An S shell is a spherical shaped shell that can hold two electrons with opposite spin
Electrons are essential components of an element and decide its reactivity and bonding. They have their own unique properties known as quantum numbers which makes them all unique, existing in a number of sub orbitals around a nucleus.
Shielding is the measure o the effect of inner sub shells of the S P D and F on their interference of the nuclear charge of the protons on the valence electron.