
Review of Short Phrases and Links 
This Review contains major "Atomic Weights" related terms, short phrases and links grouped together in the form of Encyclopedia article.
Definitions
 Atomic weights are expressed in terms of a standard atom: the isotope of carbon that has 6 protons and 6 neutrons in its nucleus.
 Atomic weights are routinely used in chemistry in order to determine how much of one chemical will react with a given weight of another.
 Atomic weights were formerly determined directly by chemical means; now a mass spectrograph is usually employed.
 The atomic weights are now based on carbon12.
 Since atomic weights are average values, molecular weights are also average values.
 Problem 2: Listed in the following chart are the atomic weights (measured in atomic mass units) for natural silver and its two isotopes.
 He made a list of these relative atomic weights for as many elements as he knew.
 These numbers were formerly called “atomic weights”, but are now called “relative atomic masses” (r.a.m.).
 Elements of higher atomic number have correspondingly heavier atomic weights; this fact could have been predicted from Prout's hypothesis.
 The most strongly bound nuclei are those with atomic weights between about 50 and 65 (the iron group).
 Over a century ago, chemists developed a purely relative scale for atomic weights.
 To get the formula weight of this compound we would add up the atomic weights.
 In the conversion of moles to kilograms we have assumed the atomic weights of hydrogen, carbon, nitrogen and oxygen are 1, 12, 14 and 16 respectively.
 However, erroneous assumptions about the formulas for other compounds led to half a century of confusion about atomic weights.
 The fact that the atomic weights are not the whole numbers as originally planned has brought about some confusion.
 T. W. Richards did important work on atomic weights (after 1883) and revised some of Stas's values.
 Dalton estimated the atomic weights according to the mass ratios in which they combined, with hydrogen being the basic unit.
 One difficulty arose from the fact that chemical formulas could be written in any one of a number of ways, if the atomic weights were unknown.
 In 1803 Dalton orally presented his first list of relative atomic weights for a number of substances.
 Its stable isotopes are 12 C (98.90%) and 13 C (1.10%). The weight of the 12 C atom is the international standard on which atomic weights are based.
 They have the same number of protons in their nuclei but a different number of neutrons (the same atomic number but different atomic weights).
 The atomic weights of the other elements were originally compared to hydrogen without specifying which isotope.
 EXAMPLE 1 Use the Table of Atomic Weights to show that the mass of a mercury atom is 2.510 times the mass of a bromine atom.
 Then the weights of a given element's isotopes are averaged to give the atomic weights found on the Periodic Table.
 Naturally occurring chemical elements are usually mixtures of isotopes so that observed (noninteger) atomic weights are average values for the mixture.
 The American chemist Theodore William Richards (18681928) ushered in a new age of accuracy in chemistry by determining the atomic weights of many elements.
 In the early 19th century, the atomic weights of chemical elements began to be measured.
 The primary decay products at atomic weights below 45 Sc are calcium isotopes and the primary products from higher atomic weights are titanium isotopes.
 By 1818, JÃ¶ns Jakob Berzelius had determined atomic weights for fortyfive of the fortynine accepted elements.
 Atomic weights of elements with atomic numbers from 1109 taken from this source.
 Periodic law: (that properties of elements are functions of their atomic weights) Dmitri Mendeleev, Russia, 1869.
 Knowing the number of atoms of helium expelled from the atom of each product, we can at once calculate the atomic weights of the products.
 The formula weight is the sum of the atomic weights of the atoms in an empirical formula.
 In the case of the HCl, we can add the atomic weights of the elements in the compound and get a molecular weight.
 His work provided a simple way to determine atomic weights and molecular weights of gases.
 Carbon12 was chosen by IUPAC in 1961 as the basis for atomic weights; it is assigned an atomic mass of exactly 12 atomic mass units.
 In contrast to atomic weights, which can be defined only approximately, atomic masses are exact constants of nature.
 Listings of atomic weights or unified atomic mass units usually average the isotopes of an element.
 Isotopes are atoms of the same element that have different numbers of neutrons in the nucleus and therefore different atomic weights (shown as a superscript.
 At normal temperatures the gases of the two isotopes differ only in their atomic weights.
 In each group of three, the atomic weight of one element fell halfway between the atomic weights of the other two elements.
 But carbon 12 is defined as having an atomic weight of exactly 12.000000  in fact this is the standard by which all other atomic weights are defined.
Atomic Weights
 Isotopes In 1961 the International Union of Pure and Applied Chemistry adopted the isotope carbon12 for basis for atomic weights.
 The molecular weight may be calculated from the molecular formula of the substance; it is the sum of the atomic weights of the atoms making up the molecule.
 The average mass of a molecule, calculated by summing the atomic weights of atoms in the molecular formula.
Categories
 Atomic Weight
 Atomic Masses
 Molecular Weights
 Carbon12
 International Union

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