1 Covalent Bonding Note: Students and classrooms with iPads should - PDF document

9 Which of the following consists of small individual molecules? A C(diamond) B SiO 2 C Cu 2 O Answer D Na SO 3 E 34

10 Which of the following substances has both ionic and covalent bonding within the crystal? A Cu B CuCO 3 C LiCl Ba D Answer BaF 2 E 35

Naming Binary Molecular Compounds Return to Table of Contents 36

Naming Binary Molecular Compounds Use prefixes to indicate the number the atoms. All end in "ide" Examples NO 2 nitrogen dioxide P 2 O 5 diphosphorous pentoxide ( penta­oxide­­>pentoxide) 37

Naming Binary Molecular Compounds Look on your reference sheets for the prefixes. The atom with the lower electronegativity is usually written first. If there is only one of the first atom, the mono­ is left off. Examples CO carbon monoxide CO 2 carbon dioxide 38

11 Chlorine monoxide is A ClO 2 B ClO OCl C D O 2 Cl Answer 39

12 Dinitrogen tetroxide is A NO 2 B N 2 O 4 C NO 3 ­ D N 4 O 2 Answer 40

13 H 2 O is A Hydrogen monoxide B Dihydrogen monoxide C Hydrogen oxide D Hydrogen dioxide Answer 41

14 SO 3 is A sulfate B sulfur oxide C sulfur trioxide D sulfite Answer 42

15 MgO is A monomagnesium monoxide B magnesium monoxide C monomagnesium oxide D magnesium oxide Answer 43

16 P 4 O 10 is A Phosphorous pentoxide B Tetraphosphorous decoxide C Phosphorous oxide D Phosphate Answer 44

Lewis Structures Return to Table of Contents 45

Lewis Structures Lewis structures are diagrams that show valence electrons as dots. Lewis structures are also known as Lewis dot or electron dot diagrams. Note that no electrons are paired until after the fourth one. 46

17 How many valence electrons does nitrogen have? A 2 B 3 C 4 D 5 Answer E 7 47

18 The Lewis structure for nitrogen is N True False Answer 48

The Octet Rule Recall that atoms tend towards having the electron configuration of a noble gas.For most atoms, that means having 8 valence electrons. The Octet Rule also applies to molecular compounds. In covalent bonding, an atom will share electrons in an effort to obtain eight electrons around it (except hydrogen which will attempt to obtain 2 valence electrons). A pair of valence Exceptions to the Octet Rule electrons that is not shared between atoms H needs 2e­ is called an unshared Be needs 4e­ pair, also known as a lone pair or a needs 6e­ B nonbonding pair. 49

How do electron dot structures represent shared electrons? An electron dot structure such as H:H represents the shared pair of electrons of the covalent bond by two dots. Shared pair of electrons H + H H H Hydrogen Hydrogen Hydrogen atom atom molecule 1s H H Hydrogen molecule 1s 50

Structural Formulas A structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded atoms. As in the example below, one shared pair of electrons is represented by one dash. Shared pair of electrons H H Hydrogen molecule H H 51

19 How many electrons are shared by two atoms to create a single covalent bond? A 2 B 1 Answer 52

Single Covalent Bonds The halogens form single covalent bonds in their diatomic molecules. Fluorine is one example. −− > F F F + F F F OR Fluorine Fluorine Fluorine molecule atom atom 1s 2s 2p Fluorine molecule 1s 2s 2p 53

Lewis Structure of H 2 O In a water molecule, each hydrogen and oxygen atom attains a noble­gas configuration by sharing electrons. The water molecule has two unshared, or lone, pairs of electrons. 2 H + O ­­> O H or O H Oxygen H Hydrogen Water atom H atoms molecule 1s 2s 2p O Water molecule 1s 1s H H 54

Lewis Structures of NH 3 In the ammonia molecule, NH 3 , each atom attains a noble­ gas configuration by sharing electrons. This molecule has one unshared pair of electrons. H H 3 H + N ­­> N H or N H H H Hydrogen Nitrogen Ammonia atom atom molecule 1s 2s 2p N Ammonia molecule 1s 1s 1s H H 55

Drawing Lewis Structures The P atom has 5 valence 1. Find the total number of valence electrons. electrons in the polyatomic ion or P molecule. A Cl atom has 7, and there are three of them. Cl Cl Cl The total number of valence electrons is: 56

Drawing Lewis Structures 2. The central atom is the least P has an electronegativity of 2.1 and Cl electronegative element has an electronegativity of 3.0 (excluding hydrogen). P will be the central atom. P The Cl atoms will surround the P atom. Cl Cl Cl The single bonds are shown as single lines. 3. Connect the other atoms to it by single bonds. 57

