Bonding in intrinsic semi-conductors
A simple explanation, not what 'really' goes on but good enough for introduction:

Covalent bonds exist between atoms, basically 2 atoms 'share' an electron.  In a pure semi-conductor crystal used for electronics, for example, silicon, all atoms are bound to 4 neighboring atoms via covalent bonds.  This should make sense given that silicon has 4 valence electrons and therefore 'wants' to bind with 4 additional electrons to fill its valence orbital.

When you dope a semiconductor you blast out some of the silicon atoms and replace them with atoms that have either 3 or 5 valence electrons.  This means that you will either have 1 extra electron or be missing an election to fill the doped atom's valence orbital.  The bonds in the crystalline structure are very strong and the dopant percentage of the structure is low so the crystalline structure in maintained despite these impurities imparting a "desire to change" the structure. 

The presence of the free electron or electron hole is what changes the nature of the material to be a donor (n-type) or acceptor (p-type).