Chemical Bonding

1. Chemical Bonding Ionic, Metallic and Covalent Bonding

2. Chemical Bonding • Atoms in compounds are held together by chemical bonds. • Chemical bonds result from the sharing or transfer of electrons between pairs of atoms.

3. Valence electrons • All elements in the same group behave similarly because they have the same number of valence electrons. • Valence electrons are the electrons in the highest occupied energy level of an element’s atoms.

4. Remember…. • The number of valence electrons in a representative element is the same as the group number. • Examples: • Na has 1 valence electron, • Mg has 2, • Al has 3, • Si has 4, • P has 5, etc.

5. Describing electron pairs in electron dot diagrams • In Lewis dot structures, “dot-dot” pairs are called unshared pairs or lone pairs. • “Dashes” are called shared pairs or bonding pairs.

6. Lewis dot structures • Only the valence electrons are shown in electron dot structures.

7. K [K+1] Practice Problems: 3) a) Draw the electron dot notation for a potassium atom. b) Draw the electron dot notation for a potassium ion. (4) a) Draw the electron dot notation for a sulfur atom. b) Draw the electron dot notation for a sulfur ion. S [ S -2]

8. The Octet Rule • In forming compounds, atoms react to gain the electron configuration of a noble gas. • An octet is 8. Remember, most noble gases have 8 valence electrons (except helium).

9. Electron configurations for cations • cation (positive ion) forms when an atom loses electrons. • Example: Sodium loses an electron to form Na+; when it does this it achieves the same electron structure as neon.

10. “Isoelectronic” • Na+ ion is isoelectronic with a neon atom… IT HAS THE SAME ELECTRON CONFIGURATION! • Na (1s2 2s2 2p6 3s1)  Na+ (1s2 2s2 2p6) + 1e- • Ne (1s2 2s2 2p6)

11. Electron configurations for anions • An anion (negative ion) forms when an atom gains electrons. • Cl- has the same electron structure as argon. • The Cl- ion is isoelectronic with the argon atom.

12. I. Ionic Bonding • Ionic bonds form from the transfer of electrons between atoms. • Metals give electrons to nonmetals. The metal forms a cation (+) and the nonmetal forms an anion (-).

13. Ionic Bonding • Ionic bonds are the forces of electrostatic attraction between positive and negative ions in ionic compounds. • Remember: opposite charges attract. • The total negative charges equal the total positive charges, so ionic compounds are neutral.

14. How to Represent an Ionic Bond Na +1 Cl -1 Electron Dot Notations: Na + Cl  [ ] [ ] Practice Problems: (1) Draw the electron dot notation for the formation of an ionic compound between aluminum and bromine. (2) Draw the electron configuration notation for the formation of an ionic compound between magnesium and nitrogen.

16. Properties of Ionic Compounds • Ionic compounds form between metals and nonmetals. • They have high melting points. • They are generally soluble in water.

17. Properties of Ionic Compounds • Ionic compounds are good conductors of electricity when melted or dissolved in water. For a compound to conduct electricity, it must have charged particles (ions) that are free to move (in liquid state or solution).

18. Properties of Ionic Compounds 5.Ionic compounds are crystalline solids at room temperature. The ions in this beautiful CuSO4 crystal are arranged in a repeating, three-dimensional pattern.

19. Ionic crystals • The coordination number of an ion is the number of opposite charged ions that surround the ion. • In NaCl, each Na+ ion is surrounded by 6 Cl- ions.

20. Crystalline Patterns

21. II. Bonding in Metals • Metals are made of closely packed cations, instead of neutral atoms. • Because of a metal’s low ionization energy, the valence electrons become mobile, and drift freely.

22. Bonding in metals • Metallic bonding is the attraction of the free-floating valence electrons for the positively charged metal ions. This force of attraction holds metals together.

23. The mobile valence electrons of metals explain why… • Metals aremalleable (can be pounded into sheets) and ductile (can be pulled into wires). The metal cations can slip past each other, being separated by the sea of free floating electrons.

