Ch. 8 Chemical Bonding: General Concepts

1. Ch. 8 Chemical Bonding: General Concepts

2. Chapter 7 Chapter 8 Chapter 9 Chapter 11 Chapter 22

3. Chapter 8: Chemical Bonding – General Concepts Coverage: all sections Overall Goals: The valence electrons of an atom are used when bonding with other atoms, making up a molecule. Chemists have common ways to represent these bonding interactions (Lewis structures) that make it easier to communicate about chemistry. With the information from this chapter, you will be able to: describe how electrons are shared or transferred between atoms and/or ions to form bonds draw Lewis structures, determine formal charges, and draw resonance structures

4. Chapter 8: Chemical Bonding – General Concepts TRANSFER OF ELECTRONS Ionic Bonding e.g. NaCl valence e- are transferred What happens What results IONS Electron Configurations of the ions • The first electrons to be lost by an atom or ion are always those from the shell with the largest value of n • As electrons are removed from a given shell, they come from the highest-energy occupied subshell first before any are removed from a lower-energy subshell. • Within a given shell, the energies of the subshells vary as follows: s <p < d < f….. etc

5. Electron Configurations Of Cations • Main Group metals lose the electrons in their highest energy subshellfirst to achieve the previously filled noble gas (the octet rule) • Group Ia: [Noble gas core]ns1 • Form 1+ ions to be isoelectronic with noble gas core element • Group IIa: [n.g.c.]ns2 • Forms 2+ ions • Group IIIa: [n.g.c.]ns2np1 • Forms 3+ ions

6. Electron Configuration of Cations • Main Group metals lose the electrons in their highest energy subshell first. Elements in group IIIa below Al also form 1+ ions. • Ga : • Ga+ : • Ga3+ :

7. Predicting Anion Configurations • Non-metals gain electrons to become isoelectronic with the next larger noble gas • O: • N:

8. Ionic Bonding Cl - Chapter 8: Chemical Bonding – General Concepts TRANSFER OF ELECTRONS e.g. NaCl Description of ionic bonding: deocalised,primarily attractive, electrostatic force. Na+ • Ionic crystals: exist in a 3D array of cations and anions called a lattice structure • Ionic chemical formulas: always written as empirical formula (smallest whole number ratio of cation to anion) - +

9. Chapter 8: Chemical Bonding – General Concepts TRANSFER OF ELECTRONS Ionic Bonding e.g. NaCl Lattice Energy, U Formation of gaseous ions from an ionic solid AxBy (s) → xB+y (g) + yB-x (g) * Empirical observations

10. Cl- Li+ Cl- Na+ U r Cl- K+ Cl- Cs+

11. F- Ca2+ Na+ 239 pm 231 pm Why?

12. Chapter 8: Chemical Bonding – General Concepts Covalent Bonding SHARING OF ELECTRONS e.g. HCl What happens sharing of valence e- What results BONDS

13. Covalent Bonding Chapter 8: Chemical Bonding – General Concepts SHARING OF ELECTRONS e.g. HCl Description of Bonding Balance between attractive (electrostatic) force between subatomic particles of neighbouring atoms that have differing charges and repulsive force between like-charged subatomic particles

14. Bond length is the distance between the nuclei of the two atoms in a bond Bond order is the number of electron pairs shared between the atoms As bond order increases, the bond length decreases and the bond energy increases, provided we are comparing bonds between the same elements Chapter 8: Chemical Bonding – General Concepts FORMATION OF BONDS Covalent Bonding e.g. HCl Important characteristics of bonds 14

15. Chapter 8: Chemical Bonding – General Concepts SHARING OF ELECTRONS Covalent Bonding e.g. HCl Bond Dissociation Energy, D • The energy needed to break one mole of a bond in a covalent molecule in the gas phase Potential Energy vs. Internuclear Distance In H2

