[Anaelisa]

Hello,

I want to calculate the cationic field strength of the Ta⁵⁺, from Ta₂O₅.
I know that the formula is Zi/(rR + rO)^2, where Zi= charge of cation, rR= radius of cation and rO= radius of anion.

I assume that the charge Zi= 2, but how could I find rR and rO?
Thank you!

[Enthalpy]

Welcome, Anaelisa!

You have written Ta⁵⁺, a credible charge in Ta₂O₅.

rR + rO is more easily accessible than each term: does it suggest a distance to you? You can deduce that sum from the crystal lattice, or from some related dimension, or if needed (lengthy hence less safe) from the crystal shape and the density.