Learning by Simulations
[CENTER]أولا: طريقة التحميل للملفات
1- Atomic Spectra
Atomic spectra may be used to quantitatively determine more than 70 chemical elements. In order to be able to detect the atomic spectrum, the atoms or ions have to be separated from one another, i.e. the atoms have to be in a gaseous state. This can be achieved by massively heating the substances to be determined (typically the temperature is several thousand degrees). When the gas heats up, it emits light of various characteristic wavelengths. For hydrogen the resulting spectrum obeys a very simple relation:
where ni and nf are positive non-zero integers with ni > nf and RH is a constant called Rydberg's constant:
RH = 1.097
Apart from chemical analysis atomic spectroscopy is of considerable importance for the investigation of stars, looking for elemental compositions which could enable life in some form. The interest in atomic emission and absorption lines can be traced back to the late 18th and the early 19th Century when Fraunhofer discovered dark lines in the spectrum of the sun light.
f you would like to experiment with spectral lines of selected elements, you can use the program AtomSpec which allows the user to select from several chemical elements. The spectrum may be zoomed to view details of the atomic spectral lines.
2- Molecular Formulas
If we know the molecular weight of a substance we implicitely know the atomic composition of that substance - provided that the molecular weight is known to an accuracy which is sufficient to distinguish between molecules having the same nominal mass.
To explain this, let's have a look at a very simple example: suppose we know that our substance has a molecular weight of 28. A quick calculation (considering only the nominal masses of the most abundant isotopes of carbon, hydrogen, oxygen, and nitrogen - 12, 1, 16, and 14, respectively) shows that there are three substances which all have the same molecular mass of 28: C2H4 (ethene), CO (carbon monoxide) and N2 (nitrogen gas).
Now, the important point is not to forget that the nominal masses are just for convenience. The actual masses of the four elements are:
12C = 12.00000
1H = 1.007825037
16O = 15.99491464
14N = 14.003074 From this you can easily calculate that the actual molecular masses of the three substance having a nominal mass of 28 are:
- C2H4 = 28.031300148
- N2 = 28.006148
- CO = 27.99491464
The smallest difference between the three masses is between CO and N2, showing a diffence of 0.0112 mass units. In other words, if we know the molecular mass to an accuracy of 0.0056 masses (half the minimum difference) we can easily distinguish between the three molecules having a nominal mass of 28.
This simple example shows the principle of finding the atomic composition by means of mass spectrometry. You simply have to determin the mass with an accuracy high enough to reduce the number of possible candidates to a single molecule. Of course, in practice the molecular masses of interest are much higher and the number of possible candidates increases exponentially if we do not take countermeasures, such as a chemical plausibility check.
The program MolForm allows to calculate all possible molecular formulas of a particular mass. The user may set the molecular mass and its tolerance (accuracy). This example shows the possible formulas for molecules containing C, H, O, N, and Cl atoms.
3- Console of a Mass Spectrometer
A mass spectrometer is a device which can perform accurate chemical analysis (both quantitative and qualitative). Although there are several different technical implementations (sector field, quadrupole, time of flight) all of these devices are based on the same idea: the molecules under investigation are first broken up and then the type and the amount of fragments are investigated. The kind of fragments provides information on the chemical structure of the original molecule, the amount of fragmented ions allows to determine the quantities. Regardless of the actual ionisation and separation technique all kinds of mass spectrometers eventually yield a mass spectrum, which is a line spectrum showing the fragment mass on the x-axis and the number of generated ions on the y-axis.
This program ms_scope simulates the console of a mass spectrometer. The user may experiment with different resolutions and mass ranges. Ambient air, pregnene, 6-methyl-5-nonen-4-one and perfluorokerosene may be put into the simulated mass spectrometer.
To be continued
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