Unit 5-L: Chemical Bonding Chapters 8 & 9 Test: November 21, 2008

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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

+ 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 MetalNonmetalRatioFormula Mg 2+ Cl 1- 1:2MgCl 2 LiS AlO NaP BaF

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

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 (CO 2 )

Covalent Bonds Exceptions to Octet Rule Beryllium (2 valence e-, full with 4 valence e-) BeI 2 Aluminum (3 valence e-, full with 6 valence e-) AlCl 3 Boron (3 valence e-, full with 6 valence e-) BH 3

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 (H 2 )

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

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 FormulaEN valuesEN DifferenceBond Type LiCl Li = 1.0 Cl = ionic CO 2 MgO CH 4 NF 3

Determining Molecular Shape Only for covalent bonds!!!!! Use VSEPR (valence shell electron pair repulsion) rules: 1.Draw the Lewis dot structure for the molecule 2.Identify the central atom 3.Count total # of electron pairs around the central atom 4.Count # of bonding pairs of electrons around the central atom 5.Count # of lone pairs of electrons around the central atom 6.Look at summary chart to identify shape

Determining Molecular Shape Example: What is the shape of water (H 2 O)? 1) Draw Lewis Dot… 2) Central atom? oxygen 3) # of total electron pairs around central atom? 4 4) # of bonding pairs around central atom? 2 5) # of lone pairs around central atom? 2 6) Refer to chart to identify shape…angular (bent)

Practice Predict the shape of the following molecules 1) BCl 3 2) CH 4 3) NH 3 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) AND 2. Must have an asymmetrical shape: trigonal pyramidal angular, 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.

Practice Label the following MOLECULES as polar covalent, nonpolar covalent, or ionic. Formula Has a polar covalent bond Has an asymmetrical shape Identity H2OH2O CH 4 NH 3 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?