The size of atom decreases from left to right in a period ,but why the size of atom of nobel gases are more than that of the atom of the elements...

As we move from left to right in a period, the number of
shells of the atom of every element remains same, but the number of electrons in the
outermost shell and the number of protons goes on increasing. This causes an increases
in the nuclear force, which brings the elctrons slightly closer to the nucleus, thereby
reducing the atomic radius and size of the atom.

Now when
we reach the noble gases, their atoms contain 2 (Helium) or 8 (all others) electrons in
the outermost shell. Due to such a high amount of electrons in the outermost shell, the
electrons develop a repulsive force among each other. This repulsive force overcomes the
increased nuclear force, causing the electrons to move away from each other, effective
increasing the atomic radius and atomic size. Hence, due to the force of repulsion
between the valence electrons, the atomic size of atoms of noble gases is slightly
greater than the size of the atoms of the elements immediately left of it in the
periodic table.