Update for 10/29/07

Dear All,

The following are the information update for today.

For Living Environment 1: No HW today.

For Regent Chemistry 1: Study for Unit #3 Exam tomorrow!

For the answers to questions on the review sheet which we did not get a chance to go over:

6) a) Sr2+ (38)
Proton # = 38 ; Neutron # = 50 ; Electron # = 36 ; Size vs. original atom: Smaller
Electron Configuration of Ion: 2-8-18-8

b) I- (53)
Proton # = 53 ; Neutron # = 74 ; Electron # = 54 ; Size vs. original atom: Bigger
Electron Configuration of Ion: 2-8-18-18-8

c) As3- (33)
Proton # = 33 ; Neutron # = 42 ; Electron # = 36 ; Size vs. original atom: Bigger
Electron Configuration of Ion: 2-8-18-8

• With these information you should be able to draw the atomic structure. Remember, number of protons do not get changed, only the number of electrons. And the electron configuration of the ion is going to be different from that of the atom (because of new # of electrons associated with ions).

7A) John Dalton's atomic model:

- What are the key points of John Dalton’s atomic Model?

1. Each element is composed of indivisible particles called atoms.
2. Atoms of a given element are identical in their physical and chemical properties. Atoms of different element would have different properties.
3. Atoms of different elements combine in simple whole number ratios to form compounds.
4. In chemical reactions, atoms are combined, separated, or rearranged, but they are not created, destroyed, or changed.

- List two ways in which his atomic model had been found to be not true.

1) Atom can be subdivided further into subatomic particles.
2) Presence of Isotopes indicate atoms of same element can have different mass.

7B) J. J. Thomson

- What is his major contribution to atomic theory?
ANS.: He discovered the electron, and proposed plum-pudding model of atom.

- What experiment he did to proved his point? What was result of the experiment? What was his conclusion?
• Experiment: Applied a magnet on a beam in the Cathode Ray Tube under high voltage.
• Results: Regardless of materials of electrodes, the cathode rays bent toward the positive charged side of magnet.
• Conclusion: Since opposite charge attract, this subatomic particles must have negative charge - electrons.

7C) Ernest Rutheford

- What is his major contribution to atomic theory?
Ans.: He discovered the presence of a small, dense, positively charged nucleus. Found that the volume of atoms consists mostly of empty spaces.

- What experiment he did to proved his point? What was result of the experiment? What was his conclusion?

• Experiment Conducted: Bombarded thin gold foils with positively charged alpha particles.
• Results: The alpha particles passed through the gold foil most of the time without change in direction; but ~ 1 out of 10,000 times, it bounced back or was greatly deflected.
• Conclusion:
1) Most part of the atom consists of empty spaces.
2) Atoms must have a small, dense, positively charged nucleus that bounces back the alpha particles without being knocked off the position.

7D) Niels Bohr

- What are key points of Bohr’s atomic model?

1. Electron could only orbit at certain specified distance from the nucleus. Each distance represents a discrete amount of energy that an electron can have.
2. The lowest energy level was called the ground state. Since electrons could not have lower energy than the ground state, it could not come any closer to the nucleus and thus would remain stable.
3. In a given orbit, an electron never radiated or absorbed energy.
4. When an atom absorbed exactly the right amount of energy, the electron would rise to a higher energy level (referred to as excited state). Conversely, if the electron fall from this excited state back to a lower energy level, it would release an energy that would appears as photon of light.

8) Ans. Orbital: a region of space around the nucleus in which there is a high probability of finding the electrons.
- Electrons are represented as electron clouds since there is no distinct orbits, and the probability of finding electrons vary gradually and have no sharp boundary.

9) Ans. When electron absorbs energy to go from the lower energy level to a higher energy level, it is said to make an energy state transition from the ground state to the excited state. Since electrons are unstable in the excited state, it would drop back to the lower energy level and releases photon with energy corresponds to the difference between the two energy levels; this would lead to the formation of characteristic line spectrum. This spectrum is different for each element and thus can be used as a “fingerprint” for the element.

Dr. Liu