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    what is the differences in the physical properties of SiO2 and SiO4?

    Silicon's in the same group as Carbon, so it has 4 outermost electrons.

    If it's attached to 4 oxygen atoms, then it becomes a tetrahedral shape. That type of configuration can't make anything but simple molecular, which has physical properties of low melting/boiling points because it relies on intermolecular forces which are weaker than covalent/ionic bonds thus requiring less thermal energy to break for a change of state.

    If it's attached to 2 oxygen atoms, then it can be arranged in a giant molecular shape, which has physical properties of very high melting/boiling points because it relies on many covalent bonds which are stronger than intermolecular forces thus requiring more thermal energy to break for a change of state.

    CO2 prefers to form simple molecular because the C=O bonds are too strong, in case you're wondering about the exception. Silica also happens to be a semi-conductor, but I'm less familiar with explaining it in a bonding/structure way.
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