Drawing Lewis Structures 4. Count each single bond as a pair (two) of electrons. 5. Add electons to the outer atoms to give each one 8 (a full shell), or just 2 electrons for hydrogen. 6. Do the same for the central atom. 7. Check: Does each atom have a full outer shell (8 except, 2 for hydrogen)? Have you used up all the valence electrons? Have you used too many electrons? 58

Drawing Lewis Structures NH 3 1. Find the total number of valence electrons in the polyatomic ion or molecule. The N atom has 5 valence electrons and each of the three H atoms has 1 so the total number of valence electrons is, 5 + 3(1) = 8 59

Drawing Lewis Structures NH 3 2. The central atom is the least H can never be the central atom electronegative element so N must be (excluding hydrogen because it can only have one bond). The H atoms will surround the N atom. 3. Connect the other atoms to it by single bonds. The single bonds are shown as single lines. H N H H 60

Drawing Lewis Structures 4. Count each single bond as a pair H N H (two) electrons. Now add electons to the outer atoms to give each one H a full shell (2 in the case of H). 5. Next, do the same for the central atom. Each H already has two electrons, so that's done. But we have to add electrons to N to 6. Check: make 8. Does each atom have a full outer shell ? H N H 7. Have you used up all the valence electrons you started with? Have H you used too many electrons? 61

20 How many total valence electrons does H 2 O have? 8 A 10 B Answer 12 C 14 D 62

21 Which element in H 2 O is the least electronegative? H A O B Answer 63

22 Which of the following is the correct Lewis Structure for H 2 O? A H O H 1. Find the total number of valence electrons: O 2. Central atom is the least electronegative: B H H 3. Connect the other atoms to it by single bonds. 4. Count each single bond as a pair of electrons. H H O C 5. Add electrons to the outer atoms to give each one 8 Answer (except H only gets 2). D 6. Add electrons to the central atom to give it 8. O 7. Check to make sure all valence electrons are used. H H 64

23 Which of the following is the correct Lewis Structure for C 2 H 6 ? A H H H H H H C C H H C C H H B Answer H H C C H H H H H H C C C H H H H H H D 65

Lewis Structures for ions If you are drawing the Lewis Structure for an ION ... A negative ion has extra electrons , add the charge of the ion to your valence electron count. ­ has 1(7) + 2(6) + 1 = 20 electrons ClO 2 A positive ion is missing electrons , subtract the charge of the ion to your valence electron count. + has 1(5) + 4(1) ­1 = 8 electrons NH 4 66

24 2­ have? How many valence electrons does CO 3 12 A 18 B C 24 Answer 26 D 67

25 How many valence electrons does H 3 O + have? 8 A 9 B C 10 Answer 11 D 68

Formal Charge The "Formal Charge" method tells us how the electrons are distributed within a molecule. For example, depending on how the electrons are shared, some atoms may have more electrons than others resulting in a semi­charged state for that atom. Formal Charge = # of valence electrons ­ # of electrons atom possesses within the lewis structure. O FC for P : 5 ­ 4 = +1 (count each bond as one) O P O FC for each O : 6 ­ 7 = ­1 (count each bond as one) O 3­ Note: The charges must add to the charge of the molecule. So for PO 4 1 P atom x +1 = +1 + 4 O atoms x ­1 = ­4 +1 + ­4 = ­3 69

Formal Charge The best Lewis structure will have the formal charge = 0 on each atom. However, if the molecule carries a charge, the more electronegative atoms should carry a charge as they have the greater attraction for electrons! The oxygen is more electronegative so it makes sense that it carries the negative charge. Each bond is counted as one in a formal charge calculation as each atom forming part of the bond contributes just one electron to that bond. [ O ­ H ] ­1 FC on O = 6­7 = ­1 FC on H = 1­1 = 0 O H 70

Formal Charge Example: Below are two possible lewis structure for the phosphate 3­ . Which Lewis structure is considered to more closely ion, PO 4 represent the actual molecule based on formal charge calculations? O O P O O O P O O O Structure 2 Structure 1 Structure 2 is superior as all formal charges = 0 whereas in structure 1, the P carries a +1 charge and each oxygen carries a ­1 charge slide for answer 71

26 Which of the following would be the formal charge on the N in the ammonium ion? A +1 B 0 C ­1 Answer D ­2 E ­3 72

27 In which of the following molecules would N carry a non­zero formal charge? HCN A B NH 3 NO 3 ­ C NO 2 ­ D NH 4 + E Answer 73

Lewis Structures Draw the Lewis dot structure for the sulfate ion, SO 4 2­ , and find the formal charge on each atom. FC on S = 6­4 = +2 FC on O = 6­7 = ­1 ­­­­­­­­­­­­­­­­­­­­­­­­­­ 1(+2) + 4(­1) = ­2 slide for answer 74

