From a structure diagram alone it is hard to determine these interactions. The reason for this is that important structural parameters are stretched or shortened in order to flatten the molecule for a 2D drawing.
In a first order approximation a molecular modelling kit based on balls and sticks could already be very helpful. It'll let you approximate the 3D ...
Unfortunately, there is no caliper-like tool to measure ionic radii directly.
They are determined based on experimental data such as crystal structure or crystal lattice energy using empirical relationships.
The exact value would inevitably deviate from one method to another.
Pauling [1, pp. 150–152] used a hard-sphere model which doesn't account for ...
Well since the ion looks like the helium atom but without 1 proton and since the helium core is approximately 4 times heavier than the hydrogen core and if we neglect the magnetic interactions between core and electrons i would expect it to be 4 times the radius of helium = roughly 125 pm