Using the Octet Rule to Write Lewis Structures

Using the Octet Rule to Write Lewis Structures 

To write a Lewis structure, we decide on the relative placement of the atoms in the molecule (or polyatomic ion)—that is, which atoms are adjacent and become bonded to each other—and distribute the total number of valence electrons as bonding and lone pairs. Let s begin by examining Lewis structures for species that obey the octet rule—those in which each atom fills its outer level with eight electrons (or two for hydrogen). 

Step 1. Place the atoms relative to each other. For compounds of molecular formula AB , place the atom with lower group number in the center because it needs more electrons to attain an octet usually, this is also the atom with the lower electronegativity. In NF3, the N (Group 5A EN = 3.0) has five electrons and so needs three, whereas each F (Group 7A EN = 4.0) has seven and so needs only one thus, N goes in the center with the three F atoms around it  

Place atom with lowest Atom placement Add A-group Sum of valence e Draw single bonds. Remaining valence e Give each atom 8e  

If the atoms have the same group number, as in SO3 or CIF3, place the atom with the higher period number in the center. H can form only one bond, so it is never a central atom. 

Draw single bonds. Subtract 2e lor each bond 

---