Step 1: Recall concept of voids.
Voids are empty spaces in close packing of spheres. Step 2: Tetrahedral void formation.
When three spheres form a triangle in one layer and a sphere from the next layer is placed above it, the resulting void is tetrahedral in shape. Step 3: Ratio.
Number of tetrahedral voids = 2 × number of spheres. Step 4: Conclusion.
Thus, tetrahedral voids are small empty spaces formed in close packing with tetrahedral geometry.