Chapter 6. Table of Contents. Section 1 Covalent Bonds. Section 2 Drawing and Naming Molecules. Section 3 Molecular Shapes. Covalent Compounds

Transcription

1 Covalent Compounds Table of Contents Section 1 Covalent Bonds Section 2 Drawing and Naming Molecules Section 3 Molecular Shapes

2 Section 1 Covalent Bonds Bellringer Make a list of the elements that form ionic bonds. Note that most ionic bonds contain a metal and a nonmetal.

3 Section 1 Covalent Bonds Objectives Explain the role and location of electrons in a covalent bond. Describe the change in energy and stability that takes place as a covalent bond forms. Distinguish between nonpolar and polar covalent bonds based on electronegativity differences.

4 Section 1 Covalent Bonds Objectives, continued Compare the physical properties of substances that have different bond types, and relate bond types to electronegativity differences.

5 Section 1 Covalent Bonds Sharing Electrons When an ionic bond forms, electrons are rearranged and are transferred from one atom to another to form charged ions. In another kind of change involving electrons, the neutral atoms share electrons.

6 Section 1 Covalent Bonds Sharing Electrons, continued Forming Molecular Orbitals A covalent bond is a bond formed when atoms share one or more pairs of electrons. The shared electrons move within a space called a molecular orbital. A molecular orbital is the region of high probability that is occupied by an individual electron as it travels around one of two or more associated nuclei.

7 Formation of a Covalent Bond

8 Section 1 Covalent Bonds Energy and Stability Energy Is Released When Atoms Form a Covalent Bond

9 Section 1 Covalent Bonds Energy and Stability, continued Potential Energy Determines Bond Length When two bonded hydrogen atoms are at their lowest potential energy, the distance between them is 75 pm. The bond length is the distance between two bonded atoms at their minimum potential energy. However, the two nuclei in a covalent bond vibrate back and forth. The bond length is thus the average distance between the two nuclei.

10 Section 1 Covalent Bonds Energy and Stability, continued Bonded Atoms Vibrate, and Bonds Vary in Strength The bond length is the average distance between two nuclei in a covalent bond. At a bond length of 75 pm, the potential energy of H 2 is 436 kj/mol. Thus 436 kj of energy must be supplied to break the bonds in 1 mol of H 2 molecules. The energy required to break a bond between two atoms is the bond energy. Bonds that have the higher bond energies (stronger bonds) have the shorter bond lengths.

11 Section 1 Covalent Bonds Electronegativity and Covalent Bonding In covalent bonds between two different atoms, the atoms often have different attractions for shared electrons. Electronegativity values are a useful tool to predict what kind of bond will form.

12 Section 1 Covalent Bonds Electronegativity and Covalent Bonding, continued Atoms Share Electrons Equally or Unequally When the electronegativity values of two bonding atoms are similar, bonding electrons are shared equally. A covalent bond in which the bonding electrons in the molecular orbital are shared equally is a nonpolar covalent bond.

13 Section 1 Covalent Bonds Electronegativity and Covalent Bonding, continued Atoms Share Electrons Equally or Unequally, continued When the electronegativity values of two bonding atoms are different, bonding electrons are shared unequally. A covalent bond in which the bonding electrons in the molecular orbital are shared unequally is a polar covalent bond.

14 Predicting Bond Character from Electronegativity Differences

15 Section 1 Covalent Bonds Electronegativity and Covalent Bonding, continued Polar Molecules Have Positive and Negative Ends In a polar covalent bond, the ends of the bond have opposite partial charges. A molecule in which one end has a partial positive charge and the other end has a partial negative charge is called a dipole. In a polar covalent bond, the shared pair of electrons is not transferred completely. Instead, it is more likely to be found near the more electronegative atom.

16 Section 1 Covalent Bonds Electronegativity and Covalent Bonding, continued Polar Molecules Have Positive and Negative Ends, continued The symbol δ is used to mean partial. δ + is used to show a partial positive charge δ is used to show a partial negative charge example: H δ+ F δ Because the F atom has a partial negative charge, the electron pair is more likely to be found nearer to the fluorine atom

17 Section 1 Covalent Bonds Polarity Is Related to Bond Strength In general, the greater the electronegativity difference, the greater the polarity and the stronger the bond.

18 Section 1 Covalent Bonds Electronegativity and Bond Types Differences in electronegativity values provide one model that can tell you which type of bond two atoms will form. Another general rule states: A covalent bond forms between two nonmetals. An ionic bond forms between a nonmetal and a metal.

19 Section 1 Covalent Bonds Properties of Substances Depend on Bond Type The type of bond that forms (metallic, ionic, or covalent) determines the properties of the substance. The difference in the strength of attraction between the basic units of ionic and covalent substances causes these types of substances to have different properties.

20 Properties of Substances with Metallic, Ionic, and Covalent Bonds

21 Section 2 Drawing and Naming Molecules Bellringer Classify the following compounds according to the type of bonds they contain: NO CO HF NaCl HBr NaI

22 Section 2 Drawing and Naming Molecules Objectives Draw Lewis structures to show the arrangement of valence electrons among atoms in molecules and polyatomic ions. Explain the differences between single, double, and triple covalent bonds. Draw resonance structures for simple molecules and polyatomic ions, and recognize when they are required.

