New Jersey Center for Teaching and Learning AP Chemistry - PDF document

Slide 1 / 47 Slide 2 / 47 New Jersey Center for Teaching and Learning AP Chemistry Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written Unit 4: Presentation B permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to Chemical Bonding: make their work available to other teachers, participate in a virtual professional learning Lewis Structures, Hybridization, and community, and/or provide access to course materials to parents, students and others. Bond Order Click to go to website: www.njctl.org www.njctl.org Slide 3 / 47 Slide 4 / 47 Molecular Bonding Chemical Bonding The nature of the bonding within a molecule influences many properties of the substance The nature of the bonding in water leads to a bent shape causing it to Some of the properties influenced by the bonding be able to dissolve many of the Melting Point and Boiling Point nutrients and chemicals life depends on. Viscosity (resistance to flow) Solubility Vapor Pressure Molar Absorptivity (light absorbed per M) Slide 5 / 47 Slide 6 / 47 Lewis Structures Lewis Structures A proper lewis structure uses only the valence electrons Covalent bonds are formed by atoms sharing electrons available from the atoms in the molecule AND distributes the between nuclei so as to have a full valence shell. electrons so each atom has a full valence shell. e-e- e-e- e- e-e- e- e-e- O O C Guidelines for writing lewis structures e- e- e-e- e-e- Guideline One: Both O and C require 8 electrons for a full valence shell (s 2 p 6 ) Determine the ordering of atoms in the molecule Shared pairs of electrons can be represented by lines and un- bonded electrons can be represented as dots. O O C

Slide 7 / 47 Slide 8 / 47 Lewis Structures Lewis Structures Guideline One: Guidelines for writing lewis structures Determine the ordering of atoms in the molecule Guideline Two: Typically, the least electronegative atom is the central atom. Determine the number of valence electrons in the molecule Cl NH 3 = 8 CCl 3 H = 26 C Cl 4 Cl C Cl S O 2 O S O Cl If the molecules is an ion, one must either subtract or But not always... often it's the less abundant atom add electrons to the valence electron count. H 2 O H O H N H 3 H N H NO 3- = 23 +1 = 24 NH 4+ = 9-1 = 8 H In hydrocarbons, the carbon atoms will form a congo line or chain... CH 3 CH 2 CH 2 OH C C C O H Slide 9 / 47 Slide 10 / 47 Lewis Structures Lewis Structures: Octet Rule Guidelines for writing lewis structures Guideline Three: Form a single bond (2 shared electrons) between all The "Octet Rule" refers to the fact that a full valence shell for elements and then distribute electrons such that all atoms most elements is a full outer s and p orbital or 8 electrons. have a full valence shell, saving the central atom for last. Some elements do not follow this as shown below. Example: H 2 O (8 ve) Notice H needs only 2 electrons H = 2 Be = 4 B = 6 for a full valence shell. H - O - H In addition, elements in period 3 or below can have expanded octets or more than 8 valence electrons. Notice that C does not have a Example: CO 2 (16 ve) full valence shell and therefore adjustments will need to be O - C - O made to this structure Slide 11 / 47 Slide 12 / 47 Lewis Structures Lewis Structures Guidelines for writing lewis structures Guidelines for writing lewis structures Guideline Four: Guideline Five: If an atom is short of an octet, additional electrons must be If all atoms have a full valence shell but valence electrons shared between the nuclei forming "Pi" bonds. remain, they are to be added to the central atom in pairs. F O - C - O 34 valence electrons F S F F O C O Extra pair of un-bonded electrons is added to central Pi bonds atom. As we will see shortly, these extra electrons influence the properties of the molecule significantly Note: Pi bonds are formed from valence electrons in "p" orbitals.

