I was thought that 1 mole of any gas contains 22.4litres of volume.but I am confused that how hydrogen and radon whose atomic size differ so much can
Have same 6.022*1023 atoms in 22.4litres only.Radon is big ,so the no. of radon atoms in 22.4 litres must be less than as compared to no. of hydrogen atoms. Please clear my doubt

One mole of gas does indeed occupy 22.4 L of volume at standard temperature and pressure. As you note a gas like radon has molecules that are bigger than hydrogen, which means they are heavier as well. So a mole of radon gas weighs more than a mole of hydrogen, but because they occupy the same volume the density of the radon gas is much higher.

As for how radon and hydrogen could occupy the same volume: keep in mind that the vast majority of the 22.4L is empty space - it's not like the molecules are crushed together at all. In fact the molecules of one mole of hydrogen gas occupy about 0.8 cubic centimeter of volume (that's the density of liquid hydrogen) - so that means the molecules of a mole of hydrogen gas actually occupy only about 0.004% of the 22.4L. So there;'s plenty of room for both large and small molecules to fit into 22.4L.