Lewis Structures Of Covalent Compounds NAB Method

Rule 1: NAB Rule 2: Place the least electronegative element at the center, except for H which is always an outer atom Rule 3: Add covalent bonds between the center atom and the outer atoms Rule 4: Add lone pairs to the outer atoms Rule 5: Add lone pairs to the center atom

Building Lewis Structures for Covalent Molecules Determine the number of electrons needed (N) by each atom to complete its octet Add up the total number of valence electrons available (A) The number of bonds (B) that come off the central atom (N-A) / 2

N = “NEEDED” 1.Calculate the “N” as the sum of the electrons necessary for all elements to achieve an octet 2.Important exceptions (as usual) H = 2 Be = 4 B = 6

NEEDED Add together the number of valence electrons for each atom in the molecule if each had a full octet. For example, CF 4 There are 5 atoms in the molecule. If each had a full octet, 5 x 8 = 40 e - s are needed

A “AVAILABLE” 1.Calculate “A” as the sum of all valence electrons available to share Ex Add together the number of valence electrons for each atom in the molecule. For example, CF 4 Carbon has 4 valence e- and each fluorine has 7 valence e- = 4 + 4(7) = 32 available e -

“B” BONDS 1.This is easy –Shared is the difference between NEEDED and AVAILABLE: N - A. –Each bond requires 2 electrons –So B = (N – A) / 2 - B is the number of bonds that will come off the central atom

BONDS For example, CF 4 The number of shared electrons is N – A = 40 – 32 = 8 shared electrons 2 electrons per bond, so 8/2 = 4 bonds

How do you choose the central atom? Many times it's readily apparent: SiBr 4 CH 4 H 2 SCO 2

Write out the elements of the molecule so that the least electronegative elements is in the center surrounded by the other elements. For example, CF 4

Place a covalent bond between the central atom and the outside atoms. Remember each covalent bond contains two electrons.

The four covalent bonds use eight of the 32 valence electrons in CF 4 There are 24 valence electrons remaining. Add electrons to the outer atoms as lone pairs to satisfy the Octet Rule. This uses 24 electrons. There are no electrons left, so this is The Lewis dot structure for CF 4

RESONANCE Consider the Lewis structure of the carbonate ion, CO The Lewis structure for this ion has a carbon-oxygen double bond, and two carbon-oxygen single bonds. But which of the three oxygens forms the double bond? There are three possibilities:

When more than one Lewis structure can be drawn, the molecule or ion is said to have resonance resonance We use a double headed arrow to show that individual structures are related by resonance.double headed arrow

HYPERVALENCY Larger atoms can exhibit hypervalency, in which the octet is expanded and there are more than eight electrons in the valence orbitals. Valence expansion (also known as hypervalency) is allowed for atoms in the 3rd period and below of the periodic table.

Hypervalency