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).
1) To understand why RuO2 is an insulator even though Ru is a metal, we need to consider the structure and bonding in the compound. In RuO2, the oxygen atoms (O) form covalent bonds with the ruthenium atoms (Ru). These covalent bonds involve the sharing of electrons between the atoms. However, the way the atoms are arranged in the crystal lattice of RuO2 affects its conductivity.
In the crystal lattice, the arrangement of the atoms forms a network of covalent bonds that overlaps to some extent. This overlapping creates an extended system of electrons known as a "band." In conducting materials, such as metals, this band is partially filled with electrons, allowing for the flow of electric current.
In the case of RuO2, the arrangement of the atoms and their associated electron orbitals results in a completely filled valence band and an empty conduction band, with a large energy gap between them. This large energy gap makes it difficult for electrons to move from the valence band to the conduction band, and thus, it behaves as an insulator.
So even though ruthenium (Ru) itself is a metal, the specific arrangement of the atoms and their bonding in RuO2 result in insulating properties.
2) TiO (titanium oxide) is considered ionic due to the difference in electronegativity between titanium (Ti) and oxygen (O). Ionic bonding occurs when there is a large difference in electronegativity between the two elements involved in the bond.
While it is true that elements in groups 1 and 2 are more likely to form ionic compounds due to their low electronegativity, other elements can also form ionic compounds if there is a significant electronegativity difference with the other element.
In the case of TiO, the electronegativity of oxygen is higher than that of titanium. This leads to a transfer of electrons from the titanium atom to the oxygen atom, forming positive titanium ions (Ti^4+) and negative oxide ions (O^2-). The resulting electrostatic attraction between these oppositely charged ions creates an ionic bond in TiO.
3) Compounds that have a combination of a metal and a metalloid, such as GaAs (gallium arsenide), can have semiconductor properties, although it is not a strict rule. The semiconducting properties of such compounds depend on their crystal structures and the specific arrangement of the atoms.
In the case of GaAs, gallium is a metal, and arsenic is a metalloid. GaAs forms a crystal lattice where gallium and arsenic atoms are arranged in an ordered manner. This arrangement and bonding give rise to unique electronic properties.
GaAs is a semiconductor because it has a small energy gap between the valence and conduction bands, making it easier for electrons to transition from one band to another. This allows GaAs to conduct electricity under certain conditions.
However, it is important to note that not all compounds with a combination of metal and metalloid exhibit semiconducting properties. Factors such as the bonding nature, crystal structure, and arrangement of atoms play a crucial role in determining the electronic properties of a compound.