A Carbon atom has four valence electrons, 
two in one 2s orbital and 2 in two 2p orbitals.  
There are three 2p orbitals and each 2p could 
hold 2 electrons.  Carbon has two 2p electrons
in two 2p orbitals.  As a result, Carbon should
only be able to form two bonds with other atoms.
However, a Carbon molecule can form up to four
bonds with other atoms.  One example is CH4.
When a Carbon atom is bonded with four Hydrogen 
atoms, one 2s electron is promoted to a vacant 
2p orbital.  The one 2s and three 2p orbitals 
will then mix and create four equivalent sp3 
hybrid orbitals.  Each one of the sp3 orbitals 
will form a å bond with each Hydrogen.  The 
resulting compound CH4 has a molecular shape 
of a tetrahedral.