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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?