Covalent Bonding and Nomenclature

1. Covalent Bonding and Nomenclature Chemical Bonding (Predicting Bond Types) Lewis (Electron) Dot Diagrams Binary Molecular Nomenclature Exceptions to the Octet Rule Coordinate Covalent Bonding Resonance Structures Molecular Shapes and Polarity Intermolecular Forces of Attraction

2. What is a chemical bond? A chemical bond is a strong attractive force between atoms or ions in a chemical compound. Back to main menu

3. Why do elements form chemical bonds? • Uncombined elements have relatively high potential energy. • Atoms will gain, lose or share valence electrons in order to chemically combine with other atoms. • By combining with other atoms, atoms decrease potential energy and create more stable arrangements. Back to main menu

4. What two factors determine whether or not a chemical bond will form? • the electron configurations of the atoms involved • the attraction the atoms have for electrons Back to main menu

5. How is the type of chemical bond formed between two atoms determined? The type of chemical bond formed depends upon the degree to which the valence electrons are shared between the atoms. Back to main menu

6. Covalent Bonding In a covalent bond, valence electrons are shared by the atoms. Covalent bonds can be nonpolar or polar. Back to main menu

7. Nonpolar vs. Polar In a nonpolar covalent bond, electrons are shared equally. Bonding which occurs between two atoms of the same element is an example of nonpolar covalent bonding. Examples: H2, Br2, O2, N2, Cl2, I2, F2 In a polar covalent bond, electrons are shared unequally. Back to main menu

8. Ionic Bonding In an ionic bond, valence electrons are transferred between atoms. One atom gains electrons to form a negative ion (anion) and the other atom loses electrons to form a positive ion (cation). Back to main menu

9. Ionic Bonding Which category of elements tends to gain electrons and form negative ions (anions)? nonmetals Which category of elements tends to lose electrons and form positive ions (cations)? metals Back to main menu

10. Using differences in electronegativity to determine bond type Electronegativity is a measure of an atom’s ability to attract electrons when chemically combining with another element. The higher the electronegativity value, the stronger the attraction the atom has for another atom’s electrons. The degree to which bonding between atoms of two elements is ionic or covalent can be estimated by calculating the difference in the elements’ electronegativities (ΔEN). Back to main menu

11. Using differences in electronegativity to determine bond type Back to main menu

12. Back to main menu

13. Example 1: What type of bond would form between an atom of nitrogen and an atom of chlorine? a. Nitrogen has an electronegativity value of 3.0. b. Chlorine has an electronegativity value of 3.0. c. The difference in the electronegativity values for nitrogen and chlorine is ΔEN = - = d. Therefore the type of bond formed would be nonpolar covalent. The electrons would be shared equally. . 3.0 3.0 0.0 Back to main menu

14. Example 2: What type of bond would form between an atom of hydrogen and an atom of chlorine? a. Hydrogen has an electronegativity value of 2.1. b. Chlorine has an electronegativity value of 3.0. c. The difference in the electronegativity values for hydrogen and chlorine is ΔEN = - = d. Therefore the type of bond formed would be polar covalent. The electrons would be shared unequally. . 3.0 2.1 0.9 Back to main menu

15. Dipole A bond formed between atoms which are not shared equally is called a dipole. • In the bond formed between hydrogen and chlorine, the chlorine would form the negative dipole (symbolized by δ-) because it has the higher electronegativity value. • The hydrogen would form the positive dipole (symbolized by δ+) because it has the lower electronegativity value. Back to main menu

16. Example 3: What type of bond would form between an atom of lithium and an atom of chlorine? a. Lithium has an electronegativity value of 1.0. b. Chlorine has an electronegativity value of 3.0. c. The difference in the electronegativity values for lithium and chlorine is ΔEN = - = d. Therefore the type of bond formed would be ionic. The electrons would be transferred between atoms . 3.0 1.0 2.0 Back to main menu

17. Example 3: What type of bond would form between an atom of lithium and an atom of chlorine? The lithium atom would lose electrons and form a positive ion, also known as a cation. The chlorine atom would gain electrons and form a negative ion, also known as an anion. Back to main menu

18. You Try It 1. Complete the following table. 0.8 Ionic 3.2 4.0 3.5 Nonpolar Covalent 0.0 3.5 2.5 Polar Covalent 0.5 3.0 Back to main menu

19. You Try It 2. For each of the bonds in question 1 that were polar covalent, identify the negative dipole (δ-) and the positive dipole (δ+). ICl Iodine is the positive dipole and chlorine is the negative dipole. Back to main menu

20. You Try It 3. Elements that exist as two atoms chemically bonded together are called diatomic elements. The diatomic elements are hydrogen, bromine, oxygen, nitrogen, chlorine, iodine, and fluorine. (You need to memorize the diatomic elements.) What type of chemical bond exists between the diatomic elements? Nonpolar covalent Back to main menu

21. You Try It 4. Using the three classifications of bonds discussed, predict the type of bond that is most likely to be present in compounds made from elements of groups 1 (1A) and 17 (7A). Ionic Back to main menu

