# Ion Hydration

Gold Member
Hi All,

What are the average distance of a ion and a water molecule when it is hydrated.

1/ for sodium?
2/ potassium?

Thanks.

Borek
Mentor
What's the context? Are you asked to estimate it? Or you just need the value?

Gold Member
I just need average values for sodium and potassium.
Contexts:
1/ with 1 water molecule until 4 water molecules (see picture for a sodium example).

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Borek
Mentor
Google "ionic radii in water". Just beware, as some of those listed can already take hydration into account.

Gold Member
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Borek
Mentor
I don't have access, so can't tell you much. But my guess is that it should at least list sources of the radii used.

Gold Member
Click on figures and tables. There are many values.

Fig. 5. (drawn to scale) The oxygen (O) and two hydrogen (H) atoms of water are shown as circles with covalent radii, d(O) and d(H). The circles on the right represent ions with aqueous ionic radii, d(i), aq = kd(A). The length of the hydration bond, d(–O) is the distance between the center of O(water) and P(i/w) (shown as dotted lines in Figs. I–V): (I) Li+, cdots, three dots, centered, Cs+: k = 0.81 = phi/2, d(–O) = 0.64 (±) 0.03 Å. d(i), aq (in Å): 1.42 (Li+) < 1.65 (Ag+ circle with dashes) < 1.74 (Na+) < 2.16 (K+) < 2.26 (Rb+) < 2.49 (Cs+). (II) F−, cdots, three dots, centered, I−: k = 1.57 = 2/phi1/2, d(–O) = 1.60 (±) 0.04 Å. d(i), aq (in Å): 1.11 (F−) < 1.48 (Cl−) < 1.74 (Br−) < 2.08 (I−). The ‘hydrogen bond’ length d(–H) = d(–O) − d(OH) = 0.63 Å. (III): Ni+2, cdots, three dots, centered, Ba+2: k = 0.80 not, vert, similar phi/2 = 0.81, d(–O) = 1.06 (±) 0.08 Å. d(i), aq (in Å) (only some are shown to avoid overcrowding): 0.98 (Ni+2) < 1.19 (Hg+2) < 1.32 (Mg+2) < 2.06 (Ba+2). (IV): Al+3, cdots, three dots, centered, Eu+3: k = 0.47 = (2/phi3), d(–O) = 1.40 (±) 0.06 Å. d(i), aq (in Å) (only some are shown): 0.55 (Al+3) < 0.63 (Rh+3) < 0.81 (Tl+3) < 0.91 (Yb+3, dashed line) < 1.08 (Eu+3, dash and dot line). (V) Lu+3, cdots, three dots, centered, La+3: k = 0.62 = phi/2, d(–O) = 1.37 (±) 0.05 Å. d(i), aq in increasing order (in Å) (only some are shown): 0.97 (Lu+3) < 1.01 (Y+3, dashed line) < 1.13 (Sm+3) < 1.17 (La+3).

Borek
Mentor
I can buy it as well, but I won't buy it just to spend money :tongue2:

Gold Member
I think the responses are in my previous message.

alxm
No, that's a silly (to the extent of bordering on crankish) paper.

And your question makes no sense. These ions do not bind consistently to any particular number of water molecules in liquid water, and the coordination structure is dependent on the coordination number. The K+-H2O distance can be anywhere between 2.5 and 3.5 Å, and that's dependent on temperature, etc. See, e.g. http://jcp.aip.org/resource/1/jcpsa6/v132/i12/p124503_s1" [Broken]. (Which, unlike the other, isn't indulging in numerology.)

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alxm
What's your point? That you know how to use google scholar/chemfinder/isi? I do too.

So if you've been reading these papers, why are you under the impression that hydration of an ion looks like the picture you posted?

Gold Member
1/ I was just asking for the relevance/quality of the papers.
2/ The picture is very simplified. It is flat but it is very convenient to draw that way. It seems that the distances from oxygen to a ion is a bit farer than my trial.

Gold Member
It is said that distance from Oxygen atom and potassium atom is around 2.5, 2.65 Angstroms.
Is it possible to find some example where this distance is far less and around 1.5 Angstrom?