Quick Review. 1. Radii/Size Trend. 2. Ionization Energy Trend. 3. Second and Third Ionization Energy. 4. Electron Affinity.

Transcription

1 Quick Review 1. Radii/Size Trend. 2. Ionization Energy Trend. 3. Second and Third Ionization Energy. 4. Electron Affinity.

2 hemical Bonding Localized Electron Model Why do elements combine to form compounds? Noble gases have completely filled s and p orbitals of electrons (ns 2 np 6 ) In general elements form compounds to achieve noble gas configuration. alled octet rule for main group elements. alled duet rule for hydrogen.

3 hemical Bonding What are two ways that elements can form compounds? (i) Ionic bonds: transfer of electrons (ii) ovalent bonds: sharing of electrons ow can we predict what type of bond forms between two elements? Difference in electronegativity tendency of atoms in bonds to attract bonding electrons to itself. In general metals have low EN values and nonmetals have high EN values. Ionic bonds are formed between elements with large electronegativity differences. ovalent bonds are formed between elements with small electronegativity differences.

4 Electronegativity Trend EN values are greater the closer the element is to F.

5 Types of ovalent Bonds In general when a bond forms between two different nonmetals there is an unequal sharing of electrons which creates a bond dipole. O δ - δ + The more electronegative atom gets more than its fair share of electrons. Electronegativity difference is not so large that all the bonding electrons are pulled away and an ionic bond is formed.

6 Arrangement of Valence Electrons Assumptions: (A) Only valence electrons are involved in the bonding of atoms to form molecules. The inner core electrons are held too tightly to interact. (B) Atoms react to form molecules so to achieve the stable noble gas electron configuration. Atoms in molecules follow the octet rule, hydrogen follows the duet rule. () In covalent compounds, atoms share electrons to form bonds in order to achieve stable noble gas configuration. In ionic compounds electrons are transferred from one atom to another in order to achieve stable noble gas configuration.

7 Drawing Lewis Structures 1. ount the total number of valence electrons. 2. Identify the central atom and place all terminal atoms around the central atom. 3. omplete the octet for all atoms in the Lewis Structure, ydrogen will always be satisfied with a single bond. 4. ount the number of valence electrons using in the Lewis structure, if it matches with the # counted in step 1 your are done! If not you must do one of the following things: (a) If you have used too many electrons add multiple bonds. (b) If you have used too few (exceptions to octet) put extra electrons on the central atom. (c) If you only have 4 or 6 valence electrons the octet rule can t be satisfied (e.g. Be 2, B 3 ). (d) If you have an odd number of electrons, complete the octet for all atoms except the least EN atom. Put 7 electrons around the least EN atom.

8 Atom ovalency Rules Organic ompounds compounds composed of a carbon backbone with atoms of N, O and bonded to the carbons. DO NOT use these rules for other compounds, just organic! Bonds Formed in Lewis Structure 4 0 N 3 1 O 2 2 alogens (F, l, Br, I) 1 3 Lone Pairs on Atom in Lewis Structure

9 Lewis Structures of Ionic ompounds

10 ovalent Lewis Structures Examples: (a)f (b)o 2 (c)o (d)f 4 (e)sl 3 + (f)so 3

11 Resonance What is the actual structure of SO 3? Experiment shows that all S-O bonds are identical in length and strength, with a bond length between that of a single and double bond. Not just one structure is an accurate representation of the molecule, the actual structure is an average of the three. You can recognize resonance when you can draw more than one valid Lewis Structure for a molecule usually involves placement of a multiple bond. You MUST draw all Lewis resonance structures to show that the actual structure is an average of the three structures.

12 Resonance The localized electron model assumes electrons are fixed but in molecules that exhibit resonance the electrons are delocalized, or free to move about the molecule. NOTE: The molecule is NOT jumping from one structure to the next, the actual structure is an average of all the resonance structures. Other examples: (a)no 2 - (b)o 2

13 Atom Organic ompounds Bonds Formed in Lewis Structure Lone Pairs on Atom in Lewis Structure 4 0 N 3 1 O 2 2 alogens (F, l, Br, I) 1 3 Acetic Acid O O affeine N O O N N N

14 Exceptions to Octet Rule 1. Molecules with fewer than 8 valence electrons: (a)be 2 (b)b 3 (c)bf 3

15 Exceptions to Octet Rule 2. Molecules/Ions with an odd number of valence electrons: (a)no 2

16 Exceptions to Octet Rule 3. Species having more than 8 valence electrons around the central atom. -rationalized by the availability of low energy d orbitals. -it is always the central atom that has more than 8 electrons around it never the terminal atoms. -row 2 elements (, O, N, F) do not have d orbitals available to them and therefore can never have more than 8 electrons around them. (a)sf 6 (b)pl 5 (c)sf 4 (d)krf 2