Question

The ionic radii of Ma+ and Xa- of MaXa ionic compounds are 0.102 nm and 0.181 nm, respectively. The ionic radii of Mb+ and Xb+ of MbXb ionic compounds are 0.061 nm and 0.175 nm, respectively. A student claimed that MaXa ionic compounds have a tetrahedral configuration with coordination number of 4 and MbXb ionic compounds have an octahedral configuration with coordination number of 6. Is this feasible? If so, state why it is feasible. If not, provide your reason why it is not possible.

Answer 1

One of the characteristics that determine the coordination number is the size of the ionic radii of the cations and anions , rC and rA respectively. Since metal elements provide electrons to be ionized , the cations are generally under anions thus the ratio rC / rA is less than unity.

This is a table representing the coordination number adopted by the cations as a function of radii ratio

# Coordination	Geometrical arrangement	rCation/rAnion
3	Triangular	0.155
4	tetrahedral	0.225
6	octahedral	0.414
8	cubic	0.732

To ratios rC/rA less than 0.155, the cation, which is very small, is bonded to two anions in a way linear. If rC/rA has a value between 0.155 and 0.225, the coordination number of the cation is 3 (triangle) This means that each cation is surrounded by three anions in the form of an equilateral triangle plane with the cation located in the center. in this way to other values.

For MaXa rC/rA= 0.102/0.181=0.564. This number is not within the range of a tetrahedron, is not feasible because the geometrical arrangement will be cubic

For MbXb rC/rA=0.061/0.175=0.348. This if possible because it is in the permitted range for an octahedron (between 0.225 and 0.414).