Lewis Structures

Lewis Structures At the conclusion of our time together, you should be able to: List the basic rules for drawing a Lewis Dot structure for a compound Use these rules to draw a Lewis Dot structure for a compound

The Lewis Model of Chemical Bonding In 1916 G. N. Lewis proposed that atoms combine in order to achieve a more stable electron configuration. Maximum stability results when an atom is isoelectronic with a noble gas. An electron pair that is shared between two atoms constitutes a covalent bond. 2 3

A. Writing Lewis Structures In writing Lewis structures we include only the valence electrons Most important requirement Atoms achieve noble gas electron configuration (octet rule, duet rule)

Covalent Bonding in H2 Two hydrogen atoms, each with 1 electron, . H can share those electrons in a covalent bond. H : Sharing the electron pair gives each hydrogen an electron configuration analogous to helium. 2 5

Covalent Bonding in F2 Two fluorine atoms, each with 7 valence electrons, F : .. . F : .. . : : can share those electrons in a covalent bond. F : .. Sharing the electron pair gives each fluorine an electron configuration analogous to neon. 2 6

The Octet Rule In forming compounds, atoms gain, lose, or share electrons to give a stable electron configuration characterized by 8 valence electrons. F : .. 2 7

to write a Lewis structure for CF4. Example Combine carbon (4 valence electrons) and four fluorines (7 valence electrons each) C . F : .. . : to write a Lewis structure for CF4. : F .. C The octet rule is satisfied for carbon and each fluorine. 4 8

Example It is common practice to represent a covalent bond by a line. We can rewrite : F .. C .. C F .. : as 4 9

A. Writing Lewis Structures

Lewis Structures with Double and Triple Bonds

Lewis Structures of Molecules with Multiple Bonds Single bond – covalent bond in which 1 pair of electrons is shared by 2 atoms e.g. Ethane, C2H6 Double bond – covalent bond in which 2 pairs of electrons are shared by 2 atoms e.g. Ethylene, C2H4 Triple bond – covalent bond in which 3 pairs of electrons are shared by 2 atoms e.g. Acetylene, C2H2

Inorganic examples C : O .. C : O .. Carbon dioxide : N C H : N C H Hydrogen cyanide 10 13

Organic examples C H C : .. H Ethylene : C H Acetylene C H 10 14

Rules for Lewis Structures 1. Make certain that the bond is a covalent bond then set up the skeleton structure as follows: The atom with the lowest electronegativity will tend to go in middle Place all the other atoms around this central atom Attach these atoms to the central atom in reasonable fashion with single bonds

Rules for Lewis Structures 2. Sum valence electrons 3. Complete octets of peripheral atoms 4. Place leftover e- on central atom 5. If necessary use multiple bonds to fill the center atom's octet.

For NF3 Like NBr3 Molecular formula Atom placement Sum of valence e- : N 5e- : F : : F : : N F 7e- X 3 = 21e- Remaining valence e- : F : : Total 26e- Lewis structure

Writing Lewis Structures for Molecules with One Central Atom SAMPLE PROBLEM: Writing Lewis Structures for Molecules with One Central Atom PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds responsible for the depletion of stratospheric ozone. PLAN: Follow the steps outlined previously SOLUTION: Step 1: Carbon has the lowest EN and is the central atom. The other atoms are placed around it. Cl Cl C F F

SOLUTION: Steps 2-4: C has 4 valence e-, Cl and F each have 7. The sum is 4 + 4(7) = 32 valence e-. Make bonds and fill in remaining valence electrons placing 8e- around each atom. : C Cl F : : :

Writing Lewis Structures for Molecules with Multiple Bonds. SAMPLE PROBLEM: PROBLEM: Write Lewis structures for the following: Nitrogen (N2), the most abundant atmospheric gas PLAN: If an atom does not have an octet, Step 5 which follows the other steps in Lewis structure construction must be done. If a central atom does not have 8e-, an octet, then an e- can be moved in to form a multiple bond.

SOLUTION: N2 has 2(5) = 10 valence e-. N . : N . : N :

How Do Lewis Structures Explain Polyatomics? A polyatomic ion is a “charged molecule”. It has valence electrons +/- “extra” electrons Can Lewis structures explain the common occurrence of polyatomics?

Lewis Structures of Polyatomic Ions Consider the Lewis structure for the cyanide ion, CN- Number of valence electrons from C and N is 4+5 = 9 Plus one from the negative charge = 10 Apply the octet rule to the various options :C≡N: -1 charge