Unit 5-L: Chemical Bonding

Chapters 8 & 9

Test: November 21, 2008

• http://www.visionlearning.com/library/module_viewer.php?mid=55

Chemical Bonds

• Definition-Force that holds 2 atoms together

• Valence Electrons and electronegativity determine the type of bonding.

• Try to achieve stable e- configuration – 8 valence e-; unless you look like ________

with ______ valence e-

– Noble Gas configuration; – Octet Rule

• Also, try to achieve lowest possible energy state

Ionic Bonding

• Occurs when electrons are completely transferred from one atom to another. – Atoms become ions after the transfer

of the e-

– Ions, having opposite charges, are held together with an electrostatic force

• Formed between a metal and a nonmetal.

Properties of Ionic Bonds

• High melting points and boiling points

• Dissolve easily in water

• Conduct electricity in solutions (electrolytes)

• Normally exist as solids with a crystal lattice structure

http://www.beyondbooks.com/psc92/3b.asp

+

Sodium Meets Chlorine

Ionic Bonding

• The simplest ratio of the ions represented in an ionic compound is called a formula unit– We use this because no single

particle of an ionic compound exists– Total # of e- gained by nonmetal

atoms = total # of e- lost by the metal, so overall charge = 0

Formulas for Binary Ionic Compounds• Compound that only contains two

elements• If more than one of an element is used,

add subscript numbers to balance the charge

Metal Nonmetal Ratio Formula

Mg2+ Cl1- 1:2 MgCl2

Li S

Al O

Na P

Ba F

Polyatomic Ions

• More than one element bonded together with a charge

• When writing formulas, if more than one of a polyatomic ion is needed (ie. needs a subscript) place the polyatomic ion in parentheses

• Example: Mg(OH)2

Representing an Ionic Bond

• Draw electron dot structures for the metal and nonmetal, – show movement of e- from metal to

nonmetal with arrow– Draw the ions electron dot structures

after the reaction arrow

• Make sure oxidation numbers add up to zero!

Ionic Bonding

Practice by writing the equations for the following compounds:

1. Magnesium & Sulfur

2. Aluminum & Oxygen

3. Magnesium & Iodine

Metallic Bonds

• Metals often form lattices in the solid state similar to ionic crystals

• Even though metal atoms have at least one valence e- they do not share or lose electrons

Metallic Bonds

• Electron sea model – all the metal atoms in a metallic solid contribute their valence e- to form a “sea” of e-

• Structure explains malleability, ductility, and conductivity of heat and electricity

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/12.html

Alloys

• Mixtures of elements with metallic properties

• Common Alloys– Brass: copper & zinc– Sterling Silver: silver & copper– Bronze: copper & tin– Stainless Steel: iron & chromium– White Gold: gold & palladium (or Ni)

Covalent Bonds

• The result of sharing valence electrons. The shared electrons are part of the complete outer shell of both atoms.– Occurs when elements are close together on the

periodic table– Between nonmetallic elements

• Molecule-formed when two or more atoms bond covalently.

• Common covalent molecule is carbon dioxide (CO2)

Covalent Bonds

Exceptions to Octet Rule• Beryllium (2 valence e-, full with 4

valence e-)

BeI2

• Aluminum (3 valence e-, full with 6 valence e-)

AlCl3• Boron (3 valence e-, full with 6

valence e-)

BH3

Covalent Bonds

• Can exist as gases, liquids, or solids depending on molecular mass or polarity

• Usually have lower MP and BP than ionic compounds

• Do not usually dissolve in water

• Do not conduct electricity

Nonpolar Covalent

• Electrons are equally shared• No difference in electronegativity

– Diatomic molecules, elements that exist as bonded pairs

– H, O, F, Br, I, N, Cl

• Also, can be due to shape of molecule– Electrons pulled equally in all directions,

polar effect cancels

(ex. I Be I )

Nonpolar Covalent Bonds

• Two Hydrogen Atoms (H2)

http://web.jjay.cuny.edu/~acarpi/NSC/5-bonds.htm

Polar Covalent Bonds

• Unequal sharing of electrons• Have poles (dipoles) – regions that are

positive & regions that are negative• Electrons are pulling toward more

electronegative element• Symbols:

δ+ δ- : show regions of partial charge

: arrow points to more electronegative

element

Polar Covalent Bonds

Water Molecule

http://web.jjay.cuny.edu/~acarpi/NSC/5-bonds.htm

Multiple Covalent Bonds

• Atoms of some elements attain a noble-gas configuration by sharing more than one pair of electrons between two atoms

• When writing structural formulas a line can represent a pair of shared electrons

Predicting Bond Type

• Look up EN values for the two elements in the bond

• Subtract the smaller EN value from the larger EN value

• If EN difference is…

0 – 0.4 nonpolar covalent bond

0.5 – 1.6 polar covalent bond

>1.7 ionic bond

Predicting Bond Type

• Identify as Ionic, Nonpolar Covalent or Polar Covalent

Formula EN values EN Difference Bond Type

LiClLi = 1.0

Cl = 3.0 2.0 ionic

CO2

MgO

CH4

NF3

Determining Molecular Shape

Only for covalent bonds!!!!!

Use VSEPR (valence shell electron pair repulsion) rules:

1. Draw the Lewis dot structure for the molecule2. Identify the central atom3. Count total # of electron pairs around the

central atom4. Count # of bonding pairs of electrons around

the central atom5. Count # of lone pairs of electrons around the

central atom6. Look at summary chart to identify shape

Determining Molecular Shape

Example: What is the shape of water (H2O)?

1) Draw Lewis Dot…2) Central atom? oxygen3) # of total electron pairs around central atom? 44) # of bonding pairs around central atom? 25) # of lone pairs around central atom? 26) Refer to chart to identify shape…angular (bent)

PracticePredict the shape of the following

molecules

1) BCl3 2) CH4 3) NH3

Lewis Dot

Central Atom

# e- pairs

# bonded pairs

# lone pairs

Shape

Polarity of Molecules

• To determine the polarity of a molecule, not a bond, you must know the type of bond and the shape.

• POLAR MOLECULES must meet 2 criteria:1. Must have a polar covalent bond (EN difference between 0.5 and 1.6)

AND2. Must have an asymmetrical shape:

trigonal pyramidalangular, or 2 element linear

• If both criteria are not met, it is not a polar molecule! It would be either a nonpolar molecule or an ionic compound.

PracticeLabel the following MOLECULES as polar

covalent, nonpolar covalent, or ionic.

FormulaHas a polar

covalent bond

Has an asymmetrical

shapeIdentity

H2O CH4

NH3

HCl

Polymers

• Large molecule (macromolecule) usually composed of a repeating pattern of atoms covalently bonded together

• Plastics are the most easily identified polymers there are many natural biopolymers such as:– proteins – nucleic acids– carbohydrates

Essential Questions

1. What types of elements combine to form ionic and covalent compounds?

2. What factors determine the shape of covalent molecules?

3. What is a polar molecule?

4. Can you identify the Lewis Dot Structures for ionic and covalent compounds?

5. How do you determine the type of bonding using electronegativity?

Review Questions

1. How many valence e- must an element have in its outer energy level to be considered stable?

2. Determine the ratio of the atoms in the following compounds

a) Al & F

b) Li & O

Review Questions

3. What type of structure has cations surrounded by anions, and anions surrounded by cations?

4. Do atoms that share a covalent bond have an ionic charge?

5. When is a bond polar?

6. When is a molecule polar?