Course Lecture Topic Information
Chem 105 Web Site
Dr. Kimberly Lawler-Sagarin
Elmhurst College
This discussion will take place the week of September 10 through September 16
Assignments for the discussion boardThis week's discussion questions are as follows:
- Q1: Based on your student number, post an answer to the following question. Please try to respond to existing threads, rather than starting new ones if someone has already started responding to the question.
- (Students 1,5) How can we explain the distinct lines that appear in an atomic spectrum? What is the difference between an excited state and a ground state for an atom?
- (Students 2,6,9) Consider the large diagram on page 68 of the CfCT text (fig 3.12). As a group explain this diagram to the class. Some possible things to consider: Explain why only some transitions give off visible light. How would you expect this picture to change for a different element? Who first proposed this model and what became of it?
- (Students 3,7,10) Consider the second point in the text's description of Dalton's Atomic Theory (section 2.4): All atoms of a given element are identical to one another and different from atoms of other elements. Dalton proposed his theory about 200 years ago. Do you believe this tenet still holds? If so why? If not, how would you modify the theory?
- (Students 4,8,11) Consider the fourth point in the text's description of Dalton's Atomic Theory (section 2.4): A chemical reaction involves the rearrangement, separation, or combination of atoms. Atoms are never created or destroyed during a chemical reaction. Dalton proposed his theory about 200 years ago. Do you believe this tenet still holds? If so why? If not, how would you modify the theory?
- (Students 2,6,9) Consider the large diagram on page 68 of the CfCT text (fig 3.12). As a group explain this diagram to the class. Some possible things to consider: Explain why only some transitions give off visible light. How would you expect this picture to change for a different element? Who first proposed this model and what became of it?
- (Students 1,5) How can we explain the distinct lines that appear in an atomic spectrum? What is the difference between an excited state and a ground state for an atom?
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Q2: A variety of websites contain information about the chemical elements (click here for links to on-line periodic tables). Look up some information on your assigned element, and share this information with the class. Pick details that you think others would be interested in. For example: How is this element used? What are its properties? Is it unique? Is it similar to other elements? Each of the elements assigned is used in fireworks displays or similar kinds of special effects to produce the colors/effects listed below. Do not simply report a list of facts from other websites, rather, write a paragraph or two discussing the element in a manner you would find interesting to read yourself.
- (Student 1) Red: strontium
- (Student 2) Red: lithium
- (Student 3) Orange: calcium
- (Student 4) Gold: carbon
- (Student 5) Yellow: sodium
- (Student 6) Green: barium
- (Student 7) Blue: copper
- (Student 8) Blue: arsenic
- (Student 9)Violet: potassium
- (Student 10) White: magnesium
- (Student 11) White: iron (sparklers)
- Q3: Question/Comment/Answer. For this question, you must comment on the discussion for Question 1 or 2 (Q1,Q2). Once again, there are several ways to address this. You may choose to ask a student about one of their postings. You can answer a question from another student. Or, you may comment on another student's posting by adding additional information. The question, comment or answer should be thoughtful and well researched (about one paragraph).
Required Readings this Week
- CfCT, Chapter 2: Sections 2.1-2.5
- CfCT, Chapter 3: Sections 3.4-3.6
- Colour, Sections on refraction and rainbows, Chapter
- About Rainbows from the National Center for Atmospheric Research.
Atoms consist of two main parts:
Atoms and Atomic Structure
Atoms are one of the main building blocks of matter. At one time, they were believed to be indivisible spheres. In our current understanding of the atom, atoms are believed to be made up of smaller, subatomic particles. This week, we will learn a little about this atomic structure.
As we learned last week, atoms come in different "flavors" called elements. These elements are collected in what is known as the periodic table and are referred to by name or by 1-2 letter abbreviations (e.g carbon, C; oxygen, O; neon, Ne; Gold, Au).
The type of atom we have is determined by the number of protons it has in its nucleus. The atomic number is equal to the number of protons and defines the identify of the element.
A few examples are shown below:
| Element | Atomic # | # Protons | # Electrons |
| carbon (C) | 6 | 6 | 6 |
| nitrogen (N) | 7 | 7 | 7 |
| argon (Ar) | 20 | 20 | 20 |
| gold (Au) | 79 | 79 | 79 |
In electrically neutral atoms the # of electrons = # of protons. The positive charge of the protons is balanced by an equal number of negatively charged electrons.
