Chapter 8 “Covalent Bonding”

Covalent Bonds covalent combination of prefix co- (Latin - “together”), valere, - “to be strong” 2 e-’s shared have strength to hold 2 atoms together particle called a “molecule”

Molecules Some elements in nature are molecules: neutral group of atoms covalently bonded Ex. - air contains O molecules, 2 O atoms joined covalently Called “diatomic molecule” (O2)

(diatomic hydrogen molecule) The nuclei repel each other, (both have + charge) How does H2 form? (diatomic hydrogen molecule) + + + +

How does H2 form? nuclei attraction to e-’s stronger than repulsion of nuclei e-’s shared covalent bond Only NONMETALS! + +

Covalent bonding w/ Fluorine atoms Covalent bonds Nonmetals hold valence e-’s don’t give away e-’s still want NGC share valence e-’s with each other = covalent bonding both atoms count e-’s for NGC Covalent bonding w/ Fluorine atoms

F F Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals F F 8 Valence electrons

single covalent bond between 2 H atoms Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals F F 8 Valence electrons single covalent bond between 2 H atoms

Molecular Compounds gases or liquids at room temperature Compounds bonded covalently called molecular compounds Molecular compounds have lower melting and boiling points Weaker bond than ionic gases or liquids at room temperature a molecular formula: Shows how many atoms of each element a molecule contains

Reminder from Ch. 7 No “molecule” of sodium chloride Ionic cmpds exist as collection of + & - charged ions arranged in repeating 3D patterns.

Molecular Compounds The formula for water is written as H2O The subscript “2” behind hydrogen means 2 atoms of hydrogen subscript 1 omitted Molecular formulas do not tell any information about structure (arrangement of various atoms).

- Page 215 These are some of the different ways to represent ammonia: 3. The ball and stick model is BEST, because it shows 3D arrangement. 1. The molecular formula shows how many atoms of each element are present 2. The structural formula ALSO shows the arrangement of these atoms!

Section 8.2 The Nature of Covalent Bonding

A Single Covalent Bond is... sharing 2 valence e-’s Only nonmetals and hydrogen. Different from ionic bond b/c they actually form molecules. Two specific atoms joined In an ionic solid, you can’t tell which atom e-’s moved from or to

How 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? 2p 1s 2s C Yes, due to electron promotion!

How 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? 2p 1s 2s

Water Another example: water is formed with covalent bonds, by using an electron dot diagram H Each hydrogen has 1 valence electron - Each hydrogen wants 1 more The oxygen has 6 valence electrons - The oxygen wants 2 more They share to make each other complete O

H O Water Put the pieces together The first hydrogen is happy The oxygen still needs one more H O

H O H Water So, a second hydrogen attaches Every atom has full energy levels Note the two “unshared” pairs of electrons H O H

Examples: Conceptual Problem 8.1 on page 220 We’ll do #7 & 8

Double and Triple Covalent Bonds Sometimes atoms share more than one pair of valence e-’s double bond: atoms share 2 pairs of e-’s (4 total) triple bond: atoms share 3 pairs of e-’s (6 total) Table 8.1, p.222 - Know these 7 elements as diatomic: Br2 I2 N2 Cl2 H2 O2 F2

Dot diagram for Carbon dioxide CO2 - Carbon is central atom ( more metallic ) Carbon has 4 valence e-’s Wants 4 more Oxygen has 6 valence e-’s Wants 2 more C O The chemistry of CO2 6:44

Carbon dioxide Attaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 short C O

Carbon dioxide Attaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 short O C O

Carbon dioxide only solution  share more O C O

Carbon dioxide The only solution is to share more O C O

Carbon dioxide The only solution is to share more O C O

Carbon dioxide The only solution is to share more O C O

Carbon dioxide The only solution is to share more O C O

Carbon dioxide The only solution is to share more O C O

O C O Carbon dioxide The only solution is to share more Requires 2 double bonds Each atom can count all the electrons in the bond O C O

