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

Chemical Bonds Q: What is a bond? A: A force that holds atoms together Octet Rule Atoms try to achieve stable electron configuration like the noble gases; 8 valence electrons *3 major types of bonds that we will study: Ionic, Metallic, Covalent

Ionic Bonds Properties - Ionic compounds… Have high melting and boiling points Dissolve easily in water Conduct electricity in solutions Exist as solids, crystal structure

Ionic Bonds Occurs when valence electrons are transferred from one atom to another – usually between a metal and a nonmetal –Metals donate electrons and form cations –Nonmetals accept electrons and form anions 2 ions bond together to form a binary compound Opposite charges make them attracted (bonded) to each other by electrostatic forces

Ionic Bonds What is the charge of… –A sodium ion? ____ –A chlorine ion? ____ –NaCl? ____ The overall charge of any ionically bonded compound is zero - this means stability! Animation:

Ionic Bonds Sodium and chlorine bond in a 1:1 ratio to form NaCl Calcium and chlorine bond in a 1:2 ratio for form CaCl 2 What determines each ratio & formula? ________________

Ionic Bonds Try to predict the ratio and resulting binary compound for each of the following ions based on their oxidation numbers: Ions Oxidation Numbers RatioFormula Magnesium & Oxygen Mg 2+ & O 2- 1:1MgO Lithium & Sulfur Aluminum & Oxygen Sodium & Phosphorus Barium & Fluorine

Polyatomic Ions Some ions contain more than one element - called a polyatomic ion The group as a whole has an overall charge Examples:  Lithium and sulfate would bond together to make Li 2 SO 4  Ammonium and sulfur would bond together to make (NH 4 ) 2 S NameFormula ammonium(NH 4 ) +1 hydroxide(OH) -1 acetate(C 2 H 3 O 2 ) -1 nitrate(NO 3 ) -1 chlorate(ClO 3 ) -1 bromate(BrO 3 ) -1 iodate(IO 3 ) -1 carbonate(CO 3 ) -2 sulfate(SO 4 ) -2 phosphate(PO 4 ) -3 *Put parentheses around polyatomic ion when it requires a subscript

Polyatomic Ions Try to predict the ratio and resulting polyatomic compound for each of the following ions based on their oxidation numbers: Ions Oxidation Numbers RatioFormula Potassium & Hydroxide K + & OH - 1:1KOH Calcium & Carbonate Barium & Nitrate Aluminum & Phosphate Ammonium & Fluorine

Metallic Bonds Bonding is collective in nature - all metal cations contribute their valence electrons to form a “sea of electrons” Examples: Aluminum, Silver, Iron

Metallic Bonds Solid metals form crystal lattices Structure and organization are responsible for the unique properties of metals such as malleability, ductility, and conductivity of heat and electricity

Alloys A mixture of elements that has metallic properties –Brass: copper and zinc –Bronze: copper and tin –Stainless Steel: iron and chromium –Sterling Silver: copper and silver

Covalent Bonds Carbon dioxide, represented by the formula CO 2, is an example of a compound formed by covalent bonds

Covalent Bonds Properties - Covalent compounds… Have low melting and boiling points Do not dissolve easily in water Do not conduct electricity in solutions Exist as gases, liquids, or solids

Covalent Bonds Occurs when valence electrons are shared between atoms – usually between nonmetallic elements Covalently bonded compound known as a molecule These shared electrons are part of the valences of all atoms involved (satisfies octet rule) Many combinations can occur between two nonmetals –Example: carbon and oxygen can form carbon monoxide (CO) and carbon dioxide (CO 2 ) Carbon Dioxide

Covalent Bonds Some nonmetal elements on the periodic table exist in nature only as pairs called diatomic molecules; bonded covalently –H, O, F, Br, I, N, Cl Hydrogen Molecule (H 2 )

Multiple Covalent Bonds Atoms of some elements attain a noble gas configuration by sharing more than one pair of electrons between two atoms When drawing these molecules, a line can represent a pair of shared electrons How many pairs of electrons does each kind of bond share? Single: _________ Double: _________ Triple: _________

Polar & Nonpolar Covalent Bonds Covalent bonds share electrons equally and unequally (“tug of war” with electrons) –Which molecule appears “balanced”? –Which one does not? Water (H 2 O) Oxygen (O 2 )

Polar Covalent Bonds Unequal sharing of electrons Occurs when atoms that share electrons have different electronegativities –Shared electrons spend a greater amount of time at the more electronegative atom Example: water (H 2 O)

Polar Covalent Bonds Water Molecule Animation

Nonpolar Covalent Bonds Equal sharing of electrons Occurs in two scenarios… –No difference in electronegativity between atoms that are sharing Example: chlorine molecule (Cl 2 ) –Shape of molecule pulls electrons equally in all directions Example: methane (CH 4 )

Bonding & Electronegativity Atoms bond according to the difference in their electronegativities Look at table of EN values and find the difference between the two atoms involved (absolute value) If EN difference is… 0 – 0.4 = nonpolar covalent bond 0.5 – 1.6 = polar covalent bond >1.7 = ionic bond

Bonding & Electronegativity

Try to predict the bond type (ionic, polar covalent, nonpolar covalent) using your EN table: FormulaEN valuesEN DifferenceBond Type LiCl Li = 1.0 Cl = ionic CO 2 MgO CH 4 NF 3

Molecular Shape Applies to covalent compounds only. Use VSEPR steps (valence shell electron pair repulsion): 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, identify shape

Molecular Shape # of e- pairs around central atom # of bonding pairs of e- # of lone pairs of e- NameShape 220linear 330 trigonal planar 440tetrahedral 431 trigonal pyramidal 422 angular (bent) 413linear

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)

Molecular Shape Try to predict the shape using VSEPR and your shape chart: 1) BCl 3 2) CH 4 3) NH 3 Lewis Dot Central Atom # e- pairs # bonded pairs # lone pairs Shape

Polar & Nonpolar Molecules Polar molecules have a partial positive and partial negative charge – called a dipole Dependent upon location and nature of the covalent bonds they contain Molecules are considered polar if they: Have a polar covalent bond - EN difference between Have an asymmetrical shape - trigonal pyramidal - linear (with 2 different elements) - angular (bent) If both criteria are not met, it is either a nonpolar molecule or an ionic compound

Polar & Nonpolar Molecules Evaluate and identify each molecule as polar covalent, nonpolar covalent, or ionic: Formula Has a polar covalent bond Has an asymmetrical shape Identity H2OH2O CH 4 NH 3 HCl