24. What happens when you pound an ionic crystal vs. a metal crystal with a hammer?

25. The mobile valence electrons of metals explain why… • Metals are good conductors of electricity. To be a conductor of electricity, charged particles must be free to move…. the electrons can flow freely within the metal.

26. Metal Alloys • Two metals can be mixed together to form alloys.

27. III. Covalent Bonding • In covalent bonding, the bonded atoms share electrons. • Molecules form as atoms share electrons in covalent bonds.

28. Drawing Lewis Structures of Covalent Molecules • Electrons are shared in such a way that each atom in the molecule satisfies the octet rule • Most atoms require 8 electrons total (its own + shared) • Hydrogen only needs two • Beryllium only needs four • Boron only needs six

29. Steps in Drawing Lewis Structure for Covalent Molecule • Set up a skeletal arrangement of the atoms, placing the least electronegative element in the center (use trend in electronegativity to determine this) • Determine how many bonds/shared pairs using N-A/2 • N= total number of electrons needed for every atom in molecule to satisfy octet (look out for H-2, Be-4, B-6) • A = total number of electrons available (sum of all atoms’ valence electrons)

30. Steps in Drawing Lewis Structure for Covalent Molecule • Connect the atoms with the number of bonds determined (a line). Each bond represents two electrons being shared. • Add enough lone pairs (dots) of electrons around each atom until each atom in the molecule satisfies the octet rule. • Keep in mind that each bond represents two electrons.

31. Remember: The octet rule applies to covalent bonding, too!

32. Draw an F2 molecule.

33. Draw an F2 molecule. • A single covalent bond forms when 2 atoms share 2 (a pair) electrons.

34. Draw an O2 molecule.

35. Draw an O2 molecule. • A double covalent bond forms when two atoms share 4 electrons (two pairs)

36. Draw an N2 molecule.

37. Draw an N2 molecule. • A triple covalent bond forms when two atoms share 6 electrons (three pairs).

38. Draw Carbon Monoxide

39. Draw carbon monoxide. • A coordinate covalent bond forms when one atom contributes both bonding electrons in a covalent bond.

40. Draw ozone.

41. Draw ozone. • Resonance occurs when 2 or more equally valid Lewis dot structures can be drawn for a molecule.

42. Sigma and pi bonds • Sigma bonds are bonds that lie directly on the bond axis (from one atom’s center to the other atom’s center). • Pi bonds do not lie on the bond axis. • A single covalent bond is 1 sigma bond. • A double covalent bond is 1 sigma + 1 pi bond. • A triple covalent bond is 1 sigma + 2 pi bonds.

43. Central vs. terminal atoms • The central atom in a molecule is the atom with the most metallic character (least electronegative). • Hydrogen cannot be a central atom because it has only one electron to use to form bonds. It must be a terminal atom.

44. Draw: • Methane, CH4 • Ammonia, NH3 • Water, H20 • Carbon dioxide, CO2 • Ethane, C2H6 • Ethylene, C2H4 • Acetylene, C2H2

45. Bond Properties • Bond order Bond order = 1 when there is a single bond (1 sigma bond) Bond order = 2 when there is a double bond (1 sigma + 1 pi) Bond order = 3 when there is a triple bond (1 sigma + 2 pi)

46. Bond Properties 2. Bond length • Single bonds are long and weak. • Double bonds are shorter and stronger. • Triple bonds are shortest and strongest.

47. Bond Properties 3. Bond energy • Bond energy increases as bond order increases. • Atoms are held more tightly when there are multiple bonds.

48. Bond Properties 3. Bond polarity • When the two atoms involved in a bond have the same value of electronegativity, the electron pair is shared equally, and the bond is described as a nonpolar bond.

49. Bond Properties 3. Bond polarity (continued) • When the two atoms in a bond have different electronegativity values, the pair is not shared equally, and it is described as a polar covalent bond. The electron pair is attracted toward the more electronegative atom, giving it a slightly negative charge.