Atomic+Particles+Electrons

Atomic structure specifically the different energy levels of electrons determines if an object is transparent or opaque.

Electrons can gain energy from light and move up a level only if the photons contains the correct amount of energy. Visible light has the right energy for opaque objects. Visible light doesn't have the right amount of energy for transparent objects.

Why is glass transparent and other substances are opaque. https://www.youtube.com/watch?v=VwRLIt6jgdM http://ed.ted.com/lessons/why-is-glass-transparent-mark-miodownik

https://io9.gizmodo.com/why-dont-electrons-just-fall-into-the-nucleus-of-an-ato-1597851164

Electrons https://www.youtube.com/watch?v=vr0HiIed-8Q