Chapter 8: Basic Concepts of Chemical Bonding

1. Chapter 8: Basic Concepts of Chemical Bonding

2. Chapter 8: Basic Concepts of Chemical Bonding Chemical Bonds • ionic bond electrons are transferred from one atom to another atom • covalent bond electrons are shared between two atoms • metallic bond electrons are free to move between all atoms

3. Chapter 8: Basic Concepts of Chemical Bonding The formation of chemical bonds involves valence electrons Lewis Symbols: “shorthand” for showing valence electrons Gilbert Lewis (1875-1946)

4. 1A 8A 2A 3A 4A 5A 6A 7A Chapter 8: Basic Concepts of Chemical Bonding Lewis dot structures: He H Li Be B C N O F Ne Na Mg Al Si P S Cl Ar For representative (main group) elements: group number = number of valence electrons

5. K Cl Cl Chapter 8: Basic Concepts of Chemical Bonding Noble gases, except helium, have 8valence electrons ns2np6 Octet Rule: When atoms react, they tend to lose, gain, or share the number of electrons required to achieve eight valence electrons (an “octet” of electrons) + K+ + [Ar] [Ar] electron configuration:

6. Chapter 8: Basic Concepts of Chemical Bonding Formation of an ionic lattice Cl- Na+ Cl- Na+ Cl- Na+

7. Chapter 8: Basic Concepts of Chemical Bonding very exothermic DH<0 + Cl- (g) Na+ (s) NaCl (s)

8. Chapter 8: Basic Concepts of Chemical Bonding charge on ions distance between charges Strength of ionic bond depends on Eel • the larger Eel, the stronger the bond • the greater the charges, the stronger the bond • the smaller the distance between the charges, the stronger the bond

9. Chapter 8: Basic Concepts of Chemical Bonding The stronger the ionic bond the the melting point higher 1261oC 66, 133 2852oC 66, 140 SrI2 +2, -1 538°C 113, 220 r1 r2

10. H H F F Chapter 8: Basic Concepts of Chemical Bondingc Covalent Bonds … … are formed when two atoms share electrons … in order to achieve “noble gas configuration” … of the nearest noble gas Each hydrogen has the electron configuration of He + H H Each fluorine has the electron configuration of Ne + F F

11. F F Chapter 8: Basic Concepts of Chemical Bonding Covalent Bonds + H H H H H H F F + F F • A shared electron pair is drawn as a dash (two electrons!) • Unshared electrons are drawn as dots

12. Triple bond Single bond Double bond C + O + O or O C O O C O or + N N N N N N Chapter 8: Basic Concepts of Chemical Bonding Single and Multiple Bonds F F + F F F F or

13. Distance between atoms (bond length) decreases Chapter 8: Basic Concepts of Chemical Bonding Single and Multiple Bonds X X Bond strength increases X X X X

14. Cl Cl Chapter 8: Basic Concepts of Chemical Bonding Bond Polarity and Electronegativity • When both atoms attract bond electrons equally, electrons are shared equally non-polar covalent bond: equal sharing of electrons

15. H Cl Chapter 8: Basic Concepts of Chemical Bonding Bond Polarity and Electronegativity • If one of the atoms attracts bond electrons more strongly, electrons are shared unequally +  - polar covalent bond: unequal sharing of electrons

16. H Cl - Na+ Cl Chapter 8: Basic Concepts of Chemical Bonding Bond Polarity and Electronegativity For comparison: +  - polar covalent bond: unequal sharing of electrons ionic bond: electrons are not shared

17. Chapter 8: Basic Concepts of Chemical Bonding How do we know when a non-polar, polar, or ionic bond is formed? The tendency of atom to attract electrons when forming a molecule is summarized in the concept of electronegativity Increase in Electronegativity

18. Chapter 8: Basic Concepts of Chemical Bonding How do we know when a non-polar, polar, or ionic bond is formed? If the difference in Electronegativity, DEN, is… …smaller than 0.5 DEN < 0.5 => nonpolar bond …greater than or equal to 0.5 AND smaller than 2.0 0.5 DEN < 2.0 => polar bond …greater than or equal to 2.0 DEN  2.0 => ionic bond

19. S Chapter 8: Basic Concepts of Chemical Bonding Among the following examples, which bond is most polar? all equally non-polar C-F (C-H is non-polar) P-Cl (furthest apart in P.T. => largest DEN) all equally non-polar O-I (furthest apart in P.T. => largest DEN) e) O-I O-N O-F

20. S C C Chapter 8: Basic Concepts of Chemical Bonding Among the following examples, which bond is shortest? H-H C-H Cl-Cl Bond length depends on (a) radii of the bonded atoms remember that the atomic radii decrease along a period in the P.T. (b) the number of bonds between atoms

21. Chapter 8: Basic Concepts of Chemical Bonding Drawing Lewis Structures of Molecules there are no Lewis structures for ionic compounds!! • If the compound contains more than 2 atoms: • how are the atoms bonded and, • if there are nonbonding electron, where are they?

22. Chapter 8: Basic Concepts of Chemical Bonding Molecules with a central atom : NH3, PCl3, CHCl3 central atom is generally the first in the molecular formula

23. H2O Chapter 8: Basic Concepts of Chemical Bonding …unless the first element is Hydrogen : H has only one valence electron => can only make one bond HCN (same order as in formula)

24. Chapter 8: Basic Concepts of Chemical Bonding Rules for Drawing Lewis Structures (1) sum valence electrons from all atoms: these are the ones that need to be distributed 8 NH3 (2) connect atoms by covalent bonds: count electrons used up 6 (3) complete "octets" of atomsbound to central atom n/a (4) place any leftovers from (1) on the central atom check that central atom has octet + 2 (5) If there are not enough electrons to give the central atom an octet, try multiple bonds n/a

25. Chapter 8: Basic Concepts of Chemical Bonding (1) sum valence electrons from all atoms: these are the ones that need to be distributed 10 CO (2) connect atoms by covalent bonds: count electrons used up 2 (3) complete "octets" of atomsbound to central atom C O + 6 (4) place any leftovers from (1) on the central atom check that central atom has octet C O + 2 (5) If there are not enough electrons to give the central atom an octet, try multiple bonds triple bond!