O C O Carbon dioxide The only solution is to share more Requires two double bonds Each atom can count all the electrons in the bond 8 valence electrons O C O

O C O Carbon dioxide The only solution is to share more Requires two double bonds Each atom can count all the electrons in the bond 8 valence electrons O C O

How covalent bonds form - Mark Rosengarden Carbon dioxide The only solution is to share more Requires two double bonds Each atom can count all the electrons in the bond 8 valence electrons O C O How covalent bonds form - Mark Rosengarden

How to draw them? Use the handout guidelines: Add up all valence e-’s Count total e-’s needed to make all atoms happy (stable) Subtract; Divide by 2 (tells you how many bonds to draw) Choose central atom (least electronegative) Start w/ most electronegative atom, fill in remaining valence e-’s to fill atoms up

N H Examples 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 = 8 NH3 wants 8+6 = 14 (14-8)/2= 3 bonds 4 atoms with 3 bonds N H

H H N H Examples Draw in the bonds; start with singles All 8 e- accounted for Everything full – DONE! H H N H

Example: HCN HCN: C is central atom N - has 5 valence electrons, wants 8 C - has 4 valence electrons, wants 8 H - has 1 valence electron, wants 2 HCN has 5+4+1 = 10 HCN wants 8+8+2 = 18 (18-10)/2= 4 bonds 3 atoms with 4 bonds – this will require multiple bonds - not to H however

H C N HCN Put single bond between each atom Need to add 2 more bonds Must go between C and N (Hydrogen is full) H C N

H C N HCN Put in single bonds Needs 2 more bonds Must go between C and N, not the H Uses 8 electrons – need 2 more to equal the 10 it has H C N

H C N HCN Put in single bonds Need 2 more bonds Must go between C and N Uses 8 electrons - 2 more to add Must go on the N to fill its octet H C N

Another way of indicating bonds Often use a line to indicate a bond Called a structural formula Each line = 2 valence e-’s H O H H O H =

Other Structural Examples H C N H C O H

A Coordinate Covalent Bond... When one atom donates both electrons in a covalent bond. Carbon monoxide (CO) is a good example: Both the carbon and oxygen give another single electron to share O C

Coordinate Covalent Bond When one atom donates both electrons in a covalent bond. Carbon monoxide (CO) is a good example: Oxygen gives both of these electrons, since it has no more singles to share. This carbon electron moves to make a pair with the other single. C O

Coordinate Covalent Bond When one atom donates both electrons in a covalent bond. Carbon monoxide (CO) The coordinate covalent bond is shown with an arrow as: C O C O

Coordinate covalent bond Most polyatomic cations and anions contain covalent & coordinate covalent bonds Table 8.2, p.224 Sample Problem 8.2, p.225 The ammonium ion (NH4+) can be shown as another example

Bond Dissociation Energies... Total energy required to break bond btwn 2 covalently bonded atoms High dissociation energy usually means compound relatively unreactive, b/c it takes hi energy to break bond

Resonance is... When more than one valid dot diagram is possible. Consider the two ways to draw ozone (O3) Which one is it? Does it go back and forth? It’s hybrid of both, shown by double-headed arrow found in double-bond structures!

Resonance in Ozone Note the different location of the double bond Neither single structure is correct, actually a hybrid of the two. To show it, draw all possible structures, and join them with a double-headed arrow.

Resonance Occurs when more than one valid Lewis structure can be written for particular molecule (due to position of double bond) resonance structures of carbonate ion (used in production of carbonated beverages). The actual structure is an avg (or hybrid) of these structures.

Polyatomic ions – note the different positions of the double bond. Resonance in a carbonate ion (CO32-): Resonance in an acetate ion (C2H3O21-):

The 3 Exceptions to Octet rule For some molecules, it is impossible to satisfy the octet rule #1. usually when there is an odd number of valence electrons NO2 has 17 valence electrons, because the N has 5, and each O contributes 6. Note “N” page 228 It is impossible to satisfy octet rule, yet the stable molecule does exist

Exceptions to Octet rule Another exception: Boron Page 228 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 high electronegativity (it is greedy), so this coordinate bond does not form #3 -Top page 229 examples exist because they are in period 3 or below

Section 8.3 Bonding Theories OBJECTIVES: Describe the relationship between atomic and molecular orbitals.

