Covalent Bonds(Ch. 8/8.1)

Cornell Notes2/1/10

What we’ve learned so far….Atoms lose/gain electrons to form cations

and anions (8 valence electrons like a noble gas!)

Charged anions and cations form IONIC BONDS to make IONIC COMPOUNDS

FK+ -

What we’re learning today…

Atoms can also SHARE electrons to form COVALENT BONDS!

Why is this important? Covalent bonds are important in many organic

molecules (living creatures) Examples:

Cell membranes are made of lipids (We’d be puddles of liquid without them!)

Proteins (We’d starve without them!)

Covalent BondThe chemical bond formed when two

atoms SHARE electrons Why do atoms form bonds? To gain the same

number of valence electrons as a noble gas!

ClClFull octet (8)! Full octet (8)!

Bonding Pair

Types of Covalent Bonds

Single Bond: ONE pair of shared electrons

Double Bond (Pi Bond): TWO pairs of shared electrons

Triple Bond: THREE pairs of shared electrons

ClCl

OO

N N

How many bonds can an atom form?

# bonds formed = # of UNPAIRED electrons

# electrons atoms keep = # electrons in PAIRS

N N3 unpaired electrons

= 3 bonds!

O 2 pairs = four electrons the atom keeps!

Example 1How many bonds can Sulfur form?

How many electrons will Sulfur keep to itself?

STwo unpaired electrons!

Four electrons will be kept by sulfur!

STwo pairs of electrons!

Two bonds can be formed!

Example 2How many bonds can Carbon form?

How many electrons will Carbon keep to itself?

CFour unpaired electrons!

Zero electrons will be kept by carbon!

CNo pairs of electrons!

Four bonds can be formed!

Example 3How many bonds can Neon form?

How many electrons will Neon keep to itself?

NeZero unpaired electrons!

Eight electrons will be kept by neon!

NeFour pairs of electrons!

Zero bonds can be formed!

Drawing Lewis Structures (Ch.

8.3)

Cornell Notes

What we have learned so far…Covalent Bonds: when two atoms SHARE pairs of

electronsWhy do atoms bond together? To get the same

number of valence electrons as a noble gas!How many bonds can an atom form?

Equal to the number of UNPAIRED electrons!

How many electrons do atoms keep to themselves? Equal to the number of PAIRED electrons!

Lewis structures are pictures of molecules Right number of valence electrons

Follows the octet rule

What we are doing today

Drawing our own Lewis structures!

Why are we doing this?

1. Organic molecules have covalent bonds!

2. We function because of covalent bonds!

3. Lewis structures tell us the SHAPE of molecules

Shape can determine how a molecule will behave

How do we draw a Lewis structure?

1. Determine what elements and how many atoms you have in a molecule from the formula

2. Draw the electron dot structures for every atom

The atom(s) with the most unpaired electrons go in the middle

3. Connect the dots! (unpaired electrons)

4. Redraw the structure neatly so lines are straight

Step 1: Determine the elements and number of atoms from the formula

Symbols tell you what elements we haveLittle numbers tell you how many atoms of

each element

Example:

H2O2 Hydrogen

atoms!1 Oxygen

atom!

Step 2: Draw the electron dot structures for EVERY atom

Put the atom(s) with the most UNPAIRED electrons in the middle!

Example: H2O

(2 hydrogens, 1 oxygen)

H H O

Step 3: Connect the dots! (unpaired electrons)

Connect one unpaired electron from atom to an unpaired electron of another atom

These lines show COVALENT BONDS BETWEEN ATOMS!

Example: H2O

H HO

Step 4: Redraw the structureLines are straightLeave paired electrons in the structure

Example: H2O

H HO H HO

“Do Now” for 2/22/10

Draw the Lewis structures for the following molecules (look in your notes or p. 254 in your text)

C2H6 (common name ‘ethane’, an alkane) C2H4 (common name ‘ethylene’, an alkene) C2H2 (common name ‘acetylene’, an alkyne)

Bonus Point: What is the special name for the group of compounds that these 3 molecules fall under?

When you are done, QUICKLY finish drawing the compounds from Friday in your notes (or on your worksheet) so we can go over them:

H2S H2O2 NH2Cl HCN

Answers for “Do Now”

C2H6 C2H4

HHH H

C C HH

C CH

H

HH

HH

C C

H

HH

H

CC H

H

H

H

“Do Now” response cont.

C2H2The name for the

group of organic compounds that all 3 of these molecules belong to is:

C CH H

C C HH

Hydrocarbons

Def: Simplest organic compounds composed of hydrogen and carbon EXCLUSIVELY.

Announcements We will be finishing Ch. 8 this week If by the end of class on Wed. you are struggling

with ionic or covalent bonds, it is highly recommended that you attend an after school review session

Response to lab ?’s are due TODAY New HW was posted in class and online last Fri and

is due on Thurs, 2/25 Exam on Ch. 7 and 8 is scheduled for THIS

Friday, 2/26, review guides on table – complete by Thursday!

