why does the atomic size suddenly increase on reaching the 18th group even though we know that the atomic size decreases on moving across the group?

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Solution

It happens mainly because the there is a balance between the electron-nucleus attractive force and electron-electron repulsive force (which exists between electron of same orbital).
When orbitals are half filled the electron-nucleus force is more prominent making the atomic radii smaller.

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Similar questions

As we know that if size of atomic radii increases, I.E decreases. But why it is said in a random topic that I.E increases down the group?

Why with travelling from left to right in a period atomic size decreases but suddenly increases in group 18

The atomic size of an atom increases across the period and decreases down the group.

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