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Helium atom, Radon Atom and water molecule sizes and concrete?

Question asked by kwilkis on Nov 25, 2015
Latest reply on Nov 12, 2019 by Bart Bergsbaken

Helium atom, Radon Atom and water molecule sizes and concrete? I am not a chemist but am trying to understand atom size vs water molecule size. Labs use the Helium atom (Helium gas) for testing porosity and permeability of materials as it is the smallest of the single atom gases, with an atomic radius of 31pm, or diameter of 62pm. The Radon atom has an atomic radius of 120pm, or a diameter of 240pm. Water (H20) has two hydrogen atoms (atomic radius of 25pm) and one oxygen (atomic radius of 60). My question is.. what is the over all size of the water molecule, in comparison to the atomic Radon atom size, in pm ?


What I am trying to understand is, and this is from a report I read…

The helium porosity values obtained for these core samples range from 7.0-10.7 %,

and reflect the presence (& relative abundance) of 1) intercrystalline micropores that are

co-mingled with the cement paste constituents, and 2) air-entrapment macropores that are

irregularly scattered within the concrete framework. The air permeability analysis

evaluates the transmission efficiency of (dry Nitrogen) gas that is forced to flow through

the void system within the concrete core cylinders. Air permeability values for the core samples range between 0.208-0.406 millidarcies (md). The air permeability

measurements assess the relative interconnectivity of all void types contained within the

samples. The porosity & permeability values reported in Table I indicate that the pore

networks contained within the concrete samples are extremely poorly

interconnected. Gas permeability values for concrete samples generally exceed fluid

permeability equivalents by as much as an order of magnitude, indicating that the concrete structure is essentially impervious to the transmission of water in an

aqueous state.”


This makes sense, the inner pathways of the crystaline concrete are broken up and restricted by the formed crystalline to the point that the Helium atom is restricted in its movement through the pores of the concrete. Therefore as the water molecule is so much larger, the concrete core samples will not allow the passage of the larger water molecule through the concrete’s pour structure, making it impervious to water.


OK, so how big is the water molecule in comparison to the Radon Atom ? Can the Radon atom pass though this concrete’s internal porous pathway. Or is this concrete impervious to radon as well ?


The Radon atom has an atomic radius of 120pm, or a diameter of 240pm. If I add up the sizes of the atoms in a water molecule, 2 Hydrogen  (@25pm) and 1 Oxygen(@ 60pm), i get 110pm atomic radius, or 220 pm diameter. That would be if they are all in a line, which they are not. They are more configured into a ball. I am sure it cannot be that simple, but it would appear that the Radon atom is larger than the water molecule. Thus making the concrete impervious to both water and Radon gases ?


Is this a correct assumption ? You help is greatly appreciated. Thanks in advance.