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Noble gases have larger radii than that of halogens.

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

Why is it like that? When we talk about radii of noble gases, what type of radius is referred? Is it van der Waals or atomic?

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2 Answers 2

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From Ref. 1:

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.

The answer lies in the way in which size measurements can be done:

enter image description here

i. Metallic/Covalent radius (left)

ii. Van der Waal radius (right)

Noble gases don't form (or don't easily form) bonds. Therefore their van der Waals radius is measured.

The rationale here is based on the fact that noble gases have very low chemical reactivity and their measured atomic radii are non-bonded. An explanation lies in the 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 minimize 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

  1. Atomic and Ionic radius

  2. Conceptual Chemistry

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  • $\begingroup$ Your last statement is righr that we can't compare 2 differently observed things....but just suppose IF we have a way to compare their ORIGINOL radius(not by bonding but the distance btw the outer electron and nucleus of an isolated atom) then also will the noble gas atom radius is more than previous halogen ? (Ignoring bonding style ) how much extend the outer shell of noble gas can expand due to inter electronic repulsion? $\endgroup$ Apr 10, 2022 at 12:26
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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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