The number of electrons required to make a shell stable depends on the shell number. The first shell can hold up to 2 electrons, the second shell can hold up to 8 electrons, the third shell can hold up to 18 electrons, and so on. In general, the nth shell can hold up to 2n^2 electrons.

The outermost shell of an atom is called the valence shell. The number of electrons in the valence shell determines the atom's chemical properties. Atoms with a full valence shell are stable and do not react easily with other atoms. Atoms with an incomplete valence shell are unstable and react easily with other atoms to fill their valence shell.

The noble gases are the elements that have a full valence shell. They are all very stable and do not react easily with other elements. The alkali metals are the elements that have one electron in their valence shell. They are all very reactive and react easily with other elements to lose their valence electron. The halogens are the elements that have seven electrons in their valence shell. They are all very reactive and react easily with other elements to gain one electron to fill their valence shell.