Lewis Dot Structures

Lewis Dots Lewis Dot structures describe the covalent bonding in molecules They describe how the valence electrons (outermost s + p electrons) are arranged around atoms and molecules Because they describe molecules they almost always refer to non-metals. They also can be used to describe molecular shapes

Ne Around atoms When describing simple atoms (not bonded), you just lay out the electrons on the four sides of an atomic symbol. One lays them out in order. Using Neon as an example (8 valence electrons) first two on the right Next three on the other three sides Last three filling in the pairs

The Logic of Dots for Atoms The Dots Ignore the kernel electrons Electron Configuration Ne first two on the right (these are the s electrons) Next three on the other three sides Last three filling in the pairs 2p 1s 2s

S (sulfur) 6 valence electrons I (iodine) 7 valence electrons Lewis Dots around other atoms More Examples: C (carbon) 4 valence electrons

Forming Molecules Atoms will covalently bond (share electrons) so that they will end up with 8 valence electrons. The shared pair counts for both atoms and makes up the bond Here is flourine bonding (7 valence each)

Bonding of 2 Fluorine atoms to make F 2 F F Each flourine atom has 7 electrons Shared, each atom Now has 8 electrons The shared pair of electrons IS the covalent bond. The bond is shown with a dashed line

Rules Each atom has a number of valence electrons (= present) Each atom needs 8 –Except hydrogen =2 –Boron = 6 –Phosphorus, arsenic and up MAY use 8 or 10 –Sulfur, selenium and up MAY use 8,10 or 12 –Chlorine, bromine and up MAT use 8,10 or 12

Calculating bonds First, the number of bonds must be calculated. Add up all valence electrons present (=P) Add all needed electrons ( = N) –Assume all atoms need 8 (except H and B) unless there aren’t enough bonds Bonds present = (N-P)/2

Calculating Bonds: Example H 2 O H present = 1(each) 2 (total) –Need = 2 (each) (4 total) O present = 6 –Need = 8 Bonds =(N-P)/2 = (12-8)/2 = 4/2 = 2 Bonds

Calculating Bonds: Example 2 CO 2 O present = 6(each) 12 (total) –Need = 8 (each) (16 total) C present = 4 –Need = 8 Bonds =(N-P)/2 = (24-16)/2 = 8/2 = 4 Bonds

Making a Lewis Dot Diagram: H 2 O Locate the central atom. Place the outer atoms ( and hydrogen) surrounding the central atom. Place a single bond between each atom and the central atom. Place extra bonds between the central atom and surrounding atoms. (no extra bonds here) Fill in the open spaces on all the atoms the unbonded pairs of dots. O HH

Making a Lewis Dot Diagram: CO 2 Locate the central atom. Place the outer atoms (oxygen) surrounding the central atom. Place a single bond between each atom and the central atom. Place extra bonds between the central atom and surrounding atoms. (two extra bonds here) Fill in the open spaces on all the atoms the unbonded pairs of dots. C OO

Coordinate Bonding The usual definition of covalent bonding involves a “shared pair” of electons. They do NOT need to come equally from both atoms Some of the bonds may be made of electrons that come from just one atom. Carbon Monoxide (C in black, O in red) C O (Note that the top bond come only From the Oxygen’s electrons)