4 1 Introduction to Covalent Bonding Covalent bonds

4. 1 Introduction to Covalent Bonding Covalent bonds result from the sharing of electrons between two atoms. • A covalent bond is a two-electron bond in which the bonding atoms share valence electrons. • A molecule is a discrete group of atoms held together by covalent bonds. 1

4. 1 Introduction to Covalent Bonding Unshared electron pairs are called nonbonded electron pairs or lone pairs. Atoms share electrons to attain the electronic configuration of the noble gas closest to them in the periodic table. • H shares 2 e−. • Other main group elements share e− until they reach an octet of e− in their outer shell. 2

4. 1 Introduction to Covalent Bonding A. Covalent Bonding and the Periodic Table Lewis structures are electron-dot structures for molecules. They show the location of all valence e−. C N O F Ne 3

4. 1 Introduction to Covalent Bonding A. Covalent Bonding and the Periodic Table How many covalent bonds will a particular atom form? • Hydrogen forms one bond • Atoms with one, two, or three valence e− form one, two, or three bonds, respectively. predicted number of bonds = 8 – number of valence e− P Se 4

4. 1 Covalent Compounds A. Covalent Bonding and the Periodic Table General rule for bonding elements (except for hydrogen, H) Number of bonds + Number of lone pairs = 4 5

Predict The Number of Bonds the Following Atoms Can Make • Br Rn • I S • O P • Ar Se 6

4. 2 Lewis Structures • A molecular formula shows the number and identity of all of the atoms in a compound, but not which atoms are bonded to each other. • A Lewis structure shows the connectivity between atoms, as well as the location of all bonding and nonbonding valence electrons. 7

Writing Lewis Structures H: Only make 1 bond Always save till the end C: 4 Bonds Likes to bond to other C’s first Likes to bond to O and N N: 3 bonds Likes to bond to C Can bond O or N if no C’s O: 2 bonds Likes to bond to C Can bond O or N if no C’s X (Halogens): Same as H Multiple Bonds: Use to finish up octets Can not violate bond rules (i. e. no triple bonded O’s) - There a few exceptions 8

4. 2 Lewis Structures A. Drawing Lewis Structures • General rules for drawing Lewis structures: 1) Draw only valence electrons. 2) Give every main group element (except H) an octet of e−. 3) Give each hydrogen 2 e−. 9

4. 2 Lewis Structures HOW TO Draw a Lewis Structure • Use the common bonding patterns from Figure 4. 1 to arrange the atoms. CH 4 NH 3 10

4. 2 Lewis Structures B. Multiple Bonds • One lone pair of e− can be converted into one bonding pair of e− for each 2 e− needed to complete an octet on a Lewis Structure. • A double bond contains four electrons in two 2 e− bonds. • A triple bond contains six electrons in three 2 e− bonds. 11

H Planning (C 3 H 4) • Avoid adding H’s to bonds that can be completed with multiple bonds: C—C—N C—C—O C—C—C 12

4. 2 Lewis Structures B. Multiple Bonds Example Draw the Lewis Structure for C 2 H 4. Step [1] Step [2] Arrange the atoms. Is Everyone happy? 13

4. 2 Lewis Structures B. Multiple Bonds Step [3] Make everyone happy by adding additional bonds 14

4. 2 Lewis Structures B. Multiple Bonds • Avoid adding H’s to bonds that can be completed with multiple bonds: Example Draw the Lewis Structure for C 3 H 4. Step [1] Step [2] Arrange the atoms. Is Everyone happy? 15

4. 2 Lewis Structures B. Multiple Bonds Step [3] Make everyone happy by adding additional bonds 16

H 2 O HF CH 5 N 17

Problems CH 2 O 3 C 2 H 4 O 2 CHNO 18

Problems CHN HNO 2 C 3 H 6 20

More Problems? C 2 H 3 N C 2 H 6 O C 3 H 3 N 21

Writing Lewis Structures H: Only make 1 bond Always save till the end C: 4 Bonds Likes to bond to other C’s first Likes to bond to O and N N: 3 bonds Likes to bond to C Can bond O or N if no C’s O: 2 bonds Likes to bond to C Can bond O or N if no C’s X (Halogens): Same as H Multiple Bonds: Use to finish up octets Can not violate bond rules (i. e. no triple bonded O’s) - There a few exceptions 22

4. 3 Exceptions to the Octet Rule • Most of the common elements generally follow the octet rule. • H is a notable exception, because it needs only 2 e− in bonding. • Elements in group 3 A do not have enough valence e− to form an octet in a neutral molecule. F F B F 23

4. 3 Exceptions to the Octet Rule • Elements in the third row have empty d orbitals available to accept electrons. • Thus, elements such as P and S may have more than 8 e− around them. O HO P OH OH O HO S OH O 24

Structure to Formula • • Use subscripts to indicate how many Carbon comes first Hydrogen second Other elements in alphabetical order 25