Section 8.3 Bonding Theories OBJECTIVES: Describe how VSEPR theory helps predict the shapes of molecules.

Molecular Orbitals are... The model for covalent bonding assumes orbitals are those of individual atoms = atomic orbital Orbitals that apply to overall molecule, due to atomic orbital overlap are molecular orbitals bonding orbital is molecular orbital occupied by 2 e-’s of covalent bond

Molecular Orbitals - definitions Sigma bond- 2 atomic orbitals combine to form molecular orbital symmetrical along axis connecting nuclei Pi bond- bonding e-’s likely above and below bond axis (weaker than sigma) Note pictures - next slide Hybridization video – 1:36

- Pages 230 and 231 Sigma bond is symmetrical along axis between 2 nuclei. Pi bond is above and below bond axis - weaker than sigma

Attractive & repulsive forces in H2 bond + nuclei repel e-’s repel Nuclei and e-’s attract Attractive forces stronger than repulsion As nuclei distances decrease, PE decreases If distance decreases more, PE increases b/c increased repulsion Bond forms w/ bond length = interatomic distance (PE minimum)

VSEPR: stands for... Valence Shell Electron Pair Repulsion Predicts 3D shape of molecules The name tells you the theory: Valence shell = outside e-’s Electron Pair repulsion = e- pairs try to get as far away as possible from each other. determines angles of bonds.

VSEPR Based on # of pairs of ve-’s, bonded & unbonded. Unbonded pair called lone pair. CH4 - draw structural formula Has 4 + 4(1) = 8 wants 8 + 4(2) = 16 (16-8)/2 = 4 bonds

VSEPR for methane (a gas): Single bonds fill all atoms. 4 pairs of e-’s pushing away The furthest they can get away is 109.5°

H C H H H 4 atoms bonded 109.5º Basic shape -tetrahedral pyramid w/ triangular base Same shape for everything with 4 pairs H 109.5º C H H H

Other angles, pages 232 - 233 Ammonia (NH3) = 107o Water (H2O) = 105o Carbon dioxide (CO2) = 180o

VSEPR models VSEPR theory video 4:52 - Page 232 Methane has an angle of 109.5o, called tetrahedral Ammonia has an angle of 107o, called pyramidal Note the unshared pair that is repulsion for other electrons.

VSEPR song – 4:33

Hybrid Orbitals Provides info for molecular bonding & shape

Hybridization w/ single bonds Orbitals combine C outer e- configuration 2s2 2p2 but one 2s e- promoted to 2p One 2s e- and 3 2p e-’s Allows bond in methane (CH4) All bonds same due to orbital hybridization Mix to form 4 sp3 hybrid orbitals

Hybridization w/ double bonds Ethene C2H4 – 1 C-C double bond and 4 C-H single bonds sp2 hybrid orbitals form from one 2s and two 2p atomic orbitals of C

Section 8.4 Polar Bonds and Molecules OBJECTIVES: Describe how electronegativity values determine the distribution of charge in a polar molecule.

Section 8.4 Polar Bonds and Molecules OBJECTIVES: Describe what happens to polar molecules when they are placed between oppositely charged metal plates.

Section 8.4 Polar Bonds and Molecules OBJECTIVES: Evaluate the strength of intermolecular attractions compared with the strength of ionic and covalent bonds.

Section 8.4 Polar Bonds and Molecules OBJECTIVES: Identify the reason why network solids have high melting points.

