Final answer:
The radius ratio, a concept in chemistry concerning the sizes of ions in ionic compounds, is almost always less than one as cations are usually smaller than anions.
Step-by-step explanation:
In the context of ionic compounds, the radius ratio is a comparison of the sizes of the cations (positive ions) to the anions (negative ions). The radius ratio is almost always less than one, which means that cations are typically smaller than anions. When forming ionic compounds, the size of the ions determines the type of crystal lattice that can form, impacting the compound's structural properties. Thus, the radius ratio is a fundamental concept in understanding the structure of ionic compounds in chemistry.