23 Section 2 Drawing and Naming Molecules Objectives, continued Name binary inorganic covalent compounds by using prefixes, roots, and suffixes.

24 Section 2 Drawing and Naming Molecules Lewis Electron-Dot Structures Valence electrons are the electrons in the outermost energy level of an atom. A Lewis structure is a structural formula in which valence electrons are represented by dots. In Lewis structures, dot pairs or dashes between two atomic symbols represent pairs in covalent bonds.

25 Section 2 Drawing and Naming Molecules Lewis Electron-Dot Structures, continued Lewis Structures Show Valence Electrons As you go from element to element across a period, you add a dot to each side of the element s symbol.

26 Section 2 Drawing and Naming Molecules Lewis Electron-Dot Structures, continued Lewis Structures Show Valence Electrons, continued You do not begin to pair dots until all four sides of the element s symbol have a dot.

27 Section 2 Drawing and Naming Molecules Lewis Electron-Dot Structures, continued Lewis Structures Show Valence Electrons, continued An element with an octet of valence electrons has a stable configuration. The tendency of bonded atoms to have octets of valence electrons is called the octet rule.

28 Section 2 Drawing and Naming Molecules Lewis Electron-Dot Structures, continued Lewis Structures Show Valence Electrons, continued When two chlorine atoms form a covalent bond, each atom contributes one electron to a shared pair. An unshared pair, or a lone pair, is a nonbonding pair of electrons in the valence shell of an atom.

29 Section 2 Drawing and Naming Molecules Lewis Electron-Dot Structures, continued Lewis Structures Show Valence Electrons, continued A single bond is a covalent bond in which two atoms share one pair of electrons The electrons can pair in any order. However, any unpaired electrons are usually filled in to show how they will form a covalent bond.

30 Section 2 Drawing and Naming Molecules Drawing Lewis Structures with Single Bonds Sample Problem A Solution Draw each atom s Lewis structure, and count the total number of valence electrons. number of dots: 14 Arrange the Lewis structure so that carbon is the central atom.

31 Section 2 Drawing and Naming Molecules Multiple Bonds For O 2 to make an octet, each atom needs two more electrons. The two atoms share four electrons. A double bond is a covalent bond in which two atoms share two pairs of electrons.

32 Section 2 Drawing and Naming Molecules Multiple Bonds, continued For N 2 to make an octet, each atom needs three more electrons. The two atoms share six electrons. A triple bond is a covalent bond in which two atoms share three pairs of electrons.

33 Section 2 Drawing and Naming Molecules Naming Covalent Compounds, continued This system of prefixes is used to show the number of atoms of each element in the molecule.

34 Section 2 Drawing and Naming Molecules Naming Covalent Compounds, continued Prefixes can be used to show the numbers of each type of atom in diphosphorus pentasulfide.

35 Section 3 Molecular Shapes Predicting Molecular Shapes Sample Problem D Solution Draw the Lewis structure for H 2 O. Count the number of shared and unshared pairs of electrons around the central atom. H 2 O has two shared pairs and two unshared pairs.

36 Section 3 Molecular Shapes Molecular Shape Affects a Substance s Properties Shape Affects Polarity One property that shape determines is the polarity of a molecule. The polarity of a molecule that has more than two atoms depends on the polarity of each bond and the way the bonds are arranged in space.

37 Section 3 Molecular Shapes Molecular Shape Affects a Substance s Properties, continued Shape Affects Polarity, continued If two dipoles are arranged in opposite directions, they will cancel each other. If two dipoles are arranged at an angle, they will not cancel each other. Compare the molecules of nonpolar carbon dioxide, CO 2, which has a linear shape, and polar water, H 2 O, which has a bent shape.

38 Molecular Shape Affects Polarity

39 Standardized Test Preparation Understanding Concepts 1. Which of these combinations is likely to have a polar covalent bond? A. two atoms of similar size B. two atoms of very different size C. two atoms with different electronegativities D. two atoms with the same number of electrons

40 Standardized Test Preparation Understanding Concepts 1. Which of these combinations is likely to have a polar covalent bond? A. two atoms of similar size B. two atoms of very different size C. two atoms with different electronegativities D. two atoms with the same number of electrons

41 Standardized Test Preparation Understanding Concepts 3. How many electrons are shared in a double covalent bond? A. 2 B. 4 C. 6 D. 8

42 Standardized Test Preparation Understanding Concepts 3. How many electrons are shared in a double covalent bond? A. 2 B. 4 C. 6 D. 8

43 Standardized Test Preparation Reading Skills 9. If water is not a good conductor of electric current, why is it dangerous to handle an electrical appliance when your hands are wet or when you are standing on wet ground?

44 Standardized Test Preparation Reading Skills 9. If water is not a good conductor of electric current, why is it dangerous to handle an electrical appliance when your hands are wet or when you are standing on wet ground? Answer: Because even a small amount of ionic compounds dissolved in water makes it a good conductor. The salts in your body or on the ground are enough to cause the water to carry a current.