Question

In a helium there are 2electron and 2 nutron so that the shell shrinks . Due to which force and why it shrinks
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Answer 1

The Force responsible is Strong Nuclear force exerted on the outermost layer by the Nucleus.Hence Atom shrinks.
Helium has 2 protons in its nucleus, as opposed to Hydrogen's one. Therefore, Helium will exert a greater pull on its electrons, pulling them closer and leading to a smaller atom.helium nucleus is twice as big as that of the hydrogen nucleus. The larger charge leads to a smaller average size for the innermost s-orbital. There is some screening of the nuclear charge felt by one electron due to the presence of the other electron, but the closed K shell also adds a little stability.

This tendency of the innermost orbitals to shrink continues up the periodic table