16. Chapter 8: Chemical Bonding – General Concepts SHARING OF ELECTRONS Covalent Bonding e.g. HCl Bond Dissociation Energy, D • D varies with bond type (single, double, triple) i.e. variable bond order • D varies with the atoms involved i.e. variable bond length

17. While electrons in a covalent bond are shared, the electrons are not evenly distributed between the two nuclei Electronegativity is a measure of the attractive force that one atom in a covalent bond has for the electrons of the bond Chapter 8: Chemical Bonding – General Concepts FORMATION OF BONDS Determining the type of bonding between two atoms Electronegativity (c)is the ability of an atom to attract toward itself the electrons in a chemical bond. 17

18. X (g) + e- X-(g) Electron Affinity - measurable, this is an potential energy relative to the gain of an electron: Electronegativity - relative

19. Chapter 8: Chemical Bonding – General Concepts FORMATION OF BONDS Determining the type of bonding between two atoms Electronegativity values can be used to predict the type of bonding that occurs between atoms electronegativity differences Dc  0---------0.4*-------------------2.0*------ polar nonpolar Ionic Covalent *Values vary from text to text

20. Chapter 8: Chemical Bonding – General Concepts FORMATION OF BONDS Bond Dipoles • Atoms with different electronegativity values share electrons unequally • Electron density is uneven, with a higher charge concentration around the more electronegative atom • Bond dipoles indicate with delta (δ) notation that a partial charge has arisen. Partial negative (δ-) charge is assigned to the more electronegative element • Such a bond is termed a polar bond • Bond dipoles can be used to determine molecular dipoles

21. Chapter 8: Chemical Bonding – General Concepts Coverage: all sections Overall Goals: The valence electrons of an atom are used when bonding with other atoms, making up a molecule. Chemists have common ways to represent these bonding interactions (Lewis structures) that make it easier to communicate about chemistry. With the information from this chapter, you will be able to: describe how electrons are shared or transferred between atoms and/or ions to form bonds draw Lewis structures, determine formal charges, and draw resonance structures

22. The element symbol is imagined to have a box around it. The valence e- are distributed about the four sides of the box. For group elements, the group number is the number of valence e-. For, Al, a group IIIa element, there are 3 valence e-. Its Lewis structure is Al Chapter 8: Chemical Bonding – General Concepts IMPORTANT BACKGROUND Lewis Symbols for atoms 22

23. VIII

24. Chapter 8: Chemical Bonding – General Concepts IMPORTANT BACKGROUND Curved Arrows Not covered in your text Valence electrons may be transferred or shared between atoms. In order to visualize the transfer or sharing of electrons it helps to learn to "push" CURVED ARROWS. It can never be stressed enough - curved arrows are TOOLS used by chemists to represent the movement of electrons not atoms! When electrons are moving between compounds/atoms, GENERALLY, they move from electron rich to electron deficient!

25. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS For chemical compounds • Predict the connectivity between atoms in covalently bondedcompounds or ions IN THE GAS PHASE (isolated) • Only works for small molecules or ions. • Predicts with about 90% success. • In the long run you will have to gain a “feel” for the correctness of a structure. • The Lewis symbols of the atoms involved are altered as per the gain or loss of e- that occurs when electrons are transferred. Ionic [ Na ]+ [ O ]2- the ionic compound K2S:

26. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS e.g: HNO3 Covalent For chemical compounds 1. Skeleton a. consider the molecular formula given b. draw skeletal structure of compound showing what atoms are bonded to each other - put least electronegative element in the center - the atom with the most available sites for bonding is placed central - consider the # of valence e's and how far a species is away from the electron configuration of a noble gas (octet/duet) - consider exceptions to the octet rule - electron deficient: Be, Group III elements, radicals - electron sufficient: nvalence ³ 3 • good start is to arrange atoms to make a • molecule with high symmetry - minimized formal charges will ultimately determine success of skeleton. • H prefers to bond to more electronegative atoms when it can

27. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Covalent e.g: HNO3 For chemical compounds 2. Count total # of valence electrons in compound - for every "+" charge take away an electron - for every "-" charge add an electron This is the total number of electrons that need to be in your final structure!!!

28. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Covalent For chemical compounds e.g: HNO3 3. Remaining electrons a. subtract the electrons used to make the skeleton from the total number of valence electrons. b. Place remaining electrons so as to fill the octets of as many atoms as possible (start with the most electronegative atoms first then proceed to the more electropositive ones).

29. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Covalent For chemical compounds e.g: HNO3 4. octet/duet Stable atoms tend to have full or exactly half-full sublevels. special stability is achieved when the valence shell of an atom equals that of a noble gas. -are all necessary octets/duets filled? If not then fill the remaining by making multiple bonds (convert a lone pair of electrons into a bonding pair of electrons). Are we done?

30. the difference between the valence e-s of an isolated atom and the number of e-s assigned to that atom in a Lewis structure formal charge on an atom in a Lewis structure total number of valence electrons in the free atom total number of nonbonding electrons ( total number of bonding electrons ) 1 - - = 2 Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Formal Charge For chemical compounds • A good structure should have : • small formal charge values (0 is best) • Lowest individual non-zero formal charge • most electronegative element is 0 or negative • no adjacent positive or negative formal charges where possible The sum of the formal charges of the atoms in a molecule or ion must equal the charge on the molecule or ion. NO LEWIS STRUCTURE IS TRULY COMPLETE WITHOUT FORMAL CHARGES BEING INDICATED

31. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Covalent For chemical compounds e.g: HNO3

32. Example: O3

33. Common Bonding Patterns C B F N O + + + + C O F N - - - - B C N O F Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Covalent For chemical compounds - 33

34. Group III can have as few as 6 surrounding electrons Be can have as few as 4 Elements that have an odd number of electrons can have only 7. Elements in the 3rd period and higher contain more than four orbitals in the valence shell, so may accommodate more than 8. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Exceptions to the octet rule For chemical compounds 34

35. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Exceptions to the octet rule For chemical compounds e.g. - more valence electrons than satisfied by octet (SF6) - minimize formal charge (H2SO4) - not enough electrons (NO) - minimize formal charge (BF3)

36. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Exceptions to the octet rule For chemical compounds Note a Lewis Diagram that is predicted for a compound in the gaseous state may not be the most stable form for the compound! Electron sufficient  - to minimize formal charge - not enough electrons - to minimize formal charge Dimerisation!

37. - - + + O O O O O O Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Resonance In a molecule e-’s in reality are delocalized.  This means that more than one Lewis diagram can describe a molecule.  EQUIVALENT LEWIS DIAGRAMS CAUTION • Structures with the same formula in which the atoms are in different arrangement are termed isomers • resonance structures keep the atoms in the same geometric configuration but the electrons are arranged differently "pushing" CURVED ARROWS is once again a useful TOOL this time to determine all possible resonance structures

38. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Resonance ANOTHER EXAMPLE (CO32-) "pushing" CURVED ARROWS is once again a useful TOOL this time to determine all possible resonance structures

39. O O O O O O O O O Which structure is correct? The molecule exist as an average of the different Lewis diagram. Resonance hybrid structure

40. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Resonance In a molecule e-s in reality are delocalized.  This means that more than one Lewis diagram can describe a molecule.  NON-EQUIVALENT LEWIS DIAGRAMS "pushing" CURVED ARROWS is once again a useful TOOL this time to determine all possible resonance structures

41. Chapter 8: Chemical Bonding – General Concepts LEWIS DIAGRAMS Resonance In a molecule e-s in reality are delocalized.  This means that more than one Lewis diagram can describe a molecule.  Rules to Remember: • Atoms never move. • You can only move electrons in multiple bonds or lone pairs. • The bonding framework of the molecule must remain intact. • ie. don’t break single bonds! • The overall charge on the system must remain the same.