Bond Polarity do covalent bonds always share equally? e-’s pulled - tug-of-war, btwn nuclei In equal sharing (such as diatomic molecules), the bond is called nonpolar covalent bond

polar bond Bond Polarity When 2 different atoms bond covalently, unequal sharing more electronegative atom stronger attraction slightly negative charge polar bond Lower case delta

Electronegativity? Linus Pauling 1901 - 1994 The ability of an atom in a molecule to attract shared electrons to itself. Linus Pauling 1901 - 1994

Bond Polarity Refer to periodic table w/ EN Consider HCl H = electronegativity of 2.1 Cl = electronegativity of 3.0 Polar bond Cl: slight - charge H: slight + charge

Partial charges Bond Polarity d+ d- d+ and d- Partial charges, much less than 1+ or 1- in ionic bond H Cl Partial charges d+ d- d+ and d-

Bond Polarity H Cl Can also be: arrow points to more EN atom KNOW Table 8.3, p.238 shows how electronegativity indicates bond type H Cl

Polar molecules Sample Problem 8.3, p.239 polar bond tends to make entire molecule “polar” areas of “difference” HCl has polar bonds, thus polar molecule molecule w/ 2 poles called dipole, like HCl

Polar molecules effect of polar bonds on polarity of entire molecule depends on molecule shape CO2 has 2 polar bonds linear nonpolar molecule

Polar molecules effect of polar bonds on molecule polarity depends on shape water has 2 polar bonds - bent shape highly electronegative O pulls e- away from H very polar! polar bond of water molecule Chemistry of water 4:46

bonding animations

Polar and non-polar covalent bonds - Mark Rosengarten

Attractions between molecules p. 240 makes solid & liquid molecular cmpds possible weakest called van der Waal’s forces - there are two kinds: #1. Dispersion forces: weakest of all, caused by motion of e- - increases as # e- increases (momentarily more on side of molecule closest to neighboring molecule, neighboring molecule’s e-’s move to opp side) halogens start as gases; bromine (l); iodine (s) – all in Group 7A

#2. Dipole interactions Occurs when polar molecules attracted to each other 2. Dipole interaction happens in water Figure 8.25, page 240 +region of one molecule attracts -region of another molecule

#2. Dipole interactions H F d+ d- H F d+ d- Occur when polar molecules attracted to each other Slightly stronger than dispersion forces Opposites attract, but not completely hooked like ionic solids H F d+ d- H F d+ d-

#2. Dipole Interactions d+ d- d+ d- d+ d- d+ d- d+ d- d+ d- d+ d-

#3. Hydrogen bonding …is the attractive force caused by hydrogen bonded to N, O, F, or Cl N, O, F, and Cl very electronegative, so this is very strong dipole And, the hydrogen shares with lone pair in molecule next to it This is strongest of the intermolecular forces

Order of Intermolecular attraction strengths Dispersion forces are weakest Little stronger are dipole interactions Strongest is H bonding All are weaker than ionic bonds

#3. Hydrogen bonding defined: When H atom is: a) covalently bonded to a highly EN atom, AND b) is also weakly bonded to unshared e- pair of nearby highly EN atom The H is left very e- deficient (only had 1 to start with!) - it shares with something nearby H is ONLY element with no shielding for its nucleus when involved in covalent bond!

Hydrogen Bonding (Shown in water) This H is bonded covalently to: 1) the highly negative O, and 2) a nearby unshared pair.

H bonding allows H2O to be a liquid at room temp

Attractions and properties Why are some chemicals gases, some liquids, some solids? Depends on type of bonding! Table 8.4, page 244 Network solids – solids where all atoms covalently bonded to each other

Attractions and properties Figure 8.28, page 243 Network solids melt at very high temps, or not at all (decomposes) Diamond does not really melt, but vaporizes to a gas at 3500 oC SiC, used in grinding, has melting pt of 2700 oC

Covalent Network Compounds Some covalently bonded substances DO NOT form discrete molecules. Diamond, a network of covalently bonded carbon atoms Graphite, a network of covalently bonded carbon atoms

Ionic/Covalent Bond Song