Lewis structure :
•• •• ••
:Cl• •N • •Cl:
•• • ••
•
:Cl:
••
The dots around the respective atoms represent the number of valence electrons each of the corresponding atoms have. Since nitrogen is more electronegative it is placed within the middle, it can also form the 3 covalent bonds, unlike chlorine which can only form 1 and thus is in the middle of the diagram. This structure has gained stability in that all of the atoms have 8 electrons in its outermost shell, if you were to count the shared electrons for each atom.