General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 5.5 Covalent Compounds: Sharing Electrons Chapter 5 Compounds and Their Bonds 2013 Pearson Education, Inc. Lectures 2013 Pearson Education, Inc. Chapter 5, Section 5 2 Covalent bonds form when atoms of nonmetals share electrons to complete octets. Valence electrons are not transferred, but shared to achieve stability. Covalent Bonds 2013 Pearson Education, Inc. Chapter 5, Section 5 3 Formation of H 2 In the simplest covalent molecule, H 2, the H atoms increase attraction as they move closer. share electrons to achieve a stable configuration. form a covalent bond. 2013 Pearson Education, Inc. Chapter 5, Section 5 4 Formation of H 2 2013 Pearson Education, Inc. Chapter 5, Section 5 5 Electron-Dot Formulas of Covalent Molecules In a fluorine (F 2 ) molecule, the F atoms share one of their valence electrons. acquire an octet. form a covalent bond. 2013 Pearson Education, Inc. Chapter 5, Section 5 6 Elements That Exist as Diatomic Molecules These seven elements share electrons to form diatomic, covalent molecules. 2013 Pearson Education, Inc. Chapter 5, Section 5 7 What is the name of each of the following diatomic molecules? H 2 _______________ N 2 _______________ Cl 2 _______________ O 2 _______________ I 2 _______________ Learning Check 2013 Pearson Education, Inc. Chapter 5, Section 5 8 What is the name of each of the following diatomic molecules? H 2 hydrogen N 2 nitrogen Cl 2 chlorine O 2 oxygen I 2 iodine Solution 2013 Pearson Education, Inc. Chapter 5, Section 5 9 The number of covalent bonds a nonmetal forms is usually equal to the number of electrons it needs to acquire a stable electron configuration. Typical bonding patterns for some nonmetals are shown in the table below. Bonding Patterns of Some Nonmetals 2013 Pearson Education, Inc. Chapter 5, Section 5 10 Electron-Dot Formulas for Some Covalent Compounds 2013 Pearson Education, Inc. Chapter 5, Section 5 11 Guide to Drawing Electron-Dot Formulas 2013 Pearson Education, Inc. Chapter 5, Section 5 12 Step 1 Determine the arrangement of atoms. In NH 3, N is the central atom and is bonded to three H atoms. Step 2 Determine the total number of valence electrons. Total valence electrons for NH 3 = 8 e Draw the Electron-Dot Formula for NH 3 2013 Pearson Education, Inc. Chapter 5, Section 5 13 Step 3 Attach each bonded atom to the central atom with a pair of electrons. Draw the Electron-Dot Formula for NH 3 2013 Pearson Education, Inc. Chapter 5, Section 5 14 Step 4 Place the remaining electrons using single or multiple bonds to complete the octets. 8 valence e 6 bonding e = 2 e remaining Use the remaining 2 e to complete the octet around the N atom. Draw the Electron-Dot Formula for NH 3 2013 Pearson Education, Inc. Chapter 5, Section 5 15 Learning Check Draw the electron-dot formula for CCl 4. 2013 Pearson Education, Inc. Chapter 5, Section 5 16 Step 1 Determine the arrangement of atoms. In CCl 4, C is the central atom and is bonded to four Cl atoms. Solution 2013 Pearson Education, Inc. Chapter 5, Section 5 17 Step 2 Determine the total number of valence electrons. Total valence electrons for Solution 2013 Pearson Education, Inc. Chapter 5, Section 5 18 Step 3 Attach each bonded atom to the central atom with a pair of electrons. Solution 2013 Pearson Education, Inc. Chapter 5, Section 5 19 Step 4 Place the remaining electrons, using single or multiple bonds to complete the octets. 