Periodic table atomic weight

See periodic table atomic weight mass number. 

CAS 1333-74-0. LL,. Nonmetallic element of atomic number 1, group IA of periodic table, atomic weight 1.0079, valence of 1. Molecular formula is LL,. Isotopes deuterium (2D), tritium (3T). Hydrogen discovered by Cavendish in 1766, named by Lavoisier in 1783. 

Hydrogen atoms have a periodic table atomic weight of 1.008u, which rounds to lu. Carbon atoms have a periodic table atomic weight of 12.01u, which ronnds to 12u. 

This result is slightly different from the periodic table atomic weight value of 35.45 because... 

Iron (Fe, from the Latin ferrum) is in group 8 (formerly transition Group VIII) of the periodic table (atomic number 26 and atomic weight 55.847). Its most important oxidation states are 2+ (ferrous) and 3+ (ferric), although higher oxidation states occur as transitory intermediates in some... 

Though individual atoms always have an integer number of amus, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element. Isotopes can have a weight either more or less than the average. The average number of neutrons for an element can be found by subtracting the number of protons (atomic number) from the atomic mass. 

Iodine [7553-56-2] I, atomic number 53, atomic weight 126.9044, is a nonmetaUic element belonging to the halogen family in Group 17 (VIIA) of the Periodic Table. The only stable isotope has a mass number of 127. There are 22 other iodine isotopes having masses between 117 and 139 14 of these isotopes yield significant radiation. 

Lead, atomic number 82, is a member of Group 14 (IVA) of the Periodic Table. Ordinary lead is bluish grey and is a mixture of isotopes of mass number 204 (15%), 206 (23.6%), 207 (22.6%), and 208 (52.3%). The average atomic weight of lead from different origins may vary as much as 0.04 units. The stable isotopes are products of decay of three naturally radioactive elements (see Radioactivity, natural) comes from the uranium series (see Uraniumand... 

Manganese [7439-96-5] atomic number 25, atomic weight 54.94, belongs to Group 7 (VII) ia the Periodic Table. Its isotopes are Mn, Mn, Mn,... 

Sihcon is a Group 14 (IV) element of the Periodic Table. This column iacludes C, Si, Ge, Sn, and Pb and displays a remarkable transition from iasulatiag to metallic behavior with increasing atomic weight. Carbon, ia the form of diamond, is a transparent iasulator, whereas tin and lead are metals ia fact, they are superconductors. SiUcon and germanium are semiconductors, ie, they look metaUic, so that a poHshed siUcon wafer is a reasonable gray-toned mirror, but they conduct poorly. Traditionally, semiconductors have been defined as materials whose resistance rises with decreasiag temperature, unlike metals whose resistance falls. 

Tantalum [7440-25-7] atomic number 73, is the heaviest element in Group 5 (VA) of the Periodic Table. This tough, ductile, silvery gray metal has an atomic weight of 180.948 amu. The element was discovered by A. K. Ekeberg in 1802 in minerals taken from Kimito, Finland, and Ytterby, Sweden (1). 

You can t duplicate these molecular weights for C7H16 and CsHa02 by using the atomic weights given in the periodic table. Those values are for the natural-abundance mixture of isotopes. The exact values are 12.00000 for C, 1.00783 for H, and 15.9949 for 0. 

Some of the important properties of the elements are given in Table 18.1. The imprecision of the atomic weights of Kr and Xe reflects the natural occurrence of several isotopes of these elements. For He, however, and to a lesser extent Ar, a single isotope predominates ( He, 99.999 863% " Ar, 99.600%) and much greater precision is possible. The natural preponderance of " Ar is indeed responsible for the well-known inversion of atomic weight order of Ar and K in the periodic table, and the position of Ar in front of K was only finally accepted when it was shown that the atomic weight of He placed it in front of Li. The second isotope of helium, He, has only been available in significant amounts since... 

To understand how the electron has been applied to explanations of the periodic table we must start with the discovery of the periodic system itself. The Russian chemist Dimitri Mendeleev announced in 1869 that the properties of elements arranged in order of increasing atomic weight appeared to repeat after certain definite intervals. Yet even as this discovery became increasingly well established, Mendeleev remained strongly opposed to any attempt to reduce or explain the periodicity in terms of atomic structure. He resisted the notion of any form of primary matter, which was actively discussed by his contemporaries, and opposed... 

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