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Looking at covalent radii I know the trend states that as you move from LEFT to RIGHT the radii decreases due to increased nuclear charge. Up until you hit the noble gasses like neon. Neon does not want to form a covalent bond with itself so we have to look at the van der Waal forces to determine its radii (which is bigger than Fluorine, disproving the trend).

Up until you reach Krypton, the noble gasses tend to be slightly larger than the halogen group.

I am having a hard time understanding why the noble gas Kr does follow the general trend for covalent radii in the fourth period, even though the noble gas Ne does not.

Would it be because of the greater shielding effect?


Covalent Radii of 2nd Period and 4th Period Halogen and Noble Gase.

F - 0.064 nm

Ne - 0.070 nm

Br - 0.114 nm

Kr - 0.109 nm

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  • $\begingroup$ Could you cite a source containing the values? $\endgroup$ – DHMO Oct 18 '16 at 10:13
  • $\begingroup$ Added covalent radii data. $\endgroup$ – chemguy2012 Oct 18 '16 at 12:58
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Since the increased atomic radius causes a decrease in the electric interaction between electrons and nucleus, the heavier noble gas elements, such as Kr and Xe can form compounds with other elements. (KrF2, XeF4, ... &c.)

The covalent radius is quite meaningless in for elements such as Ar and Ne since its compounds seldom exist, but Kr and Xe compounds can occur with relatively high stability, so its 'covalent radius' can be meaningful.

That 0.109nm is probably a covalent radius.

Something like measuring a Kr-F bold length in KrF2 and then subtracting the covalent radius of a fluorine atom.

Trivially, the Van der Waals radius tends to be bigger than the covalent one, the noble gas atom radius seems as if it does not follow the trend, but from Krypton which the covalent radius is measurable, the data follows the general trend in periodic table.

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