Section 8.1 Molecular CompoundsOBJECTIVES:• Distinguish between the melting points and boiling points of molecular compounds and ionic compounds.• Describe the information provided by a molecular formula.Bonds are… Forces that hold groups of atoms together and make them function as a unit. Two types: 1) Ionic bonds – transfer of electrons (gained or lost; makes formula unit) 2) Covalent bonds – ________________________ of electrons. The resulting particle is called a ___________________________________Covalent Bonds• The word covalent is a combination of the prefix co- (from Latin com, meaning “with” or “together”), and the verb valere, meaning “to be strong”.• Two electrons shared together have the strength to hold two atoms together in a bond.Molecules• Many elements found in nature are in the form of molecules:• a ______________________________________________________________ joined together by covalent bonds.• For example, air contains oxygen molecules, consisting of two oxygen atoms joined covalently• Called a “_______________________________________________________” (O2)How does H2 form?(diatomic hydrogen molecule)• The nuclei repel each other, since they both have a positive charge (like charges repel).
+ + + +• But, the nuclei are attracted to the electrons• They _______________________ the electrons, and this is called a “_____________________________________________”, and involves only ____________________________________ + +Covalent bonds• Nonmetals hold on to their valence electrons.• They can’t give away electrons to bond.• But still want noble gas configuration.• Get it by _______________________________________________with each other = ___________________________________________• By sharing, both atoms get to count the electrons toward a noble gas configuration.• Fluorine has seven valence electrons (but would like to have 8)• A second atom also has seven• By sharing electrons…• …both end with full orbitals
Molecular Compounds• Compounds that are bonded covalently (like in water, or carbon dioxide) are called ________________________________________________• Molecular compounds tend to have relatively __________________________ melting and boiling points than ionic compounds – this is not as strong a bond as ionic• Thus, molecular compounds tend to be _____________________________________ at room temperature • Ionic compounds were solids• A molecular compound have a ___________________________________________ o Shows how many atoms of each element a molecule contains• The formula for water is written as H2O• The ___________________________________ behind hydrogen means there are 2 atoms of hydrogen; if there is only one atom, the subscript 1 is omitted• Molecular formulas do not tell any information about the structure (the arrangement of the various atoms).
Section 8.2 The Nature of Covalent BondingOBJECTIVES: • Describe how electrons are shared to form covalent bonds, and identify exceptions to the octet rule. • Demonstrate how electron dot structures represent shared electrons. • Describe how atoms form double or triple covalent bonds. • Distinguish between a covalent bond and a coordinate covalent bond, and describe how the strength of a covalent bond is related to its bond dissociation energy. • Describe how oxygen atoms are bonded in ozone.A Single Covalent Bond is... • A sharing of two valence electrons. • Only _____________________________ and ____________________________ • Different from an ionic bond because they actually form molecules. • Two specific atoms are joined. • In an ionic solid, you can’t tell which atom the electrons moved from or toHow to show the formation… • It’s like a jigsaw puzzle. • You put the pieces together to end up with the right formula. • Carbon is a special example - can it really share 4 electrons: 1s22s22p2? o Yes, due to electron promotion! • Another example: lets show how water is formed with covalent bonds, by using an electron dot diagramWater Each hydrogen has 1 valence electronH - Each hydrogen wants 1 more The oxygen has 6 valence electrons - The oxygen wants 2 more They share to make each other completeO
• Put the pieces together • The first hydrogen is happy • The oxygen still needs one more • So, a second hydrogen attaches • Every atom has full energy levelsMultiple Bonds • Sometimes atoms share more than one pair of valence electrons. • A ____________________________________________ is when atoms share two pairs of electrons (4 total) • A ____________________________________________ is when atoms share three pairs of electrons (6 total) • Table 8.1, p.222 - Know these 7 elements as _____________________________:Dot diagram for Carbon dioxide• CO2 - Carbon is central atom ( more metallic )• Carbon has 4 valence electrons• Wants 4 more• Oxygen has 6 valence electrons• Wants 2 moreCO
Carbon dioxide • Attaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 short • Attaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 short• The only solution is to share more• Requires two double bonds• Each atom can count all the electrons in the bond How to draw them? Use these guidelines: 1) Add up all the valence electrons. 2) Count up the total number of electrons to make all atoms happy. 3) Subtract; then Divide by 2 4) Tells you how many bonds to draw 5) Fill in the rest of the valence electrons to fill atoms up.
Example • NH3, which is ammonia • N – central atom; has 5 valence electrons, wants 8 • H - has 1 (x3) valence electrons, wants 2 (x3) • NH3 has 5+3 = _______ • NH3 wants 8+6 = 14 • (14-8)/2= ___________________ • 4 atoms with _________ bonds • Draw in the bonds; start with singles • All 8 electrons are accounted for • Everything is full – done with this oneExample: HCN • HCN: C is central atom • N - has _____ valence electrons, wants _____ • C - has _____ valence electrons, wants _____ • H - has _____ valence electron, wants _____ • HCN has _____+_____+_____ = _____ • HCN wants _____+_____+_____=_____ • (_____-_____)/2= _____ bonds • 3 atoms with _____ bonds – this will require multiple bonds - not to H howeverHCN • Put single bond between each atom • Need to add 2 more bonds • Must go between C and N (Hydrogen is full)
• Uses 8 electrons – need 2 more to equal the 10 it has • Must go on the N to fill its octetAnother way of indicating bonds • Often use a line to indicate a bond • Called a ________________________________________________ • Each line is ________ valence electronsOther Structural ExamplesA Coordinate Covalent Bond... • When one atom donates both electrons in a covalent bond. • Carbon monoxide (CO) is a good example:
Coordinate covalent bond • Most polyatomic cations and anions contain covalent and coordinate covalent bonds • Table 16.2, p.445 • Sample Problem 16.2, p.446 • The ammonium ion (NH41+) can be shown as another exampleBond Dissociation Energies... • _________________________________________________________________ _________________________________________________________________ __ • High dissociation energy usually means the chemical is relatively _________________________________, because it takes a lot of energy to break it down.Resonance is... • When ____________________________________ valid dot diagram is possible. • Consider the two ways to draw ozone (O3) • Which one is it? Does it go back and forth? • It is a hybrid of both, like a mule; and shown by a double-headed arrow • found in double-bond structures!Resonance in Ozone • Neither structure is correct, it is actually a hybrid of the two. To show it, draw all varieties possible, and join them with a double-headed arrow.Resonance
• Occurs when more than one valid Lewis structure can be written for a particular molecule (due to position of double bond) • These are resonance structures of benzene. • The actual structure is an average (or hybrid) of these structures.The 3 Exceptions to Octet rule • For some molecules, it is impossible to satisfy the octet rule#1. usually when there is an ________________________________ of valence electrons – NO2 has 17 valence electrons, because the N has 5, and each O contributes 6. • It is impossible to satisfy octet rule, yet the stable molecule does exist• Another exception: Boron • Page 451 shows boron trifluoride, and note that one of the fluorides might be able to make a coordinate covalent bond to fulfill the boron #2 -But fluorine has a ________________________________________ (it is greedy), so this coordinate bond does not form • #3 -Top page 229 examples exist because they are in period 3 or beyond