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I say that xenon radius is smaller comparing to beryllium, but my friend says otherwise.

What I think is that xenon is down in periodic table comparing to beryllium (3 periods down) and it's far toward the right (16 groups ahead). So, the nuclear pull dominate (16 by 3 which is very big) and therefore xenon's radius is smaller than beryllium's.

My friends said that xenon is down in periodic table compared to beryllium and hence it's bigger and the nuclear pull effect has only little effect.

So who is right ?

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You cannot really effectively compare the atomic radii of xenon and beryllium empirically; the atoms are too far apart. Once you remove yourself from direct neighbours, it is generally not possible to estimate relative sizes of atoms.

The closest hint you can get are calculated atomic radii on Wikipedia’s data page. These show that xenon is slightly smaller ($\pu{108 pm}$ to $\pu{112pm}$) but the difference is smaller than the expected error. So, for all intents and purposes they have the same size.

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  • $\begingroup$ How about if they are not far apart, such as Beryllium with Sulfur or Beryllium with Phosphorus ? Magnesium with Yttrium or Magnesium with Niobium ? $\endgroup$ – Jason C Sep 2 '17 at 6:54
  • $\begingroup$ @JasonC Unless they are directly above/below/left/right of each other or top-right/bottom-left (in all cases of which you can draw a series of linear larger or smaller relationships), they are too far apart. $\endgroup$ – Jan Sep 2 '17 at 7:00
  • $\begingroup$ @Jan The radius given for Xenon is Van Der Waal radius and the radius given for Be is atomic radius. Will it be fair to say Xenon is slightly smaller based on this data? $\endgroup$ – Arishta Sep 2 '17 at 9:41
  • $\begingroup$ @Eloise No, it's the calculated atomic radius in both cases. en.wikipedia.org/wiki/… $\endgroup$ – aventurin Sep 2 '17 at 13:37
  • $\begingroup$ I have been taught that it is not possible to measure atomic radius for noble gases. To measure the atomic radius of Xe, we need a compound in which Xe-Xe bond, which does not exist. Then how is this atomic radius of Xe even calculated? $\endgroup$ – Arishta Sep 2 '17 at 14:05
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If you want to compare the radius of 2 elements, you can usually do it when the elements are either on the same column (group), or the same row (period).

When you move down the column, the element becomes bigger, because it has more energy shells. For example, Oxygen has a smaller radius than Sulfur.

When you move right the row, the element has larger nuclear charge (more protons) for the same number of energy shells. Therefore, each electron is pulled stronger and the radius decreases. For example, Fluorine has a smaller radius than Oxygen.

Now, your elements, Berylium and Xenon, are not in the same column nor row. Nevertheless, Xenon is much lower than Berylium so we might assume that it has a larger radius.

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  • $\begingroup$ So in trend of atomic radius by group or period, if we comparing with two atom with different colomn and period, we should compare both by how low they are ? For instance, Magnesium and Scandium. Since Scandium is lower than Magnesium by 1 period, hence Scandium has bigger radius. Since Sulfur has lower period than Berylium, hence Sulfur has bigger atom radius. Am i correct ? $\endgroup$ – Jason C Sep 2 '17 at 6:22
  • $\begingroup$ No. For example, Strontium has a radius of 200 picometer, and Potassium has a radius of 220 picometer, even though they are in different columns and different rows, and Potassium is in a higher row (values from "Atomic Radius", Wikipedia). However, when there is a very big difference we can say. $\endgroup$ – Unknown Sep 2 '17 at 6:28
  • $\begingroup$ So Berylium is bigger than Sulfur because Sulfur is lower than Berylium by 1 period and 4 groups ahead, so nuclear pull dominate. But you said earlier that Xenon is bigger than Berylium. So what should i do if given two element with different group and period. (I'm still learning with trend of group and period, so maybe ignore the numerical value). Also what do you mean by "big difference"? Big difference on period ? $\endgroup$ – Jason C Sep 2 '17 at 6:36
  • $\begingroup$ I assumed that Xenon has a larger radius. However, I couldn't find online a measured atomic radius of Xenon. Only a calculated value. And according to the calculated one, Beyllium has a larger radius (112pm) than Xenon (108pm). This shows exactly my point. We cannot compare them. We can only guess. And I was wrong. There is no real value for "big difference", only speculation. $\endgroup$ – Unknown Sep 2 '17 at 6:52

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