The number of electrons needed to fill a valence shell and achieve stability varies depending on the period in the periodic table. The number of electrons required corresponds to the period number itself.
Here's a breakdown of the number of electrons needed to fill the valence shell for each period:
1st Period: The first period contains only two elements, hydrogen (H) and helium (He). Hydrogen requires one more electron to fill its valence shell, while helium already has a filled valence shell with two electrons.
2nd Period: The second period contains elements from lithium (Li) to neon (Ne). Elements in this period require two more electrons to fill their valence shells.
3rd Period: The third period contains elements from sodium (Na) to argon (Ar). Elements in this period require eight more electrons to fill their valence shells.
4th Period: The fourth period contains elements from potassium (K) to krypton (Kr). Elements in this period also require eight more electrons to fill their valence shells.
5th Period: The fifth period contains elements from rubidium (Rb) to xenon (Xe). Elements in this period require eight more electrons to fill their valence shells.
6th Period: The sixth period contains elements from cesium (Cs) to radon (Rn). Elements in this period require eight more electrons to fill their valence shells.
7th Period: The seventh period contains elements from francium (Fr) to oganesson (Og). Elements in this period also require eight more electrons to fill their valence shells.
It's important to note that the transition metals, which occupy the d-block in the periodic table, have a more complex electron configuration, and their valence shells are not always completely filled.