Chemical Bonding

1. Chemical Bonding Unit 5

2. Introduction to bonding

3. How do atoms interact? • Atoms are generally found in nature in combination held together by chemical bonds. • A chemical bond is an electrical attraction between the nuclei and valence electrons of different atom that binds the atoms together so that they behave as one unit. • There are two types of chemical bonds: ionic, and covalent.

4. Atom-atom interactions • There are 2 ways to look at atom-atom interactions • 1) Balancing the opposing forces of repulsion and attraction • As the atoms come closer together there is a repulsion between the negative e- clouds of each atom • Simultaneously there is an attraction between the positive nuclei and the negative electron clouds

5. Introduction to bonding • As the optimum distance is achieved that balances these forces, there is a release of potential energy • The atoms vibrate within the window of maximum attraction/minimum repulsion • The more energy released the stronger the connecting bond between the atoms

6. Introduction to bonding

7. Introduction to bonding • 2) Atoms want to achieve the most stable arrangement of valence electrons. • By rearranging the electrons so that each atom achieves a noble gas-like arrangement of its electrons creates a pair of stable atoms (only occurs when bonded)

8. Introduction to bonding • To achieve a stable arrangement one or more valence electrons are transferred between two atoms • Basis for ionic bonding • Sometimes valence electrons are shared between two atoms • Basis for covalent bonding

9. Why do Elements React? • Gilbert Lewis formulated that atoms react in order to achieve a more stable electron configuration. • Maximum stability occurs when an atom is isoelectronic with a noble gas. (has a full octet) • ns2np6, 8ve- • When elements react it is only their valence shells that interact and therefore it is only the valence electrons that are of interest to us • To keep track of how many valence electrons each element has and that the total number of valence electrons remains constant chemists use Lewis dot symbols

10. Valence electrons recap • Valence electrons are electrons that exist the highest energy level. • These are electrons with the highest principle quantum number, n. • Valence electrons are the electrons that participate in bonding • Valence electrons can be determined by the electron configuration • Remember that all elements within the same column have the same number of valence electrons • i.e.. All alkali metals have 1 ve-

11. Lewis dot symbols • Lewis dot symbol consists of a symbol of an element an one dot for each valence electron in the element.

12. Predicting the bond type • A good predictor for which type of bonding will develop between a set of atoms is the difference in their electronegativity's. • The greater the difference between the electronegativity's, the less equal the exchange of electrons between two atoms • This leaves us with three different levels of interaction: pure covalent (nonpolar covalent), polar covalent, and ionic

14. Determining the type • When the electronegativity difference between two atoms is greater than 2.0 the bond is ionic. • When the difference is less than 0.3 the bond is considered pure (nonpolar) covalent. • When the difference is between 0.3 and 1.6 the bond is considered polar covalent.

15. Determining the type • So what about the range between 1.6 and 2.0? • If one of the atoms is a metal we will define it as ionic. • If both atoms are nonmetals we will define it as polar covalent. • These ranges aren’t conclusive, they are our attempts to explain observed patterns in nature

16. Determining the type • The take home lesson on electronegativity and bonding is this: • The closer together the atoms are on the P.T., the more evenly their e- interact, and so are more likely to form a pure covalent bond • The farther apart they are on the P.T., the less evenly their e- interact, and are therefore more likely to form an ionic bond. • In between the extremes exists varying degrees of polar covalent interactions

17. Rule of thumb • metal w/nonmetal = usually ionic • nonmetal w/nonmetal =usually covalent

18. What bond type is formed? • Let’s consider the compound Cesium Fluoride, CsF. • The electronegativity value (EV) for Cs is .70; the EV for F is 4.00. • This is a difference of 3.30 • What type of bond is it?

