Explanation:
Group 18 elements - Helium, Neon, Argon, Krypton, Xenon. They all are noble gases.
Atomic radius - The distance from the centre of the nucleus to the outermost shell is called the atomic radius.
The outermost shell of a helium atom is the K shell, which is an element of the first period. Similarly, neon in the L shell, Argon in the M shell, Krypton in the N, and so on.
As you can see in the attached image, K shell is close to the nucleus, hence the distance is shorter, and N shell is away from the nucleus, hence the distance is longer. That means the atomic radius of helium is smaller than that of Krypton and xenon, so we can say that on moving down the group, the atomic radius decreases.
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