write a note on radius ratio rule
Answers
Explanation:
In condensed matter physics and inorganic chemistry the cation-anion radius ratio (also: radius ratio rule [1]) is the ratio of the ionic radius of the cation to the ionic radius of the anion in a cation-anion compound. This is simply given by {\displaystyle r_{C}/r_{A}}.
According to Pauling's rules for crystal structures, the allowed size of the cation for a given structure is determined by the critical radius ratio.[2] If the cation is too small, then it will attract the anions into each other and they will collide hence the compound will be unstable due to anion-anion repulsion; this occurs when the radius ratio drops below 0.155.
At the stability limit the cation is touching all the anions and the anions are just touching at their edges (radius ratio = 0.155). For radius ratios greater than 0.155, the compound may be stable.
All the details of the Radius Ratio (RR) rule are as follows:
- The "radius of cation" to the "radius of anion" gives the Radius Ratio.
- The rule is that the larger the size of a cation, the larger its radius ratio and the larger the "ligancy" in the case of a crystal lattice.
- It refers to the relation between the radius, coordination number, and structural arrangement of a molecule.
- Anions are generally larger than that of a cation in size.
- So, in crystal lattice Anions take the main area, while cations fill up the void.
- Thus to check if an ionic crystal structure is stable, the RR is determined, so that it can tell whether an ion can retain a cation within it.
- Radius ratio can be represented as Radius ratio = r⁺/r⁻; Where r⁺ = radius of a cation and r⁻ = radius of an anion.
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