19. Ionic Bond

20. Gaining/losing electrons • Elements with low ionization energies (metals) tend to form cations by losing valence electrons • Groups 1 and 2 are the elements that are most likely to form cations • Elements with high electron affinities (non-metals) tend to form anions by gaining valence electrons • Groups 16 and 17 are the elements that are most likely to form anions

21. Example • Reaction between Li and F • This occurs in 3 steps • The ionization of Li • The acceptance of the electron by F • The electrostatic attraction between the ions

22. Ionic Bonds • An ionic bond holds together the ions in an ionic compound • An ionic compound is the product of a cation and an anion • An ionic bond is most often between a metal and a nonmetal

23. Properties of ionic bonding • Ionic compounds tend to be crystalline solids. • The crystals are all different colors and textures, but they all tend to be hard and brittle • They can be broken to leave a clean smooth surface • Ionic compounds tend to have high m.p. • But when melted, they conduct electricity

24. Properties of ionic bonding • Many ionic solids are soluble in water where they are dissolved into separate ions. • Dissolved ions in water make water conductive • These are called electrolytes. • Most covalent compounds are not electrolytes (acids are an exception)

26. Naming Ionic Compounds • Naming ionic compounds requires practice and a solid understanding of the rules • In order to help make this easier we will split ionic compounds into 3 types. Please note that these three types are not real and are used only to help you learn how to name compounds. • Type 1 Binary compounds using group 1 and 2 metals • Type 2 Binary compounds using transition metals • Type 3 Ternary compounds using polyatomic ions

27. Type 1

28. Type 1 Type 1 naming consists of binary compounds. Binary compounds contain two elements, a metal and a nonmetal The metals in type one ONLY comes from the representative group metals, primarily those in groups 1 and 2 Metals lose their valence electrons to become cations Cations are named by the element name Nonmetals gain valence electrons to become anions Anions are the element name with the ending changed to -ide

29. Common Type 1 ions

30. Let’s try it

31. Steps for writing formulas type 1 Name to Formula Example: Potassium Chloride • Write the metal as an cation • Write the nonmetal as an anion • Balance the charge • (use the crisscross method)

32. Lets Try it

33. Steps for naming type 1 Formula to Name Example: BaS • Write the name of the metal • Write the name of the nonmetal • Change the ending of the nonmetal to –ide • Put the two names together

34. Lets Try it

35. Type 2

36. Type 2 Type 2 naming consists of binary compounds. The metals in type 2 ONLY come from the transition metals (groups 3-12) These metals are trickier that those in groups previously discussed because they have multiple oxidation states. This means that the number of electrons they can lose changes depending on : what they are to and the conditions under which the bonding occurs

37. Multiple oxidation states • An example of a transition element that has multiple oxidation states is Iron • Iron can lose between 1 and 6 electrons to become: • Fe+, Fe2+, Fe3+, Fe4+, Fe5+, Fe6+ • How do we know which oxidation state Iron has when writing formulas or the names of compounds????? • We use the Stock system which uses Roman numerals to denote the oxidation state of the metal. • AS FAR AS WE ARE CONCERNED ONLY TRANSITION METALS, TIN, AND LEAD WILL HAVE MULTIPLE OXIDATION STATES EXEPT Zn2+,Ag+, and Cd2+ WHICH ONLY FORM ONE ION.

38. Roman Numerals

39. Common Type 2 ions

40. Lets try it

41. Steps for writing formulas type 2 Name to Formula Example: Manganese (IV) oxide • Write the metal as an cation using the Roman numeral as the charge • Write the nonmetal as an anion • Balance the charge • (use the crisscross method, reduce if necessary)

42. Let’s Try it

43. Steps for writing Names type 2 Formula to Name Example: TiCl4 • Write the name of the metal • Write the name of the nonmetal • Change the ending of the nonmetal to –ide • Determine the roman numeral using the anion as a guide • Put the two names together

44. Lets Try it

45. Type 3

46. Type 3 Type 3 naming consists of ternary compounds. Ternary compounds are made of 3 or more different elements. 2 or more of these elements combine to form a polyatomic ion. The cations are almost always metals with the exception of ammonium (NH4+) The anions are usually the polyatomic ions for example • (CO32-; NO3- ; NO2- ; OH- ; PO43-)

47. Common polyatomic ions

48. Steps for writing formulas type 3 Name to Formula Example: Aluminum Acetate • Write the metal as an cation (using the Roman numeral as the charge if a type 2 metal) • Write the nonmetal as an anion. (Look up the polyatomic ion on your polyatomic ion sheet) • Balance the charge • (use the crisscross method, reduce if necessary)

49. Lets Try it

50. Steps for writing Names type 3 Formula to Name Example: FePO4 • Write the name of the metal • Write the name of the nonmetal. (find it on your polyatomic ion sheet) • Determine the roman numeral using the anion as a guide (if the metals is a transition metal, tin or lead) • Put the two names together