Final answer:
SO3, or sulfur trioxide, is not an ionic compound as it consists of sulfur and oxygen atoms that are covalently bonded, unlike K2O, Al2O3, and CaO which are composed of metal and nonmetal ions bonded ionically.
Step-by-step explanation:
Among the options listed (a-K2O, b-Al2O3, c-CaO, d-SO3), the formula that is not an example of an ionic compound is d-SO3. This is because it represents sulfur trioxide, which is a covalent compound consisting of sulfur and oxygen atoms. Ionic compounds are formed when metals transfer electrons to nonmetals, and the resulting cations and anions attract each other due to opposite charges. On the other hand, covalent compounds form when nonmetals share electrons.
K2O consists of potassium ions (K+) and oxide ions (O2-), Al2O3 consists of aluminum ions (Al3+) and oxide ions (O2-), and CaO consists of calcium ions (Ca2+) and oxide ions (O2-), all of which are ionic compounds, as they are composed of metals and nonmetals. SO3, however, comprises only nonmetals and the atoms are bound by covalent bonds, not ionic.