![]() ![]() It was exactly equal before the redefinition of the mole in 2019, and is now only approximately equal, but the difference is negligible for all practical purposes. The mole was defined in such a way that the molar mass of a compound, in g/mol, is numerically equal to the average mass of one molecule, in daltons. However, for historical reasons, molar masses are almost always expressed in g/mol. In the International System of Units (SI), the coherent unit of molar mass is kg/ mol. The molar mass is an intensive property of the substance, that does not depend on the size of the sample. The formula weight is a synonym of molar mass that is frequently used for non-molecular compounds, such as ionic salts. The difference is that molecular mass is the mass of one specific particle or molecule, while the molar mass is an average over many particles or molecules. The molecular mass and formula mass are commonly used as a synonym of molar mass, particularly for molecular compounds however, the most authoritative sources define it differently. The molar mass is appropriate for converting between the mass of a substance and the amount of a substance for bulk quantities. Most commonly, the molar mass is computed from the standard atomic weights and is thus a terrestrial average and a function of the relative abundance of the isotopes of the constituent atoms on Earth. The molar mass is an average of many instances of the compound, which often vary in mass due to the presence of isotopes. The molar mass is a bulk, not molecular, property of a substance. R Range in isotopic composition of normal terrestrial material prevents a more precise Ar(E) being given the tabulated Ar(E) value and uncertainty should be applicable to normal material.In chemistry, the molar mass ( M) of a chemical compound is defined as the ratio between the mass and the amount of substance (measured in moles) of any sample of said compound. Substantial deviations in the atomic mass of the element from that given in the table can occur. M Modified isotopic compositions may be found in commercially available material because it has been subjected to an undisclosed or inadvertent isotopic fractionation. The difference between the atomic mass of the element in such specimens and that given in the table may exceed the stated uncertainty. G Geological specimens are known in which the element has an isotopic composition outside the limits for normal material. †Commercially available Li materials have atomic weights that range between 6.939 and 6.996 if a more accurate value is required, it must be determined for the specific material. However, three such elements (Th, Pa, and U) have a characteristic terrestrial isotopic composition, and for these an atomic mass is tabulated. Table 21.1 The Basics of the Elements of the Periodic Table Name The web offers many interactive periodic table resources. Some elements, such as lithium, can have varying atomic masses depending on how their isotopes are isolated. The atomic masses of other elements, especially radioactive elements, are not known as precisely. The atomic masses of some elements are known very precisely, to a large number of decimal places. The atomic masses in Table 21.1 "The Basics of the Elements of the Periodic Table" represent the number of decimal places recognized by the International Union of Pure and Applied Chemistry, the worldwide body that develops standards for chemistry. ![]() However, some applications (especially nuclear chemistry see Chapter 11 "Nuclear Chemistry") require more decimal places. Or two decimal places are necessary for the atomic mass. One item on most periodic tables is the atomic mass of each element. The Internet is a great place to find periodic tables that contain additional information. What follows here is a relatively simple version. Periodic tables are available with a variety of chemical and physical properties listed in each element’s box. ![]() Many of the physical and chemical properties of the elements are either known or understood based on their positions on the periodic table. The periodic table is arguably the best tool in all of science no other branch of science can summarize its fundamental constituents in such a concise and useful way. The periodic table listsĬhemical elements, arranged by atomic number (that is, the number of protons in the nucleus). In this chapter, we present some data on the chemical elements. Chapter 21 Appendix: Periodic Table of the Elements ![]()
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