The 2-8-8 rule you mentioned is a simplified representation used to illustrate the distribution of electrons in the valence shell for the first three rows (periods) of the periodic table. It is based on the observation that the first two rows have a maximum of 2 and 8 valence electrons, respectively, and the third row can accommodate up to 8 valence electrons in its valence shell as well.
However, it's important to note that the 2-8-8 rule is an oversimplification and doesn't accurately represent the electron configurations of elements beyond the second period. The electron configuration of an atom is determined by the Aufbau principle, which takes into account the filling of orbitals according to their increasing energy levels.
In reality, the third shell (n=3) can accommodate a maximum of 18 electrons, not just 8. This is because the third shell contains three subshells: the s, p, and d subshells. The s subshell can hold a maximum of 2 electrons, the p subshell can hold a maximum of 6 electrons, and the d subshell can hold a maximum of 10 electrons. When you add up these capacities, you get a total of 18 electrons for the third shell.
So, the 2-8-8 rule is a simplified pattern applicable only to the first three periods of the periodic table, and it doesn't hold true for all elements and electron configurations.