Covalent Bonding Chapter 6, Section 2

How does a covalent bond form?

There is a balance between the attraction of oppositely charged particles & the repulsion of like charged particles –FYI – bond length is the distance btwn atoms How does a covalent bond form?

A bond is formed when... a distance between the nuclei is reached in which –repulsion and attraction forces are equal –potential energy is at the lowest point possible –at the bottom of the curve on PE graph

Energy in Bonds When atoms form bonds, they become more stable and release energy That same amount of energy must be absorbed to break the bond (ie. bond energy) The higher the bond energy, the stronger the bond

Bond Energies Do you see a trend between bond length and bond energy?

Octet Rule The ultimate goal of the atoms in a compound is to look like a noble gas Each atom will gain, lose, or share electrons so that they have 8 electrons in their outer energy level Check out examples on p. 169

Exceptions to Octet Rule Hydrogen is happy with 2 electrons Boron can be happy with 6 electrons Sulfur & phosphorus can actually get MORE than 8 electrons (more on that later)

e- dot diagrams Shows the number of valence electrons on an s- or p-block element Draw dots around element symbol to match number of valence electrons

Lewis Structures e- dot diagrams for compounds –Dots show unshared electrons –Dashes show shared electrons

So how do you draw a Lewis structure? 1) Determine the type & # of atoms in a molecule 2) Determine the number of valence e- on each atom 3) Add up total # of valence e- 4) Arrange the atoms with the first element in the middle and other elements radiating off. If carbon is present it automatically goes in the middle 5) Connect the elements with dashes 6) Subtract shared electrons from total electrons 7) Distribute remaining electrons as unshared pairs first to outer elements (follow the octet rule) and then to center element.

WOW!!! That last slide had a LOT of words!!!

Let’s draw a Lewis Structure! Draw the Lewis Structure for CH 3 Cl duet

That’s all for now... We will finish up the rest of section 2 next class! We will finish up the rest of section 2 next class! Homework: p. 197 #38 & 39

November 1, 2010 Get out your homework and show it to me. We will grade it as soon as the bell rings. We will finish section 2 notes after grading.

There are a few different types of covalent bonds... Single covalent bond = 2 e- are shared –Weakest & longest –Ex. H 2 Double covalent bond = 4 e- are shared –Stronger & shorter –Ex. O 2, CO 2 Triple covalent bond = 6 e- are shared –Strongest & shortest –Ex. N 2, C 2 H 2

Multiple Bonds Example What is the Lewis structure for silicon dioxide?

Multiple Bonds Example 2 What is the Lewis structure for hydrocyanic acid (HCN)?

Polyatomic Ion Lewis Structures Follow the same steps for drawing a normal Lewis structure with the following changes: –If the polyatomic ion is negative, add the charge to the total number of valence e- –If the polyatomic ion is positive, subtract the charge from the total number of valence e- –Draw brackets around the entire diagram with the charge written as a superscript

Polyatomic Ions Example: CN - Example: CN -

NH 4 + : ammonium ion

Polyatomic Ion Example What is the Lewis structure for sulfate (SO 4 2- )?

Polyatomic Ion Example What is the Lewis structure for hydroxide (OH - )?

Another example Draw the Lewis structure for ozone (O 3 ).

ooooooo...resonance structures Sometimes, molecular compounds can’t make up their minds how they want to be –O–O–O–One Lewis structure just is not enough In these situations, 2+ Lewis structures are drawn and the molecule actually is an average of all of them Ex. Ozone (O3)

Covalent Network Bonding a different type of covalent bonding that is more like ionic bonding lots of nonmetal atoms covalently bonded together in a network that looks like a crystal example: –diamond –silicon dioxide –graphite