Isotopes and Mass Number
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All atoms of a particular element have the same number of protons, but the number of neutrons may vary.
Isotopes are atoms of the same element with different numbers of neutrons.
- Isotopes have the same number of protons and the same atomic number - the only difference is the number of neutrons.
- We keep track of these different isotopes using the total number of protons and neutrons - the mass number.
The mass number is put after the element name to specify the isotope. For example, the element carbon (C) has three different isotopes: carbon-12, carbon-13, and carbon-14. We also use shorthand symbols for these where the mass number is placed as a superscript before the element symbol as in: 12C, 13C, 14C.
| element | atomic number |
number protons |
number neutrons |
mass number |
| carbon-12 (12C) | 6 | 6 | 6 | 12 |
| carbon-13 (13C) | 6 | 6 | 7 | 13 |
| carbon-14 (14C) | 6 | 6 | 8 | 14 |
Self-Test
Using the rules you have learned so far, try your hand at filling in the following table.
| name | symbol |
atomic number |
mass number |
# protons | # neutrons | # electrons |
| 16O |
|
16 | ||||
| potassium | 40 | |||||
| 2 | 2 | |||||
| 32 | 28 | |||||
| 6 | 8 |
Check out the answers here.
Average Atomic Mass
Atoms are very, very small. One atomic mass unit (amu), the approximate mass of a proton or neutron, is equivalent to 1.66 x 10-27 kg! In the laboratory we typically work with macroscopic amounts of material. Because atoms are so small, we work with many atoms at a time. Just a few grams of a substance may have on the on the order of 1023 atoms!The average atomic mass of an element is reported relative to the carbon-12 standard. An atom of carbon-12 (12C)is defined to have a mass of exactly 12.000000 amu. We assign values to other isotopes based on their mass relative to 12C. For example, carbon-13 (13C) has a mass of 13.003355 amu. This is quite close to its mass number of 13, but not exactly. Similarly, lithium has two naturally occuring isotopes: lithium-6 (6Li), with a mass of 6.015123 amu and lithium-7 (7Li) with a mass of 7.016005 amu.
There are many situations in which we need to know the mass of the elements we are working with. When there are several naturally occuring isotopes of an element, an average atomic mass must be used for this purpose. The average atomic mass takes into account that your sample of lithium chloride contains both 6Li and 7Li.
The average atomic mass is determined by weighting each isotope's mass by its abundance. For example, chlorine (Cl) has two naturally occuring isotopes: 35Cl and 37Cl. In nature, 75.77% of chlorine is 35Cl. 35Cl has a mass of 34.97 amu. The remaining 24.23% of naturally occuring chlorine is 37Cl, with a mass of 36.97 amus.
The average atomic mass is given by:
Average Atomic Mass (Cl) = (0.7577)(34.97 amu) + (0.2433)(36.97 amu) = 35.49 amu
The mass of each isotope is multiplied by its fractional abundance (the decimal representation of the percent abundance). For lithium, the two isotopes 6Li and 7Li are found in the proportion 7.42% 6Li to 92.58% 7Li. The average atomic mass is given by:
Atomic Number and Average Atomic Mass in the Periodic Table
The atomic number and average atomic mass are presented in the periodic table. The atomic number is an integer and usually appears above the element symbol. Here, the atomic numbers are shown in red. The average atomic mass is a decimal number and is typically placed below the element symbol, as shown in the periodic table "squares" below.
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Concept Check - Average Atomic Mass
Question: What is the average atomic mass of bromine? Bromine has two naturally occuring isotopes: Br-79 (50.69%), with a mass of 78.92 amu, and Br-81 (49.31%), with a mass of 80.92 amu.
Electron Configuration
In our modern model of the atom, all the electrons in the atom do not have the same energy.
Electrons occupy different energy levels.
These energy levels are discrete (or not continuous) - that is, only certain values for the energy are allowed. Electrons in the lower energy levels are generally closer to the nucleus. Each energy "shell" is given a number (1, 2, 3, etc.) designated by the letter "n".
