why doesnt the radius ratio rule work for TlCl3??? the division would be the radius of the cation / radius of the anion. For Tl3+ and Cl-...88pm/181pm = .49 which would correspond to a 'zinc blende' crystal structure and not a CsCl type crystal as the rest of the internet says...any explanations? does it have to do with covalence?
The radius ratio rule is a general guideline used to predict the crystal structure of ionic compounds based on the ratio of the radii of the cation and anion. According to this rule, if the ratio is between 0.225 and 0.414, the compound is expected to have a zinc blende crystal structure, whereas a ratio of 0.732 corresponds to the CsCl type crystal structure.
However, it's important to note that the radius ratio rule is a simplified model and does not take into account other factors that can influence crystal structure. In the case of TlCl3, the discrepancy with the predicted crystal structure can be attributed to factors such as covalence and coordination geometry.
In TlCl3, Tl is a transition metal ion with a partially filled d-orbital. This can result in covalent bonding between Tl and Cl atoms, leading to some sharing of electron density. Covalent bonding impacts the crystal structure and can disrupt the simple ionic lattice formation predicted by the radius ratio rule.
Furthermore, the coordination geometry of the Tl3+ ion plays a role in the crystal structure. In TlCl3, the Tl3+ ion adopts a distorted trigonal pyramidal geometry, with three chlorine ions surrounding the central Tl ion. This coordination environment is different from the simple cubic arrangement expected in a CsCl-type structure.
In summary, the failure of the radius ratio rule to predict the crystal structure of TlCl3 can be attributed to factors like covalent bonding and the coordination geometry of the Tl3+ ion. Crystal structures are complex and depend on various considerations beyond the simple radius ratio between cations and anions.