold up to 8, third M shell can hold up to 18and the fourth N shell can hold upto 32 electrons. This rule of arrangement of electrons according to the shell is known 2n2 rule where n means number of shell.
For example: There is one proton in the nucleus of hydrogen atom, which moves in K shell path. It has no neutron.