© IUPAC (International Union of Pure and Applied Chemistry). Source: Adapted from Pure and Applied Chemistry 78, no. 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. 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 A.1 The Basics of the Elements of the Periodic Table Name For example, see PTable and the Accessible Syngenta Periodic Table of Elements. 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 A.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 15 “Nuclear Chemistry”) require more decimal places. For many applications, only one or two decimal places are necessary for the atomic mass. The molar mass is an average of many instances of the compound, which often vary in mass. 1 The molar mass is a bulk, not molecular, property of a substance. One item on most periodic tables is the atomic mass of each element. 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. What follows here is a more complex version of the periodic table than what was presented in the chapter “Atoms, Molecules, and Ions.” The internet is a great place to find periodic tables that contain additional information. 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, introduced in Chapter 3 “Atoms, Molecules, and Ions,” lists all the known chemical elements, arranged by atomic number (that is, the number of protons in the nucleus). In this appendix, we present some data on the chemical elements. I think in general though, the number of sig figs tends to be determined by the values given in the problem not the molar masses, because values aren't typically given with more than 4 sig figs.Appendix A: Periodic Table of the Elements (your example works here because the given values you used in your problem were less than 4 sig figs). Therefore, if you use the 4 sig fig value typically given on a table, you wouldn't be able to determine the answer to your problem to any more than 4 sig figs without being slightly inaccurate. For example, the atomic mass of carbon is written as 12.01 u on most tables, but if you look up the molar mass of carbon online, you'll find that a more exact value is 12.0107 u. Although those values are known, they are not exact. I'd have to disagree with you there Jaesal. For example, if it said 1.53 mol of CO2, then you would use 3 sig figs throughout the problem, but when calculating the molar mass of CO2 you can use whatever the value is on the periodic table without correcting for sig figs. Instead you would determine the number of sig figs from any given or measured values in the problem. JaesalSoma1E wrote:I'm pretty sure it would not matter how many sig figs are on the periodic table because those values are known.
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