The ionic substance strontium oxide, SrO, forms from the reaction of strontium metal with molecular oxygen. The arrangement of the ions in solid SrO is analogous to that in solid NaCl:
(b) Based on the ionic radii in Figure 7.8, predict the length of the side of the cube in the figure (the distance from the center of an atom at one corner to the center of an atom at a neighboring corner).

Key Concepts

Ionic Compounds and Crystal Lattice Structure

Ionic compounds, like strontium oxide (SrO), consist of positively and negatively charged ions arranged in a regular, repeating pattern known as a crystal lattice. This structure maximizes the attractive forces between oppositely charged ions while minimizing repulsive forces, resulting in a stable solid. The arrangement of ions in SrO is similar to that in NaCl, where each ion is surrounded by ions of opposite charge.

Ionic Radii

Ionic radii refer to the effective size of an ion in a crystal lattice, which can vary based on the ion's charge and coordination number. For cations, the radius typically decreases with increasing positive charge due to greater nuclear attraction, while for anions, the radius increases with negative charge. Understanding ionic radii is crucial for predicting the dimensions of the unit cell in ionic compounds.

Unit Cell and Edge Length

The unit cell is the smallest repeating unit in a crystal lattice that reflects the overall symmetry and structure of the entire crystal. In the case of ionic compounds like SrO, the edge length of the cube can be determined by the sum of the ionic radii of the cation and anion. This distance represents the length from the center of one ion to the center of a neighboring ion, which is essential for calculating the dimensions of the crystal structure.