How To Write A Periodic Table

Transcription

1 Spring 2008 hemistry 2000 Midterm #1A / 50 marks INSTRUTINS 1) Please read over the test carefully before beginning. You should have 5 pages of questions and a periodic table. 2) If you need extra space, use the bottom of the periodic table page, indicate that you are doing so next to the question and clearly number your work. 3) If your work is not legible, it will be given a mark of zero. 4) Marks will be deducted for incorrect information added to an otherwise correct answer. 5) You have 90 minutes to complete this test. 1. A small amount of phosphorus is added to a sample of silicon to improve its ability to conduct electricity. [5 marks] (a) (b) Is this an example of a p-type semiconductor or an n-type semiconductor? n-type Explain how the addition of phosphorus increases conductivity. Phosphorus has one more valence electron than silicon. As a result, it provides a donor band (containing electrons) close in energy to the conduction band of the silicon. Electrons in the donor band require less energy to be excited into the conduction band of silicon (compared to the electrons in the valence band of silicon). The electrons excited into the conduction band can conduct electricity. For comparison p-type semiconductors (not expected to be mentioned on test) Dopant (Al, Ga, etc.) has one less valence electron than silicon. As a result, it provides an acceptor band close in energy to the valence band of the silicon. Electrons in the valence band of silicon require less energy to be excited into the acceptor band (compared to the conduction band of silicon). The holes left behind in the valence band (by the excited electrons) can conduct electricity.

2 2. (a) Energy Values for the Atomic rbitals of xygen and ydrogen Atoms ydrogen xygen 1s Ry Ry Ry Ry [10 marks] Use the energy values in the table above to help you develop a valence molecular orbital diagram for the hydroxide ion ( - ). Label all orbitals on your diagram and include electrons. It is not necessary to draw pictures of the orbitals for this question. 1σ is the core orbital on oxygen (from 1s). (b) According to your M diagram, what is the bond order for this ion? 1 Identify any nonbonding valence molecular orbitals. 2σ and 1π (d) Would you expect - to be paramagnetic or diamagnetic? diamagnetic

3 3. alcium carbide, a 2, is a reactive salt used in a number of industrial processes including the production of acetylene. Use molecular orbital theory to describe the bonding of the carbide anion ( 2 ). [17 marks] (a) omplete the valence molecular orbital diagram below by i. drawing and naming the atomic orbitals, ii. drawing and naming the molecular orbitals, and iii. adding electrons to the appropriate molecular orbitals 3σ* 1π* Energy 3σ 1π 2 2-2σ* 2σ (b) Write the complete orbital occupancy (i.e. electron configuration) for 2. (1σ) 2 (1σ*) 2 (2σ) 2 (2σ*) 2 (1π) 4 (3σ) 2 Would you expect the carbide anion to have a larger or smaller bond dissociation energy than 2? Justify your answer. Larger The bond order for 2 2- is 3. The bond order for 2 is 2. Bond dissociation energy increases with increasing bond order. (d) Give formulas for two neutral diatomic molecules that are isoelectronic with. 2 N 2 and

4 4. The valence π molecular orbital diagram shown below can be used to describe the π bonding in 3, + or 3. Each of these compounds consists of a triangle of carbon atoms with one hydrogen attached to each carbon. [10 marks] E π (a) i. Draw all valid resonance structures for the + cation. π ii. What is the average bond order for each - bond in the 3 3 cation according to your resonance structures? 1⅓ (b) i. Fill the π-ms in the diagram above with the appropriate number of π electrons for the + cation. See M diagram ii. What is the average π bond order for each - bond in the + cation according to the M diagram? ⅓ iii. What is the average bond order for each - bond in the + cation according to M theory? 1⅓ n the π-m diagram above, draw a picture of the lowest energy π-m. See picture next to M diagram

5 5. onsider the bonding in Br 2 according to valence bond theory. [8 marks] (a) Draw a Lewis structure for Br (b) What is the hybridization of the carbon atom in Br 2? sp 2 learly indicate which atomic orbitals combine to make each σ bond in Br 2. σ(sp 2 )+Br(4p) σ(sp 2 )+() σ(sp 2 )+Br(4p) (d) learly indicate which atomic orbitals combine to make each π bond in Br 2. π()+()