32 valence e 8 bonding e = 24 e remaining Use the remaining 24 e to complete the octets around the Cl atoms. Solution 2013 Pearson Education, Inc. Chapter 5, Section 5 20 Exceptions to the Octet Rule Not all atoms have octets. Some can have less than an octet, such as H, which requires only 2 electrons, B, which requires only 3 electrons, and Be, which requires only 4 electrons. Some can have expanded octets, such as P, which can have 10 electrons, S, which can have 12 electrons, and Cl, Br and I, which can have 14 electrons 2013 Pearson Education, Inc. Chapter 5, Section 5 21 Single and Multiple Bonds In many covalent compounds, atoms share two or three pairs of electrons to complete their octets. In a single bond, one pair of electrons is shared. In a double bond, two pairs of electrons are shared. In a triple bond, three pairs of electrons are shared. 2013 Pearson Education, Inc. Chapter 5, Section 5 22 Step 1 Determine the arrangement of atoms. In CS 2, C is the central atom and is bonded to two S atoms. Draw the Electron-Dot Formula for CS 2 2013 Pearson Education, Inc. Chapter 5, Section 5 23 Step 2 Determine the total number of valence electrons. Total valence electrons for Draw the Electron-Dot Formula for CS 2 2013 Pearson Education, Inc. Chapter 5, Section 5 24 Step 3 Attach each bonded atom to the central atom with a pair of electrons. A pair of bonding electrons (single bond) is placed between each S atom and the central C atom. Draw the Electron-Dot Formula for CS 2 2013 Pearson Education, Inc. Chapter 5, Section 5 25 Step 4 Place the remaining electrons using single or multiple bonds to complete the octets. 16 valence e - 4 bonding e = 12 e remaining The remaining 12 electrons are placed as six lone pairs of electrons on both S atoms. However, this does not complete the octet for the C atom. Draw the Electron-Dot Formula for CS 2 2013 Pearson Education, Inc. Chapter 5, Section 5 26 Step 4 Continued: Double and Triple Covalent Bonds: To complete the octet for the C atom, it needs to share an additional lone pair from each of the S atoms, forming a double bond with each S atom. Draw the Electron-Dot Formula for CS 2 2013 Pearson Education, Inc. Chapter 5, Section 5 27 A Nitrogen Molecule has a Triple Bond In a nitrogen molecule, N 2, each N atom shares 3 electrons, each N atom attains an octet, and the sharing of 3 sets of electrons is called a triple bond. 2013 Pearson Education, Inc. Chapter 5, Section 5 28 Resonance structures are two or more electron-dot formulas for the same arrangement of atoms. related by a double-headed arrow ( ). written by changing the location of a double bond between the central atom and a different attached atom. Resonance Structures 2013 Pearson Education, Inc. Chapter 5, Section 5 29 Sulfur dioxide has two resonance structures. Step 1 Determine the arrangement of atoms. In SO 2, the S atom is the central atom. O S O Step 2 Determine the total number of valence electrons. Total valence electrons for Writing Resonance Structures for SO 2 2013 Pearson Education, Inc. Chapter 5, Section 5 30 Step 3 Attach each bonded atom to the central atom with a pair of electrons. Writing Resonance Structures for SO 2 O S O O S O or 2013 Pearson Education, Inc. Chapter 5, Section 5 31 Step 4 Place the remaining electrons using single or multiple bonds to complete the octets. The remaining 14 electrons are drawn as lone pairs of electrons to complete the octets of the O atoms, but not the S atom. Writing Resonance Structures for SO 2 2013 Pearson Education, Inc. Chapter 5, Section 5 32 Step 4 Continued: To complete the octet for S, an additional lone pair from one of the O atoms is shared to form a double bond. Because the shared lone pair of electrons can come from either O atom, two resonance structures can be drawn. Writing Resonance Structures for SO 2 2013 Pearson Education, Inc. Chapter 5, Section 5 33 FNO 2, a rocket propellant, has two resonance structures. One is shown below. What is the other resonance structure? Learning Check 2013 Pearson Education, Inc. Chapter 5, Section 5 34 FNO 2, a rocket propellant, has two resonance structures. One is shown below. What is the other resonance structure? Solution