4. 4 Resonance A. Drawing Resonance Structures • Resonance structures are two Lewis structures having the same arrangement of atoms but a different arrangement of electrons. • Two resonance structures of HCO 3−: • Neither Lewis structure is the true structure of HCO 3−. 27

4. 4 Resonance A. Drawing Resonance Structures • The true structure is a hybrid of the two resonance structures. • Resonance stabilizes a molecule by spreading out lone pairs and electron pairs in multiple bonds over a larger region of space. • A molecule or ion that has two or more resonance structures is resonance-stabilized. 28

4. 5 Naming Covalent Compounds HOW TO Name a Covalent Molecule 29

4. 5 Naming Covalent Compounds HOW TO Name a Covalent Molecule Example Name each covalent molecule: (a) NO 2 Step [1] (b) N 2 O 4 Name the first nonmetal by its element name and the second using the suffix “-ide. ” (a) NO 2 (b) N 2 O 4 30

4. 5 Naming Covalent Compounds HOW TO Name a Covalent Molecule Step [2] Add prefixes to show the number of atoms of each element. • Use a prefix from Table 4. 1 for each element. • The prefix “mono-” is usually omitted when only one atom of the first element is present, but it is retained for the second element. • If the combination would place two vowels next to each other, omit the first vowel. mono + oxide = monoxide (not monooxide) 31

4. 5 Naming Covalent Compounds HOW TO Name a Covalent Molecule Example Name each covalent molecule: (a) NO 2 Step [2] (b) N 2 O 4 Name the first nonmetal by its element name and the second using the suffix “-ide. ” (a) NO 2 (b) N 2 O 4 32

Name the Following Compounds PBr 3 H 2 O SO 3 NCl 3 P 2 S 5 33

Name the Following Compounds Selenium dioxide Carbon tetrachloride Dinitrogen Pentoxide Nitrogen Monoxide Phosphorous Triiodide 34

SF 6 Carbon Tetrabromide Dinitrogen Monoxide P 4 O 10

4. 6 Molecular Shape • The Lewis structure shows connection, but nothing about actual shape. • To determine the shape around a given atom, first determine how many groups surround the atom. • A group is either an atom or a lone pair of electrons. • Use the VSEPR theory to determine the shape. • The most stable arrangement keeps the groups as far away from each other as possible. 36

4. 6 Molecular Shape A. Two Groups Around an Atom • Any atom surrounded by only two groups is linear and has a bond angle of 180 o. • An example is CO 2: 37

4. 6 Molecular Shape B. Three Groups Around an Atom • Any atom surrounded by three groups is trigonal planar and has bond angles of 120 o. • An example is H 2 CO (formaldehyde): 38

4. 6 Molecular Shape C. Four Groups Around an Atom • Any atom surrounded by four groups is tetrahedral and has bond angles of 109. 5 o. • An example is CH 4 (methane): 39

4. 6 Molecular Shape C. Four Groups Around an Atom • If the four groups around the atom include one lone pair, the geometry is a trigonal pyramid with bond angles of 107 o, close to 109. 5 o. • An example is NH 3 (ammonia): 40

4. 6 Molecular Shape C. Four Groups Around an Atom • If the four groups around the atom include two lone pairs, the geometry is bent and the bond angle is 105 o (i. e. , close to 109. 5 o). • An example is H 2 O: 41

4. 6 Molecular Shape 42

Predict the Shape of all non-H Atoms 43

Predict the Shape of all non-H Atoms 44

4. 7 Electronegativity and Bond Polarity • Electronegativity is a measure of an atom’s attraction for e− in a bond. • It tells how much a particular atom “wants” e−. 46

For Each Pair Indicate Which atom is more electronegative H or F Na or O Mg or Ba Al or Na I or Cl 47

4. 7 Electronegativity and Bond Polarity • If the electronegativities of two bonded atoms are equal or similar, the bond is nonpolar. • The electrons in the bond are being shared equally between the two atoms. 48

4. 7 Electronegativity and Bond Polarity • Bonding between atoms with different electronegativities yields a polar covalent bond or dipole, a partial separation of charge. 49

4. 7 Electronegativity and Bond Polarity 50

Classify the following bonds as nonpolar, polar covalent, or ionic H—Br O—H Na—S C—C S—Cl Li—F 51

4. 8 Polarity of Molecules The classification of a molecule as polar or nonpolar depends on: • The polarity of the individual bonds • The overall shape of the molecule o Nonpolar molecules generally have: • No polar bonds • Don’t orient themselves in a magnet • Individual bond dipoles that cancel Polar molecules generally have: • One or more polar bonds • Orient in a magnet (+ faces -, - faces +) • Individual bond dipoles that do not cancel 52

4. 8 Polarity of Molecules To determine the polarity of a molecule with two or more polar bonds: 1. Identify all polar bonds based on electronegativity differences. 2. Determine the shape around individual atoms by counting groups. 3. Decide if individual dipoles cancel or reinforce. 53

Polarity 54

Which of the following is polar? Which is non polar?