[Floreaa]

Yesterday I was at a chemistry question and because of the following question that I answered wrongly, i wasn't the first bu the second.

the question was:

Which of the following elements has a smaller atomic radius:
a) Sodium
b) Magnesium
c) Fluorine
d) Neon

My answer was d because of the fact that the atomic radius decreases along the period, but they said that it is d) because of the fact that noble gases have the biggest atomic radius in their period. After a little bit of search on the internet I've found out that Ne has the radius equal to 72 and fluorine to 71. but some other sources mention the oposite. How could it actuallly be? 

[ARGOS++]

Dear Floreaa;

I can feel with you!, because different sources report absolutely different atomic radii for all of them!:
      http://en.wikipedia.org/wiki/Atomic_radius#Empirically_measured_atomic_radius 
      http://environmentalchemistry.com/yogi/periodic/atomicradius.html
(And I believe I used different sources to yours, too!)

But "all" I visited report Ne with the smallest radius of them.

I hope to have been of help to you.
Good Luck!
                    ARGOS⁺⁺

[Floreaa]

But "all" I visited report Ne with the smallest radius of them.

Dear ARGOS++,

When you said radius were you refering to atomic or covalent radius?

Dailychem.

[cliverlong]

But "all" I visited report Ne with the smallest radius of them.

Dear ARGOS++,

When you said radius were you refering to atomic or covalent radius?

Dailychem.

Isn't a problem here about defining what the radius of an atom is?

1) If we consider an isolated atom and we accept that the electrons are in fixed energy levels but that
(wiki orbital)" This (atomic orbital) function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus"

that is, we cannot point to a region or space where the atom ends and hence determine a radius

2) If the atoms are in a 3D ionic (or other crystalline) lattice we can use X-ray diffraction to determine accurately inter-ionic distance
(from wiki) X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and scatters into many different directions. From the angles and intensities of these scattered beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, "

Notice mean position. But is this atomic radius? No, because the ions have gained or lost electrons which affect their "radius"

3) I'm sure there is a technique to determine interatomic distances between atoms in covalent molecules but I don't have time to look it up.



so I will write it isn't possible to definitively write about atomic radius - but (and I don't know how you measure it) the wave-function probablistic "there isn't a definitive radius" approach is maybe the most sound

Have a look here

http://winter.group.shef.ac.uk/orbitron/AOs/1s/e-density.html

and click around for different electron density plots for different orbitals to try and visualise this idea of electron probability density.


Clive

[ARGOS++]

Dear Floreaa;

I don’t remember each individual of the ~10 references I visited exactly, so I can only tell that they used the term "atomic radius".   But maybe there were also some with "covalent atomic radius".

What I’m pretty sure is that they must have used different methods for estimating/calculating the radius, because the "rel. standard deviation" for the radius of Fluorine, but also for Ne, was something in the range 5% - 10%, what is a quite good indication for.

Similar you may see on the page:
    http://en.wikipedia.org/wiki/Covalent_radius_of_fluorine#Discovery_of_bond_length

Good Luck!
                    ARGOS⁺⁺