-Resonance structures are all the possible Lewis structures for a molecule. In chemistry, a formal charge (FC) is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity. It is a technique to identify which resonance structure is the more correct structure. The most correct Lewis structure will be the structure where the formal charges are evenly distributed throughout the molecule. © chemistry.about.com

-The formal charge of any atom in a molecule can be calculated by the following equation:

FC=GT-B=U

FC= Formal Charge

GT= Sum of all Group Nos. of the Molecules in a chemical formula

B= No. of Bonds (shared ē)

U= Unshared ē

 

Exceptions to the Octet Rule:

 

*OCTET RULE- It is a chemical rule of thumb that states that atoms of main-group elements tend to combine in such a way that each atom has eight(8) electrons in its valence shell, they either transfer or share to have the same electronic configuration as a noble gas.

Atoms that already have 8 valence electrons (the noble gases minus helium) tend to not react with other atoms, since they're already in their most stable configuration and stand to gain nothing, stability-wise, from forming chemical bonds. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.

 

The valence electrons can be counted using a Lewis electron dot diagram as shown for carbon dioxide. The electrons shared by the two atoms in a covalent bond are counted twice, once for each atom. In carbon dioxide each oxygen shares four electrons with the central carbon, two (shown in red) from the oxygen itself and two (shown in black) from the carbon. All these four electrons are counted in both the carbon octet and the oxygen octet. ©Wikipedia.org

 

 

 

 

 

 

 

 

Incomplete Octet- Central Atoms having less than the stable number of ē which is 8.

 

 

 

 

 

 

 

 

 

 

 

 

BCL3= (Boron-Group 3, Chlorine-Group 7)

FC   =3+7(3)=24-6=18 ē (It has only 6ē, TRIGONAL PLANAR, AX3)

 

 

 

 

 

 

 

                     

 

 

 

 

 

 

 

Expanded Valence Molecule- Central Atoms having greater than the stable number of ē which is 8.

 

SF6= (Sulfur-Group 6, Fluorine-Group 7) Sulfur Hexafluoride

FC=6+7(6)=48-12=36 ē (It has 12ē, OCTAHEDRAL, AX6)

                

 

Odd ē Molecule- Central Atoms having odd number ē.

NO= (Nitrogen-Group 5, Oxygen-Group 6) Nitric Oxide

FC=5+6=11-2=9 ē (It has 9ē, Linear, AX2)

 

ACTIVITY:

 

Find out the FC then draw the Lewis Structure of the following and identify whether if it is an Incomplete, Expanded or an Odd octet.

 

 

 

 

 

 

 

 

 

 

 

 

written by: Mariquita Villorante