atomic radii vs atomic volume

why doesnt the atomic radii and the atomic volume follow the same trend.

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atomic volume would be calculated from the atomic radii.
V=(pi)r[sup]2[/sup]

Do you perhaps mean ionic radii, molar volume etc?

Or could it be that (individual) p-orbitals, for example, have a maximum radius, but not being spherical, they will have a smaller volume.

But all 3 p-orbitals kinda add up to a sphere, so it makes no difference?

I've never been asked such a question..

in my chemistry book there is a chart for atomic radii and atomic volume but they dont correspond to each other and im trying to figure out why. ive found several websites that have the same info my book does. i found a formula that is supposed to give you the atomic volume and it is
V[sub]M[sub]=A/p
so if you could help further explain it it would be much appreciated

i guess it would help if i would put what the letters mean
Vm=average volume per 103*NO of atoms in the structure, where ‘NO’ is Avogadro's number (6.022 x 1023/mol)
A= atomic mass in kg/kmol
p= density kg/m3
so i hope this helps clarify a little

If you work through just the units V[sub]m[/sub]= A/p , your units will be m[sup]3[/sup]/kmols which is molar volume (read the unit: how many cubic meters per mole).

The red lights started going off when I saw density. For example, what happens when you calculate V[sub]m[/sub] for diamond and then compare to that of graphite? Its the same element (Carbon) so A is 12, but the density changes between the two (graphite is approx. 2g/cm^3 and diamond is approx. 3g/cm^3). Why? because even though these materials have the same atoms, the bonding nature in the compound is different which is related to the radii of the atoms and how "stuffed into the box" they are.

So in other words, the atomic volume figure you see in your text book, is not related to the isolated single atomic radius as the graph of "atomic radius" is. The atoms change when you bring them together into a compound, and this follows no specific trend across any given period. There are trends down a group, because groups of atoms typically have the same crystal structure. On the left hand side on the link below, you can click on the groups and see these trends.

http://www.webelements.com/webelements/scholar/properties/definitions/molar-volume.html that is a quick concise explanation I found on this site. Here is a clip of the important parts.

Notes
The molar volume is also known as the atomic volume.
.....
The molar volume depends upon density, phase, allotrope, and temperature. Values here are given, where possible, for the solid at 298 K.

that formula was not in my book but on the internet and in my books they dont give a formula for atomic volume... the charts given in the book for atomic radii and atomic volume do not correspond but have different trends i am using abeka books if that helps any i do see what u r saying about the molar mass though but when u figure the atomic volume for a element it does not follow the same trends as the atomic radii does and i cant figure out why
the equation you gave V=(pi)r2 is based on the assumption the atoms are all perfectly spherical and if the equation is the one they used in the book then they really screwed up because based on the equation they should follow the same trend

Volume is based on a bulk effect of many atoms and has to do with how well the atoms pack together. Helium has a small radius, but I would expect it to have a large volume. Have you ever seen solid helium? It doesn't pack well.

Your book, in my opinion, mis-names the "Molar volume" to atomic volume. It gives the wrong impression on what this intrinsic volume represents. Its not the volume of 1 atom, its the volume of a collection of atoms which are interacting of sorts. Each collumn (Groups on the periodic table) of atoms "interacts" differently depending on how they bond in the solid state (which would then depend on the valency). How the atoms interact determines how dense the final product will be. Which, in the equation you gave, is a variable.

Density does not follow a trend, therefore, there should not be any general trend.

The density is dependant upon:
1) the radius of the atom (where the "ionic radius" becomes usefull, not the atomic radius)
2) the crystal structure

Knowing these two things, I can calculate a density/molar volume of anything. The crystal structure is more important of the two, because it really follows no specific trend. And to add to the headache, elements have allotropes such as Carbon (graphite, diamond, fullerenes).

So to compare the atomic radius with molar volume across the PT, is like comparing apples to burritos.

WebElements: the periodic table on the WWW [http://www.webelements.com/]

Copyright 1993-20010 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.