21. Select two substances that have an atomic crystal lattice in the solid state. 1) SiO_(2) 5) Al 2) KBr 3) Al_(2)O_(3) 4) Ca Answer (score: 1):

The correct answer is 1) $SiO_{2}$ and 3) $Al_{2}O_{3}$.<br /><br />Atomic crystal lattice refers to the arrangement of atoms in a crystal where each atom is bonded to its neighboring atoms by strong covalent bonds. In this type of crystal lattice, the atoms are arranged in a regular repeating pattern, forming a three-dimensional structure.<br /><br />$SiO_{2}$ (silicon dioxide) and $Al_{2}O_{3}$ (aluminum oxide) are both examples of substances with an atomic crystal lattice in the solid state. In $SiO_{2}$, each silicon atom is covalently bonded to four oxygen atoms, forming a tetrahedral structure. In $Al_{2}O_{3}$, each aluminum atom is covalently bonded to three oxygen atoms, forming a trigonal planar structure.<br /><br />On the other hand, KBr (potassium bromide) and Ca (calcium) do not have an atomic crystal lattice in the solid state. KBr is an ionic compound, where potassium and bromine atoms are held together by strong electrostatic forces of attraction. Ca is a metallic element, where the atoms are arranged in a metallic crystal lattice.