Atomic Masses of the Elements

The change from non-metallic to metallic properties of the Group V elements as the atomic mass of the element increases is shown in their reactions with alkalis. 

As it happens, naturally occurring fluorine consists of a single isotope, ijF. It ibllows that the atomic mass of the element fluorine must be the same as that of F-19,19.00 amu. The situation with most elements is more complex, because they occur in nature as a mixture of two or more isotopes. To determine the atomic mass of such an element, it is necessary to know not only the masses of the individual isotopes but also their atom percents (isotopic abundances) in nature. 

The weighted-average atomic mass of the element iridium is just slightly more than 192 u. The mass of the first isotope is a bit less than 191 u. Hence, the mass of the second isotope must more than 192 u that isotope must be 193 Ir. 

Mass spectrometry is based upon the separation of charged ionic species by their mass-to-charge ratio, m/z. Within the general chemical context however, we are not used to taking into concern the isotopes of the elemental species involved in a reaction. The molecular mass of tribromomethane, CHBrs, would therefore be calculated to 252.73 g mol using the relative atomic masses of the elements as listed in most periodic tables. In mass spectrometry we have to leave this custom behind. Because the mass spectrometer does not separate by elements but by isotopic mass, there is no signal at m/z 252.73 in the mass spectmm of tribromomethane. Instead, major peaks are present at m/z 250, 252, 254 and 256 accompanied by some minor others. 

The relative molecular mass, M, or molecular weight is calculated from the relative atomic masses of the elements contributing to the empirical formula. [3]... 

ISOTOPES There are a total of 46 isotopes of thulium. One of these, Tm-169 is the only stable isotope of thulium and accounts for the total atomic mass of the element. All the other isotopes are artificially produced and radioactive and have half-lives ranging from a few microseconds to two years. 

What are the atomic number and atomic mass of the element formed when 2gf Po emits a beta particle What are they if the polonium emits an alpha particle ... 

The formula mass of a substance is the sum of the atomic masses of the elements in its chemical formula. The atomic mass is the mass of a single atom. 

The top number in the symbols is the mass number and the bottom number is the atomic number. The atomic mass of the element hydrogen is the weighed average of the mass numbers of the three isotopes according to several tables ... 

Aluminium oxide ceramics contain elements with an atom mass which is virtually identical to the atom mass of the elements in bone. The crystal lattice contains Al3+ and O 2 ions. The aluminium oxide is not stable in the air as it wants to combine with components in the air. That is why the surface of a crystal only contains O2-ions. These attract substances with a large dipole from the environment, like for instance water molecules and protein-containing body substances. Since the aluminium oxide implant in the body is covered by a layer of protein molecules, the body does not recognize the implant as nonself, and consequently the defence mechanism is not activated. 

An isotope is an atom of an element that has the same number of protons as another atom of that element but a different number of neutrons. When the percent abundance of the isotopes in a sample of an element are known, the average atomic mass of the element can be calculated. 

A simple periodic table contains the symbols, atomic numbers and the relative atomic masses of the elements. Additionally, detailed periodic tables containing some physical and chemical properties (such as melting point, boiling point, oxidation state) are also made. 

In this table the significant figures in all columns represent the accuracy with which the atomic masses of the elements are known. 

The process referred to is He - jLi + The calculation of the mass change only requires the whole number atomic masses of the elements since, if we add two electrons to each side, we would have enough for a whole helium atom on the left and a whole lithium atom on the right. 

The terrestrial composition of an element is 50.7% as an isotope with an atomic mass of 78.9 u and 49.3% as an isotope with an atomic mass of 80.9 u. Both isotopes are stable. Calculate the atomic mass of the element. 

It is time for another look at the periodic table. Recall from Unit 1 that the larger of the two numbers in each square is called the atomic mass of the element. Molar mass is the sum of the atomic masses of all the atoms in the formula of a compound or molecule. Remember to round these atomic masses to the nearest whole number. 

A mass spectrometer that measures masses very accurately, usually to 1 part in 20,000. This high precision allows calculation of molecular formulas using the known atomic masses of the elements, (p. 545)... 

If you obtain the isotopic abundance of an element from mass spectrometer data or a table, you can calculate the average atomic mass of the element. You do this by calculating the weighted average of each isotope s mass. A weighted average takes into account not only the values associated with a set of data, but also the abundance or importance of each value. 

In section 5.1, you learned about the average atomic mass of an element. Then, in section 5.2, you learned how chemists group particles using the mole. In the final section of this chapter, you will learn how to use the average atomic masses of the elements to determine the mass of a mole of any substance. You will learn about a relationship that will allow you to relate the mass of a sample to the number of particles it contains. 

Table 6.2 lists the relative atomic masses of the elements in the alphabetical order of chemical symbols. The values have been recommended by the IUPAC Commission on Atomic Weights and Isotopic Abundances in 1991 [44] and apply to elements as they exist naturally on earth. 

Many atomic masses were determined as a direct result of Dalton s postulates and the work that they stimulated, and scientists attempted to relate the atomic masses of the elements to the elements properties. This work culminated in the development of the periodic table by Dmitri Mendeleyev (1834-1907) (Figure 3.8) and independently by Lothar Meyer (1830-1895). Because Mendeleyev did more with his periodic table, he is often given sole credit for its development. 

---