Atomic mass determination

Atomic masses determination by chemical and physical means... 

Because MS identifies components on the basis of their atomic or molecular mass and, in the case of organic compounds, fragmentation pattern, it is a particularly useful and powerful detector. The molecular mass, which is commonly referred to as the molecular weight, is a particularly useful piece of information when trying to identify a compound. For elements, atomic mass determination can also determine the isotope present, which in turn can be used to identify the source and movement of an element through the environment. 

In a chemical atomic mass determination, the tin content of 3.7692g SnCl4 was found to be 1.7170g. If the atomic mass of chlorine is taken as 35.453, what is the value for the atomic mass of the tin isolated during this experiment ... 

The mass of iodine in a certain compound is almost exactly twice that of the only other element—copper. Using their atomic masses, determine the formula of the compound. 

How is average atomic mass determined from isotopic masses ... 

When applied precisely, the gas density method may be used for atomic mass determinations, particularly for the lighter elements. Even crude gas density experiments may be used, in conjunction with chemical composition data and known atomic masses to establish the molecular mass, and hence the molecular formula of a gaseous compound. 

From the atomic masses determined by the mass-spectrograph, we have... 

The problem can, however, be considered in a somewhat higher degree of approximation than this. Two kinds of evidence exist on the one hand, the precise values of the atomic masses, determined by the mass spectrometer for individual isotopes of the elements and, on the other hand, the occurrence of the different types of radioactivity in various nuclei, natural and artificial. 

Thus, Mosander s activities led to the originally two-element division into a six-element division. The cerium compounds are yellow at the higher oxidation level and colourless at the lower oxidation level, lanthanum compounds are white, didymium compounds are red, yttrium and erbium compounds are white, terbium compounds are pink. Chemists existed, of course, who disputed the existence of these elements. Unequivocal identification of elements was, however, possible in later times only. In the period in question, the main characteristics on the basis of which a substance could be qualified as a new element were separability, colour, crystal shape and reactivity. Even atomic mass determinations were largely uncertain, particularly in the group of the rare earth elements, it will be seen in the... 

Since most elements occur as mixtures of isotopes with different masses, the atomic mass determined for an element represents the average relative mass of all the nat-uralfy occurring isotopes of that element. The atomic masses of the individual isotopes are approximately whole numbers, because the relative masses of the protons and neutrons are approximately 1.0 amu each. Yet we find that the atomic masses given for many of the elements deviate considerably from whole numbers. 

Until recently almost all atomic-mass determinations were made by the chemical method. This method consists in determining the amount of the element that will combine with one gram-atom of oxygen or of another element with known atomic mass. One example, which was important in the development of the theory of radioactivity, follows. 

---