Relative atomic masses of the elements

Element Symbol Atomic number Relative atomic mass (1971) Log of relative atomic mass 

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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]... 

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. 

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. 

Gravimetric analysis is the process of converting an element into a definitive compound, isolating this compound from other constituents in a sample and then weighing the compound (Box 20.1). The weight of the element can then be calculated from the formula of the compound and the relative atomic masses of the elements involved. You need to be able to weigh accurately, by difference, a substance to four decimal places (see p. 23). 

You should already know (Sections 1.3.3 and 1.3.4) how to calculate atomic masses from the numbers of protons and neutrons in the nucleus, and that isotopes are taken into account when determining the relative atomic masses of the elements. 

Table of relative atomic masses of the elements, 1981 [Pure Appl. Chem. 55,1101 (1983), alphabetical order, based on the relative atomic mass, = 12]... 

In 1869, the Russian chemist Dmitri Mendeleev used Newlands s observation and other information to produce the first orderly arrangement, or periodic table, of all 63 elements known at the time. Mendeleev wrote the symbol for each element, along with the physical and chemical properties and the relative atomic mass of the element, on a card. Like Newlands, Mendeleev arranged the elements in order of increasing atomic mass. Mendeleev started a new row each time he noticed that the chemical properties of the elements repeated. He placed elements in the new row directly below elements of similar chemical properties in the preceding row. He arrived at the pattern shown in Figure 2. 

Relative atomic masses have no units because they are ratios of two masses measured in whatever units we choose (grams, kilograms, pounds, and so forth). The relative molecular mass of a compound is the sum of the relative atomic masses of the elements that constitute it, each one multiplied by the number of atoms of that element in a molecule. For example, the formula of water is H2O, so its relative molecular mass is... 

Daltons), assigned to the isotope of carbon. It is a relative molecular mass because it is a sum of all the relative atomic masses of the elements contained within the molecule. 

Each substance is identified by the usual form of its chemical formula and by its most commonly employed chemical name. In some cases, with in-termetallic compounds in particular, the name corresponds to a list of the chemical elements in the formula. The molar masses (grnol ) are calculated from the relative atomic masses of the elements taking A = 2 for C and are reported at the beginning or end of the first line. 

Two naturally occurring isotopes of an element have masses and abundance as follows 54.00 amu (20.00%) and 56.00 amu (80.00%). What is the relative atomic mass of the element ... 

Appendix E Chemical Names, Symbols and Relative Atomic Masses of the Elements... 

Taking the laboratory preparation of copper(ii) sulfate from copper(ii) oxide and sulfuric acid as an example, this is an acid-base reaction. This calculation is explained later in the chapter and involves calculating the mass of one mole of each substance from the formula and the relative atomic masses of the elements. 

The molar mass of molecules or formula units is calculated by adding together the relative atomic masses of the elements in the molecule. 

Molecular weight is an incorrect scientific term The numerical value being referenced is a relative molecular mass. It is a mass, measuring a quantity of matter, not the influence on that matter by an external gravitational field. It is a relative mass, not an absolute mass measured in grams, because it is expressed relative to the standard atomic mass of 12.000000 units (daltons), assigned to the 12C isotope of carbon. It is a relative molecular mass because it is a sum of all the relative atomic masses of the elements contained within the molecule. 

To do this you divide the number of grams of each element by the relative atomic mass of the element. In this example there would be... 

For the sample c is the concentration of sample solution (mlm) w the mass of sample solution Ar the relative atomic mass of the element being determined X the isotope abundance (atom%) of isotope 1 T the isotope abundance (atom%) of isotope 2. 

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