1)RuO2 is molecular, but why is it insulator even though Ru is metal?
2)Why is TiO ionic? I thought only group 1 and 2 would make ionic compound
3)If the the compound has metal and metalloid like GaAs, is it always semiconductor
For the same reason most oxides are not conductors. Metals conduct electricity because the electrons in metals are "a sea of electrons" that can move freely. When you make a compound, like an oxide (Al2O3, RuO2, CuO, Na2O, just name almost anything), those electrons are tied up.
2. Ti has an electronegativity of about 1.5 and that of oxygen is about 3.5; therefore, the difference is about 2 on a scale where 1.9 is about the 50-50 mark for covalent vs ionic compounds. Generally we all those compounds with a difference greater than 1.8 or 1.9 ionic although they would more correctly be called polar covalent. TiO2, however, is more ionic that it is covalent.
3. Those metals near the stair step between metals and non-metals on the periodic table are the semiconductors (As, Ge, Ga, Si, etc).posted by DrBob222