Slide 13 / 47 Slide 13 (Answer) / 47 1 Which of the following molecules would have 10 valence 1 Which of the following molecules would have 10 valence electrons in the lewis structure? electrons in the lewis structure? A NH 4 + A NH 4 + B CN- B CN- Answer C H 2 O C H 2 O B D NO 2 - D NO 2 - E N 2 O E N 2 O [This object is a pull tab] Slide 14 / 47 Slide 14 (Answer) / 47 2 How many valence electrons can be used in the lewis 2 How many valence electrons can be used in the lewis structure for NO + ? structure for NO + ? A 6 A 6 B 8 B 8 Answer C C 10 C 10 D 12 D 12 E None of these E None of these [This object is a pull tab] Slide 15 / 47 Slide 15 (Answer) / 47 3 Which of the following molecules has a central atom with 3 Which of the following molecules has a central atom with an expanded octet? an expanded octet? A SO 2 A SO 2 B SCl 2 B SCl 2 Answer C PF 3 C PF 3 D D XeF 2 D XeF 2 E CO 32- E CO 32- [This object is a pull tab]

Slide 16 / 47 Slide 16 (Answer) / 47 4 Which of the following molecules would require Pi bonds 4 Which of the following molecules would require Pi bonds in the lewis structure? in the lewis structure? I. NO 3- I. NO 3- A I only A I only II. CO 32- II. CO 32- B II only III. HCN B II only III. HCN Answer C III only C III only E D I and II only D I and II only E I, II, and III E I, II, and III [This object is a pull tab] Slide 17 / 47 Slide 17 (Answer) / 47 5 How many unbounded pairs of electrons are on the 5 How many unbounded pairs of electrons are on the central atom in ClO 3 -? central atom in ClO 3 -? A 1 A 1 B 2 B 2 Answer C 3 C 3 A D 4 D 4 E None of these E None of these [This object is a pull tab] Slide 18 / 47 Slide 18 (Answer) / 47 6 Which of the following molecules would have a lewis 6 Which of the following molecules would have a lewis structure most similar to CO 2 ? structure most similar to CO 2 ? A SO 2 A SO 2 B CS 2 B CS 2 Answer C NO 2- C NO 2- B D CO 32- D CO 32- E H 2 O E H 2 O [This object is a pull tab]

Slide 19 / 47 Slide 19 (Answer) / 47 7 Below is a skeleton for the lewis structure for 7 Below is a skeleton for the lewis structure for alphaketoglutarate, a kreb's cycle intermediate. After alphaketoglutarate, a kreb's cycle intermediate. After finishing the lewis structure, how many Pi bond are finishing the lewis structure, how many Pi bond are needed to complete the structure? needed to complete the structure? A 0 A 0 B 1 H H O B 1 H H O O O O O Answer C - C - C - C - C C - C - C - C - C D C 2 O C 2 O O O H H H H D 3 D 3 E 4 E 4 [This object is a pull tab] Slide 20 / 47 Slide 20 (Answer) / 47 8 Which of the following would contain the largest number 8 Which of the following would contain the largest number of Pi bonds? of Pi bonds? A CH 4 A CH 4 B CO 32- B CO 32- Answer C C 2 H 2 C C 2 H 2 C D SF 6 D SF 6 E C 3 H 6 E C 3 H 6 [This object is a pull tab] Slide 21 / 47 Slide 22 / 47 Resonance Structures Resonance Structures When "Pi" bonds can be formed in more than one location, the The bonds involved in resonance are equivalent in strength and electrons are thought to be shared across all of the possible in length. locations. This is shown by writing resonance structures. In essence, the pi bond electrons are shared across all of the O bonds in which we find resonance. One pi bond is needed but could N be formed from electrons shared O O O by any of three oxygens. O O N N N O O O O O O Resonance structures EQUALS O O O O Pi bond electrons shared N N N across all three bonds. N O O O O O O O O

Slide 23 / 47 Slide 23 (Answer) / 47 9 Which of the following molecules demonstrate resonance 9 Which of the following molecules demonstrate resonance structures? structures? I. NO 2- I. NO 2- A I only A I only II. CH 3 COO- (both O attached to C) II. CH 3 COO- (both O attached to C) B II only B II only III. CH 3 CH 2 OH III. CH 3 CH 2 OH C III only C III only Answer D D I and II only D I and II only E I, II, and III E I, II, and III [This object is a pull tab] Slide 24 / 47 Slide 24 (Answer) / 47 10 How many resonance structures would be needed to 10 How many resonance structures would be needed to represent SO 3 ? represent SO 3 ? A 0 A 0 B 1 B 1 Answer C 2 C 2 D D 3 D 3 [This object is a pull tab] Slide 25 / 47 Slide 25 (Answer) / 47 11 All bonds that demonstrate resonance are equal in length 11 All bonds that demonstrate resonance are equal in length but not in strength. but not in strength. True True False False Answer False [This object is a pull tab]