It forms a bond with four oxygen atoms even though it has only there electrons in his outter shell?

  • $\begingroup$ One should understand that the rules to complete outer shells apply to individual molecules (although they don't always get followed). Solids are a different matter - look at any number of binary phase diagrams and you can find multiple phases with different ratios of one element to the other. The extended nature of bonding in solids is just different. $\endgroup$ – Jon Custer Aug 29 '18 at 12:54