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Noble gases have a larger radius than the halogens.

Sometimes it is greater than the radius of group I elements.

Why it is like that? When we talk about radius of noble gases what type of radius is referred? Is it Van der Walls or atomic?

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Unlike a ball, an atom doesn't have a fixed radius. The radius of atom can only be found by measuring the distance between the nuclei of two touching atoms and then halving that distance.

According to my findings, I think the reasoning lies in the way in which the measurements are done:

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i. Metallic/Covalent radius (left)

ii. Van der Waal radius (right)

It is believed that Noble gases don't form (or rather don't easily form bonds). This way only Van der Waals radius is measured.

  • The rationale here is based on the fact that noble gases have very low chemical reactivity and their atomic radii are non bonded.

  • Another explanation lies on electronic configuration of noble gases. They have completely filled outer orbitals with increased repulsion.

In addition, following Hund's rule, in a set of degenerate orbitals, spin-paired electrons exhibit more repulsion compared to singly occupying orbitals which have parallel spins (and consenquently less repulsion).

This also implies that increased electron repulsion between the completely-filled orbitals (as seen in noble gases which all have spin paired electronic configuration) has an effect of increasing "Van der Waals radius" as electrons tend to move as further away as possible to minimise repulsion.

However there is some disparity when comparing Noble gases radii with Group 1 elements (metallic radii) or Halogens (covalent radii) because you are comparing two different unrelated properties, nevertheless it makes sense to compare radii between different noble gases.

References

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Although its bit misleading to compare the van der waal radius of the noble gases which is mostly calculated for them but still ; Since the attraction between the noble gas molecules is the weakest i.e the van der waal attractions so this causes only a touch or gap between two molecules of gap and as you may know , this distance divided by 2 shall give you radius.

In group 1 ionic radius are measured which have strong bond but still the largest in the period ( except the noble gas) cause here we see " the nuclear attraction or the $Z_{effective} $ " increases on moving left to right. And hence atomic size decreases.

A bit more clarity about types of radius measurements would help : http://www.chemguide.co.uk/atoms/properties/atradius.html

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