Lewis Structures Draw the Lewis dot structure for the hydronium ion, H 3 O + and find the formal charge on each atom. FC on O = 6­5 = +1 FC on H = 1­1 = 0 * note how in this case the more electronegative atom (O) is carrying a + charge relative to H. This demonstrates the theory is imperfect. 75

Draw a Lewis Structure CO 2 F H N Cl Slide for Answer C P C O O Si O S We ran out of electrons, but B carbon does not have an octet Se yet! Xe Now What? I 76

Double and Triple Covalent Bonds Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons. A bond that involves two shared pairs of electrons is a double covalent bond. A bond formed by sharing three pairs of electrons is a triple covalent bond. 77

Double and Triple Covalent Bonds Carbon Dioxide, CO 2 1. Determine the # of valence electrons. 1 (4) + 2 (6) = 16 e ­ 2. Form Single Bonds This leaves 12 electrons, 6 pairs O C O 3. Place lone pairs on oxygen atoms to give each 8. O C O 78

Carbon Dioxide, CO 2 4. Check: We had 16 electrons to O C O work with; how many have we used? 5. There are too many electrons in our drawing. We must form DOUBLE O C O BONDS between C and O. Instead of sharing only 1 pair, a double bond shares 2 pairs. So one pair is taken away from each atom O C O and replaced with another bond. 79

Covalent Bond Length 80

Covalent Bond Energy Bond Type Bond Energy Bond Type Bond Energy N N C C 163 kJ 348 kJ N N 418 kJ C C 614 kJ N N 941 kJ C C 839 kJ It requires more energy to break double and triple bonds compared to single bonds. Triple bonds are the strongest of the three. 81

Covalent Bond Energies 82

Covalent Bonds Comparison Bond Bond Type of Electrons Strength Length Bond shared weak long 2 4 intermediate intermediate strong 6 short 83

28 As the number of bonds between a pair of atoms increases, the distance between the atoms: A increases B decreases C remains unchanged Answer D varies, depending on the atoms 84

29 As the number of bonds between a pair of atoms increases, the strength of the bond between the atoms: A increases B decreases C remains unchanged Answer D varies, depending on the atoms 85

30 As the number of bonds between a pair of atoms increases, the energy of the bond between the atoms: A increases B decreases C remains unchanged Answer D varies, depending on the atoms 86

31 How many electrons are shared by two atoms to create a single bond? Answer 87

32 How many electrons are shared by two atoms to create a double bond? Answer 88

33 How many electrons are shared by two atoms to create a triple bond? Answer 89

34 Using Lewis structure drawings, determine which molecule below would have the shortest bond length between atoms? A O 2 B F 2 C Cl 2 Answer D CO I 2 E 90

35 Which of the following molecules would have the longest C­O bond length? Use Lewis structures. A CO B CO 2 C H 2 CO D CH 3 OH E The lengths are all the same Answer 91

Writing Lewis Structures If you run out of electrons before the central atom has an octet……form multiple bonds until it does. 92

Bonding of O 2 Oxygen molecule O + O ­­> O O or O O Oxygen Oxygen Oxygen atom atom molecule 1s 2s 2p Oxygen O molecule O 1s 2s 2p 93

Draw a Lewis Structure CO F H Slide for Answer N Cl C P O C Si O Carbon has the lower electronegativity, so we will consider it the "central" atom... S B Se Xe I 94

Coordinate Covalent Bonds 95

Coordinate Covalent Bonds In carbon monoxide, oxygen has a stable configuration but the carbon does not. C + O −−> C O Carbon Carbon Oxygen atom monoxide atom 1s 2s 2p C O Carbon monoxide molecule 1s 2s 2p 96

Coordinate Covalent Bonds A coordinate covalent bond is a covalent bond in which one atom contributes both bonding electrons. In a structural formula, you can show coordinate covalent bonds as arrows that point from the atom donating the pair of electrons to the atom receiving them. In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms. Carbon has 4 valence electrons, oxygen has 6. 97

F 2 Draw a Lewis Structure Slide for Answer F F H N Cl C P F F Si O S B Se Xe I 98

Diatomic Molecules A molecule is a neutral group of atoms joined together by covalent bonds. Air contains oxygen molecules. A diatomic molecule is a molecule consisting of two atoms. Certain elements do not exist as single atoms; they always appear as pairs. When atoms turn into ions, this NO LONGER HAPPENS! H 2 H H Hydrogen Nitrogen Remember: Oxygen N N Fluorine HONClBrIF N 2 Chlorine Bromine O 2 Iodine O O 99

36 On the periodic table below, mark which elements exist as diatomic molecules. Note the pattern. 100