22. You Try It 5. Using the three classifications of bonds discussed, predict the type of bond that is most likely to be present in compounds made from elements of groups 16 (6A) and 17 (7A). Polar Covalent Back to main menu

23. You Try It 6. Arrange the following chemical bonds in order of least covalent to most covalent: H-H, H-Cl, H-Br, Li-Cl Li-Cl, H-Cl, H-Br, H-H Back to main menu

24. Drawing Lewis Dot Diagrams for Atoms The electrons that play the most important role in determining whether or not a chemical bond will form are the valence electrons. In a Lewis dot diagram, dots are placed around the chemical symbol of an element to illustrate the valence electrons. The chemical symbol represents the nucleus of the atom. Back to main menu

25. Drawing Lewis Dot Diagrams for Atoms Examples Back to main menu

26. Drawing Lewis Structures for Covalent Compounds Types of Covalent Bonds Single Covalent Bond – one pair of valence electrons is shared. Double Covalent Bond - two pairs of valence electrons are shared. Triple Covalent Bond - three pairs of valence electrons are shared. Back to main menu

27. Example 1: H2 H H H H The two hydrogen atoms will form a single, nonpolar covalent bond. Back to main menu

28. Example 2: O2 O O O O The two oxygen atoms will form a double, nonpolar covalent bond. Back to main menu

29. Example 3: N2 N N N N The two nitrogen atoms will form a triple, nonpolar covalent bond. Back to main menu

30. Example 4: HCl H H Cl Cl Back to main menu

31. Example 5: NH3 H N H H Back to main menu

32. You Try It Back to main menu

33. Structural Formulas Structural formulas can also be used to show the arrangement of atoms in molecules. In a structural formula, dashes are used to represent shared pairs of electrons. Back to main menu

34. Structural Formulas Example: H2 – H H H H Back to main menu

35. Structural Formulas Example: H2S – H S H S H H Back to main menu

36. Structural Formulas Example: N2 N N N ≡ N Back to main menu

37. Binary Molecular Nomenclature Compounds formed when atoms covalently bond are called molecular compounds. Binary molecular compounds are generally composed of two nonmetallic elements. When two nonmetallic elements combine, they often do so in more than one way. For example carbon can combine with oxygen to form carbon dioxide, CO2 and carbon monoxide, CO. Back to main menu

38. Naming Binary Molecular Compounds Prefixes are used to show how many atoms of each element are present in each molecule of the compound. mono- 1 di- 2 tri- 3 tetra- 4 penta- 5 hexa- 6 hepta- 7 octa- 8 nona- 9 deca- 10 Back to main menu

39. Naming Binary Molecular Compounds The names of molecular compounds have this form: (prefix + element name) (prefix + element root + ide) Back to main menu

40. Naming Binary Molecular Compounds The prefix mono is usually omitted if there is just a single atom of the first element. Example: CO2 is carbon dioxide not monocarbon dioxide. If the vowel combinations o-o or a-o appear next to each other in the name, the first of the pair is omitted to simplify the name. Example: N2O is dinitrogen monoxide not dinitrogen monooxide. Back to main menu

41. You Try It Name the following compounds. • CBr4 • Cl2O7 • N2O5 • BCl3 • PCl5 • NO • Carbon tetrabromide • Dichlorine heptoxide • Dinitrogen pentoxide • Boron trichloride • Phosphorus pentachloride • nitrogen monoxide Back to main menu

42. Writing Formulas for Binary Molecular Compounds To write the formula for a binary molecular compound you simply write down the number of atoms of each element indicated by the name. Example: Carbon tetrachloride CCl4 Back to main menu

43. You Try It Write formulas for the following binary molecular compounds. • dinitrogen tetrahydride • carbon disulfide • iodine heptafluoride • sulfur dioxide N2H4 CS2 IF7 SO2 Back to main menu

44. Writing Formulas for Binary Molecular Compounds A few molecular compounds have common names that all scientists use in place of formal names. CH4 is methane H2O is water NH3 is ammonia You need to memorize these. Back to main menu

45. Exceptions to the Octet Rule Some molecules are stable even though the atoms do not all obtain an octet. There are three common exceptions to the octet rule. Back to main menu

46. Exception #1 In some molecules the central atom has less than eight valence electrons. This is called an incomplete octet. Incomplete octets are common in covalent compounds in which the central atom is beryllium, boron or aluminum. Back to main menu

47. Exception #1 Example: BeH2 Beryllium has only four electrons around it. Back to main menu

48. Exception #2 Molecules almost always have an even number of electrons, allowing electrons to be paired, but there are some exception in which there are an odd number of electrons. These exceptions usually involve nitrogen. Back to main menu

49. Exception #2 Example: NO You will not be expected to draw exceptions with odd numbers of electrons in this course. Back to main menu

50. Exception #3 In some molecules the central atom has more than eight valence electrons. This is called an expanded octet. Some common central elements that have expanded octets are sulfur, chlorine, bromine, iodine, xenon, phosphorus, and arsenic. Back to main menu