Chemical bonding

1. Chemical bonding LA Charter School Science Partnership 10 Dec 2011 Nick Klein

2. Today’s Talk • Intro & Biography • Part 1: Chemical bonding on an atomic level • Part 2: Lewis dot structures, ionic & metallic bonding • Part 3: Covalent bonding

3. Today’s Talk cont’d • I’ve split this talk into three (roughly) even parts. We will take a brief (1-2min) break between sections. Please feel free to take notes, handle the props, think of questions for after the talk, etc.

4. About Me • Third year PhD student in Earth Sciences • BA Biology and Chemistry from Augustana College, SD • I study global impacts of marine trace chemistry • Thesis research: production of marine halocarbons Field sampling on Lake Tahoe

5. My new toy!

6. Part 1: Bonding on an atomic level • What is chemical bonding? • Let’s define it • A chemical bond is a strong attraction between two or more atoms • Electrons are given/taken or shared • Can be ionic, metallic, or covalent in character • When the chemical bonds in a molecule are broken or rearranged, this is called a chemical reaction

7. Part 1: Bonding on an atomic level • Atoms are composed of a nucleus and orbiting electrons • Nucleus contains protonsand neutrons • Electrons are negatively charged, protons positive. Neutrons are not charged (neutral).

8. Part 1: Bonding on an atomic level • The story of bonding is the story of electrons • How electrons are exchanged determines the type of bonds formed • No matter how many electrons an atom has, only the very outermost participate in bonding; these are called valence electrons

9. Part 1: Bonding on an atomic level • You can easily determine the number of valence electrons an element has by counting the number of columns in the periodic table from left to right (with one big exception!)

10. Part 1: Bonding on an atomic level 8 1 Things get weird. Skip these.The number of valence electrons varies and there’s no real pattern. 7 4 3 5 6 2

11. Part 1: Bonding on an atomic level • Each column is called a period. Elements within the same period (column) have the same number of valence electrons and have similar chemical characteristics! • When elements form chemical bonds, they want to have a full valence of electrons to be stable and happy. The magic number is usually 8 – the Octet Rule

12. Part 1: Bonding on an atomic level 8 1 7 4 3 5 6 2

13. Break!

14. Part 2: Lewis Dot Structures • Scientists write the number of valence electrons using Lewis dot structures • We write the symbol for the element and then add a dot for each valence electron • Let’s try a couple examples!

15. Part 2: Lewis Dot Structures

16. Part 2: Lewis Dot Structures • Potassium is in period 1 and has 1 valence electron K

17. Part 2: Lewis Dot Structures • Oxygen has 6 valence electrons. This gets a little trickier, though. There are four orbitals and room for two electrons in each... but same-charges repel! O

18. Part 2: Lewis Dot Structures • How might these two elements bind chemically? Is K more likely to want to take 7 electrons from another atom, or give away 1? Is O more likely to take 2 electrons or give away 6? • It’s also useful to look at electronegativity- a measure of how badly an element wants electrons (how “greedy” it is)

19. Part 2: Lewis Dot Structures

20. Part 2: Ionic bonding • Oxygen has an electronegativity of around 3, which is one of the highest, and potassium is maybe 0.5 • O wants electrons very badly, K wants to get rid of them! • They will undergo ionic bonding – a type of chemical bonding where electrons are completely removed from one atom and given to another, meaning the atoms have an electrical charge and are called ions

21. Part 2: Ionic bonding +1 K -2 O +1 K

22. Part 2: Ionic bonding • Even though these ions are not held tightly to each other, opposites attract and many ionic compounds form very orderly arrangements of ions called crystals

23. Part 2: Ionic bonding K2O Potassium oxide

24. Part 2: Ionic bonding

25. Part 2: Ionic bonding -1 +1 Na Cl

26. Part 2: Ionic bonding NaCl Sodium chloride (table salt)

27. Part 2: Ionic bonding

28. Part 2: Ionic bonding -1 F +2 Ca -1 F

29. Part 2: Ionic bonding CaF2 Calcium fluoride

30. Part 2: Ionic bonding • Ionic compounds are very hard and brittle with high melting points • They are poor conductors of heat and electricity as solids, but good at conducting electricity when dissolved in a liquid • Most are very soluble in water (like dissolves like!)

31. Part 2: Metallic bonding • The transition metals (the middle group that we skipped when talking about valence!) can participate in a special form of bonding called metallic bonding • In metallic bonds, atoms share electrons with all neighboring atoms and electrons are able to flow freely throughout the material

32. Part 2: Metallic bonding • Metallic bonding produces compounds that are usually shiny, hard, very good conductors of heat and electricity • Ductile (can be drawn into wires) and malleable

33. Break!

34. Part 3: Covalent bonding • This is where things get complicated! • We’ve looked at ionic bonding, where electrons are given or taken • In covalent bonding (think co-valence), two electrons are shared, creating a tight bond between two atoms • Carbonespecially likes to form covalent bonds. Why might this be?

35. Part 3: Covalent bonding

36. Part 3: Covalent bonding

37. Part 3: Covalent bonding • Carbon has 4 valence electrons. It could give up 4... or it could lose 4. Or it could share! • Carbon doesn’t want electrons nearly as badly as oxygen or nitrogen • Carbon can actually form up to four covalent bonds, which can lead to very large and complex molecules... the basis of all life!

38. Part 3: Covalent bonding O C O

39. Part 3: Covalent bonding O C O Each line represents two electrons being shared

40. Part 3: Covalent bonding • Why is water a “bent” molecule? O H H

41. Part 3: Covalent bonding • Why is water a “bent” molecule? O H H think 3D!

42. Part 3: Covalent bonding • Think back to the last workshop. What causes a molecule to be polar or nonpolar? • A bond can be more or less polar, too. • Look back at CO2 for an example

43. Part 3: Covalent bonding - + - O C O

44. Part 3: Covalent bonding • The bonds in CO2 are polar since oxygen is more electronegative • However, the molecule is not polar overall since it is linear, and one end of it is not more negative or positive than the other • The carbon is “buried” • Water has polar bonds and is polar overall because it is “bent”

45. Part 3: Covalent bonding • Often when chemists write the structure of an organic compound (one containing carbon), we save time and space by not writing in the letters for carbon and hydrogen, since we know those two elements form the backbone of all organic molecules • This is called a skeletal formula

46. Part 3: Covalent bonding propane

47. Part 3: Covalent bonding acetic acid (vinegar)

48. Part 3: Covalent bonding • When elements other than just carbon and hydrogen are in an organic molecule, these give the molecule special properties and are called functional groups • You’re not expected to know the functional groups, but as we look at some of them, think about the bonding and polarity of the bonds. You’ll build many of them in the activity!

49. Part 3: Covalent bonding R OH Isopropyl alcohol (rubbing alcohol) The “R” means “any chain of carbons.” Alcoholsare good at making other molecules inert and damageproteins, so are often used as disinfectants.

50. Part 3: Covalent bonding O H R formaldeyhyde Aldehydes are generally very fragrant. Many perfumeand other fragrances contain aldehydes.