Atomic weights defined

Atomic Weight. As of this writing (ca 1994) the definition of atomic weights is based on carbon-12 [7440-44-0], the most abundant isotope of carbon, which has an atomic weight defined as exactiy 12 (21). 

Isotopes answer a puzzle that troubled chemists ever since Dalton proposed his atomic theory. Dalton said that the key property of an atom is not its size or shape but its weight. Each element is characterized by an atomic weight defined relative to that of hydrogen. The fact that these relative atomic weights were usually... 

Table 1 Atomic Parameters Computed with Different Weighting Schemes X Represents the Relative Electronegativity Y the Relative Covalent Radii VwZ the Atomic Number Z Atomic Parameter VwX the Atomic Parameters Based on Relative Electronegativity VwY the Atomic Weights Defined on Relative Covalent Radii...

Atomic weights were formerly defined relative to one atom of H and then to 0. 

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. 

Faraday s Law of electrolysis states that the amount of chemical change, ie, amount dissolved or deposited, produced by an electric current is proportional to the quantity of electricity passed, as measured in coulombs and that the amounts of different materials deposited or dissolved by the same quantity of electricity are proportional to their gram-equivalent weights (GEW) defined as the atomic weight divided by the valence. The weight in grams of material deposited, IF, is given by... 

The formula weight of a substance is equal to its number of grams per mole. Avogadro s number is the number of atomic mass units in 1 g. It is defined in that manner so that the atomic weight of an element (in amu) is numerically equal to the number of grams of the element per mole. Consider helium, with atomic weight 4.0 ... 

The probability that the inner shell vacancy will de-excite by one or other of these processes depends on the energy level of the initial vacancy and the atomic weight of the atom. The fluorescent yield, co, is defined as the number of X-ray photons emitted per unit vacancy, and is a measure of the... 

Act = 226.5 the naming of the parents follows the nomenclature employed by Fajans in which the element uranium is referred to as UrI. A place in the periodic table is then defined by a specific row and a specific column. From the data, one then readily obtains the row and column of each of the elemental daughters. Since the characterization of decay products emphasized the fact that many of these products are chemically indistinguishable from one another, it is of course expected that many of the places in the last two rows of the periodic table are occupied by more than one elementary material. What is surprising is that these chemically equivalent materials have different atomic weights, some differing by as much as eight units. 

Atomic weight—the relative weights of atoms hydrogen is the lightest and is defined to be one carbon is 12 and oxygen is 16, meaning they are 12 and 16 times as heavy as hydrogen. 

Chlorinity was first defined in 1902 and, hence, was affected by subsequent refinements in measurement of the atomic weight of chlorine. To make chlorinity independent of any such future changes, a new definition of chlorinity was adopted in 1937, e g., the mass of silver required to precipitate completely the halogens in 0.3285234kg of sample seawater. ... 

As mentioned in the previous section titled A Short History of Chemistry, many scientists identified elements, determined their characteristics, similarities, and differences, and designed symbols for them. Using unique experiments, scientists devised ways to define the structure of atoms and determine atomic weights, sizes, and electrical charges as well as energy levels for atoms. 

At one time, the hydrogen atom with one proton and no neutron was used as the standard to define 1 atomic mass unit (1 amu). Today, chemists use carbon-12, the most abundant isotope of carbon for the standard amu, which is defined as 1/12 of the C-12 atom. Therefore, the actual atomic weight for an element is in average mass units (numbers), taking into account all the isotopes (atoms) of that element. 

The atomic number, which appears above each element symbol, represents the meaningful order in the periodic table. When an element is referred to by an integer, this number means the atomic number, not the atomic weight. Thus, element 27 is cobalt (whose atomic number is 27), not aluminum (whose atomic weight is 27). In Chapters 2 and 3, these two concepts are more carefully defined for now, simply bear in mind the distinction between atomic number and atomic weight. 

For nonmolecular substances like sodium chloride, the use of the word mole, with its connotation of molecules, is inappropriate. A comparable unit, the gram formula weight, is used it is defined as the sum in grams of the atomic weights of all the atoms in the chemical formula of the substance. For sodium chloride (NaCl), one gram formula weight is calculated as... 

A particular element is defined by its atomic number - the number of protons in the nucleus (which will equal the number of electrons surrounding the nucleus in a neutral atom). For example, iron is the element of atomic number 25, meaning that every iron atom will have 25 protons in its nucleus. Chemists use a one or two-letter symbol for each element to simplify communication iron is given the symbol Fe, from the old Latin word for iron, ferrum. The sum of the protons plus neutrons found in a nucleus is called the mass number. For some elements only one mass number is found in nature. Fluorine (atomic number 9, mass number 19) is an example of such an element. Other elements are found in nature in more than one mass number. Iron is found as mass number 55 (91.52%), 54 (5.90%), 57 (2.245%), and 58 (0.33%). These different mass numbers of the same element are called isotopes, and vary in the number of neutrons found in the nucleus. Atomic weight refers to the average mass found in nature of all the atoms of a particular element the atomic weight of iron is 55.847. For calculation purposes, these... 

The elements in the modern table increase in atomic weight as one goes from left to right and from top to bottom - but the weights do not advance in even steps. The progression is defined not so much by atomic weight as by atomic number. This is defined as the number of protons in the respective atomic nuclei. 

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