You do NOT need to complete the graphic organizer for steps to naming covalent compounds (Ch. 8.2 HW). You may for EC if you like.

You NEED your book Tues-Thurs. No book = no chair for you! (until you have a chance

to correctly answer a ? at least)

Subscripts, Superscripts and Coefficients

Al3(SO4 )2coefficient

subscripts

superscripts

ALUMINUM SULFATE

SO43- Al2+

5

Title: Ch. 8 Overview Cornell Notes

Review: What is the difference between a compound and a molecule?

Compound: term used to describe elements that have ionic bonds

chemical combination of 2 or more different elements

can be broken down into simpler substances has properties different than those of the

elements which make it up

Molecule: term used to describe atoms of either the same or different elements that share e- (are covalently bonded)

Strength of covalent bondsBond Length

Def: Distance from the center of 1 nucleus to the center of another between bonded atoms.

As # of shared e- pairs _________, bond length_________.

As bond length _________, the strength of the bond _________.

Bonds and energy If you form a bond energy is released. If you break a bond energy must be added, or

used. The energy required to break a bond is called

bond-dissociation energy.

increasesdecreases

increasesdecreases

Molecular Structures Quickly sketch the graphic below of the different

types of models for showing molecules in your notes.

Lewis structures for Polyatomic ions

Though polyatomic ions as a unit form ionic bonds, the elements that make them up are covalently bonded to each other.

Main difference between a molecule and a polyatomic ion: difference in the # of e- that are available to bond (hence the + or – charges)

Steps:1. Determine the # of e-

Look at the # of valence e- each atom should have and add them together to get the total # of e-

S O4 2-

6 (x1) 6(x4)+ = 30 total 2. Check the charge on the polyatomic atom

on your cheat sheet Add it to your # of e- if the charge is - Subtract it from your # of e- if the charge is +

30 + 2 = 32 total valence e-

3. Determine which will be the central atom (Remember – it will be the atom that wants to make the most bonds!)

• Both S and O have ____ valence e-, so both want to make ____ bonds.• So what do we do??

• The element with the lowest electronegativity will be the central atom.• Remember: electronegativity is an atom’s ability to attract e-• Look at p. 194 in Ch. 6.3 for the trend…which you SHOULD

have memorized!

• General rules:• H will always be a terminal atom• 1st element listed will usually be the

central atom

26

In this case, it is S

4. Draw single bonds around the central atom Connect it to all surrounding atoms

5. Add the remaining e- around the terminal atoms

Fulfill the octet rule (unless H or He!)

S O4 2-

S OO

OO

8 e- used in the single bonds

How many do we still need to place?

24 e-

Is the octet rule satisfied?(Does each atom have 8 e-?)

Yep!

“Do Now” for 2/231. Which of the following molecules has the

strongest bonds? 1st draw the Lewis structures for the molecules 2nd look back over your notes to see how to determine

bond strength. (p. 246 in text if you were absent)

a.) CN- b.) CH3NH2Hint: the C and the N are

central atoms

C N C NH

HH

H

H

As the # of shared pairs of e- increases, the bond length decreases, which means the bond is stronger.

Naming Covalent MoleculesCh. 8.2

Binary Molecular Compounds (2 nonmetal atoms)1. Name the first element in the

compound2. Name the 2nd element using the

root and the suffix “ide” (Just like in monatomic ionic compounds!)

3. Use prefixes to denote the # of each of the elements.

Prefixes (Memorize these! p. 248)

# of Atoms Prefix

1 Mono (only used for Oxygen)

2 di

3 tri

4 tetra

5 penta

6 hexa

7 hepta

8 octa

9 nona

10 deca

Let’s Practice! (Answer in your notes)

1. CO2

2. SO2

3. CCl4

3. P2O5

Carbon

Sulfur

Carbon

phosphorous

oxide

oxide

oxide

chloride

di

di

tetra

pentdi

Naming Binary Acids(H + 1 other element) Write these down!!

1. Add the prefix “hydro” to the name of the 2nd element

2. Remainder of 1st word is the root of the 2nd element with the suffix “ic”

3. The 2nd word is always acid.

HCl

Hydrochloric acid

Naming Oxyacids (H + an oxyanion) Oxyanion = polyatomic ion that contains 1 or

more oxygen atoms.

Write these down!!1. Identify the oxyanion present.2. 1st word of name begins w/ the root of

the oxyanion (and the prefix “per” or “hypo” if it is part of the name)

3. The suffix of the 1st word is: “ic” if the oxyanion ends w/ “ate” “ous” if the oxyanion ends w/ “ite”

4. The 2nd word is always “acid”.

HNO3

Nitrateic acidNitr

Naming Oxyacids

What should go in the missing boxes?

Let’s Practice! (Answer in your notes)1. Say whether it is a binary or an oxyacid

2. Write the name of the acid following the proper steps

1. HI

2. HClO3

3. HClO2

4. H2SO4

5. H2S

Hydroiodic acidBINARY

BINARY

OXYACID

OXYACID

OXYACID

Chloric acid

Chlorous acid

Sulfuric acid

Hydrosulfuric acid