Electrons can jump between energy levels by gaining or losing energy. One of the ways they can do this is by absorbing or emitting light.
Each electron energy shell can only hold a certain number of electrons.
The higher the energy level, the larger the number of electrons it can hold.
| Electron Shell (n) | Maximum # Electrons (2n2) |
| 1 | 2 |
| 2 | 8 |
| 3 | 18 |
| 4 | 32 |
Fireworks
Last week, we learned that Kurt Nassau, in his book The Physics and Chemistry of Color, identified 15 different causes of color. Two of these, incandescence and gas excitations (a type of luminescence) are responsible for the color of fireworks.Incandescence
Incandescence is the emission of light from a warm or hot object. We encounter many examples of incadescence in our daily lives. A common example is the (incandescent) light bulb. A tungsten filament is sealed into an evacuated glass bulb. When heated, the filament releases light.
Other examples of incandescence are the "red hot" coals in a barbeque grill, the "red hot" or "white hot" pieces of glowing metal on might see the hands of a blacksmith in an old western movie, or even just the heated carbon particles in a candle flame.
On the left is a picture of one of our chemistry majors, Mike Zickus, burning magnesium metal. Magnesium metal (Mg) combines with oxygen (O2)to form magnesium oxide (MgO). The brilliant white light is produced from the high temperatures generated by the heat released in the reaction. Magnesium powder is used in many types of fireworks because of this property. This explains the intense light given off by fireworks, but to describe the colors, we must look to luminescence.
Luminescence
Lumninescence is defined as the emission of light without heat. That is, any emission of light is not soley due to high temperatures. There are many forms of luminescence, some of which we will discuss later in the semester. The bright colors in firework displays are caused by the excitations of atoms or small molecules.
The textbook has a section on "Atomic Spectra and Energy Levels" (section 4.2), which discusses how the electronic energy levels of an atom can result in the absorption or emission of light. Essentially, an electron in a lower energy level can be promoted to a higher energy level by adding energy (such as during the explosion of a firework rocket). Now, the atom has excess energy, and we say it is in an excited state. However, this "excited" atom has a very limited lifetime. It quickly returns back to the lower energy state. One way it can do this is by emitting a photon.
The wavelength of light emitted has an energy that corresponds to the energy difference between the two levels. This determines the color we see. Atoms of different elements have different spacings between energy levels, leading to different colors. Thus, a variety of different substances are used to produce the colors we see in a fireworks show.
Here is a picture of methanol (wood alcohol) burning in the presence of various metal salts. The metal salts lend bright colors to the flames. Click on the picture for a larger image.
Both incandescence and luminescence are discussed in The Chemistry of Firework Colors - an About.com page about firework colors. This page identifies the main causes of color in fireworks and the compounds that are used to produce the different colors.
Rainbows
Another cause of color identified by Nassau is dispersive refraction. Refraction is the bending of light, something that happens when light passes from one medium to another. Check this out by observing a straw submerged in a glass of water - the straw appears to be "broken" at the surface of the water, because the light hitting the submerged part of the straw is bent (refracted) by the water, whereas the light hitting the top part is not."Dispersive refraction" means the light is dispersed or spread out into different wavelengths. When entering a new medium, different wavelengths of light are bent to different extents. This means the different wavelengths of light are fanned out in space, like we see when light passes through a prism.
About Rainbows is an excellent source on how rainbows are formed, and required reading for this week.
References
- Nassau, K. The Physics and Chemistry of Color: The Fifteen Causes of Color; 2nd ed.; Wiley: New York 2001.
- Scientific Instrument Services, Inc. "Alphabetic Listing of Elements, Exact Masses and Isotopic Abundances" http://www.sisweb.com/referenc/source/exactmaa.htm (accessed February 2005).
- Helmenstin, A. M. "The Chemistry of Firework Colors" http://chemistry.about.com/library/weekly/aa062701a.htm (accessed February 2005).
Note: Firework photos were taken at the Batavia fireworks show in 2003 and 2007.
Optional: for more information see: How Fireworks Work - from HowStuffWorks.com. This site goes into much more detail about the pyrotechnical aspects of fireworks. There is much more information here than we will be going into, but check it out if you are interested.
On-line Periodic Tables
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