# Sizes of atoms chart

1. Sep 20, 2007

### PH7SICS

The following link shows how to calculate the size of an atom, where can I find a chart where this has been done for all the elements in the periodic table? I know you can get charts of atomic radius but those are based on the distance between the nucleus of two bonded atoms and therefore the radius is slightly less then the actual radius. Basically I,m looking for a chart which most accurately displays the sizes of atoms.

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/atomsiz.html

2. Sep 20, 2007

### Astronuc

Staff Emeritus
3. Sep 20, 2007

### Gokul43201

Staff Emeritus
4. Sep 20, 2007

### tabchouri

Last edited: Sep 21, 2007
5. Sep 20, 2007

### DaveC426913

6. Sep 20, 2007

### Gokul43201

Staff Emeritus
The atomic sizes are derived from various bond lengths. The noble gases do not form bonds with other elements or among themselves (at least not very well) because of their inertness, making it very difficult to determine a covalent radius.

7. Sep 20, 2007

### cesiumfrog

Is a H2 molecule really that much smaller than, well, every other atom and diatom?

8. Sep 21, 2007

### DaveC426913

Ah. That's what I suspected.

So, these aren't really reps of the diameter of the outermost shell i.e. the "physical" size, they're reps of the operating diameter of the atom?

9. Sep 21, 2007

### Gokul43201

Staff Emeritus
Actually, the He-atom is smaller than the H-atom, as should be expected. Also, since atomic radii can be very crudely expected to go like some rapidly increasing function of the principal quantum number of the valence shell (the significant variation along a period is evidence for the crudeness), the relative sizes down a group show the largest variation at the top of the group. For simplicity, if the radius went like n2, then we'd expect R(H)/R(Li) ~ 1/4 (measured value 0.26) , while R(Na)/R(K) ~ 9/16 (measured value 0.8) and R(Rb)/R(Cs) ~ 25/36 (measured value 0.9).

10. Sep 21, 2007

### Gokul43201

Staff Emeritus
That depends on how well-developed your intuition is.

Going right along a period involves a small increase in the atomic number. Increasing the number of valence electrons might be expected to increase the atomic radius, but increasing the number of protons at the same rate should then be expected to cause a decrease in radius. So, to a first order in intuition, there should be no change in radius along a period.

A better intuition to use is that of the effective (screened) nuclear charge. Hopping by one atom to the right increases the number of protons and electrons by 1. The additional electron can not completely screen the charge of the additional proton, resulting in a net increase in the nuclear attraction to the outer electrons.

11. Sep 21, 2007

### PH7SICS

The data given my web elements varies considerably depending on how the atomic radius was derived. I don't know if any of those values given were obtained using the calculation in the link I provided. I notice that all the values have been rounded up so I'm guessing its not the most accurate chart. In any case dose anyone know which value is most accurate ie which method?

12. Sep 22, 2007

### cesiumfrog

Does that really explain why the chart you mentioned depicts H as only a point, compared to the other atoms depictions? I wondered if they were trying to represent the size of a hydrogen ion.

13. Sep 22, 2007

### Gokul43201

Staff Emeritus
I suspect that's either an artifact of the picture resolution or an error of carelessness. The H-atom ought to be only a little smaller than the F-atom (last ball in the second period). There would be no sense in representing the size of the H+ ion (a proton) in that chart.