Relative atomic mass scale

Atomic mass relative to C =12. Atomic weight of elements is given on the same scale. 

TTie following values have been obtained by the most precise mensurements with the mass-spectrograph for atomic masses relative to = 16, i.e. on the physical scale (Aston, Mass spectra and isotopes , Arnold, 1942, p. 116)... 

Elements Atomic mass Relative electronegativity (Poling scale) Ionization energy, kj-mole" Charge Ionic radius at coordination number 6,r k. Ionic potential... 

Atomic mass relative to =12. Atomic weight is given on the same scale. Half-life in decimal notation, ps = microseconds ms = milliseconds s = seconds m = minutes h = hours d = days and y = years. 

This table is scaled to the relative atomic mass Ar(13C) = 12. 

Relative isotopic, atomic and molecular masses are measured on a scale in which the mass of an atom of carbon-12 is exactly 12 atomic mass units (a.m.u.). 

The masses of the atoms are the basis for all stoichiometric calculations. Long before the actual masses of atoms were known, a relathe scale of masses, called the atomic ueightscale, was devised. Since 1961, this relative scale has been based on the assignment of the value 12.00000 to the most common isotope of carbon. The table on the inside back cover lists approximate values of the... 

This small quantity is not easy to work with so, as you saw in Chapter 3, a scale called the relative atomic mass scale is used. In this scale an atom of carbon is given a relative atomic mass, An of 12.00. All other atoms of the other elements are given a relative atomic mass compared to that of carbon. 

You have already seen in Chapter 3 how we can compare the masses of all the other atoms with the mass of carbon atoms. This is the basis of the relative atomic mass scale. Chemists have found by experiment that if you take the relative atomic mass of an element in grams, it always contains 6 x 1023 or one mole of its atoms. 

O relative atomic mass the average mass o( an atom on a scale where an atom of carbon-12 has a mass of 12 exactly... 

By fixing the scale of mass number to proton excess at 8 1, the latter quantity is mapped on a 44 x 44 square lattice, gauged on atomic number. The isotopes of any element map on to straight lines perpendicular to OZ, with Ru on the diagonal. The relative proton excess, 1 — tN/Z, varies... 

Notes Scaled to the relative atomic mass, Ar(12C) = 12, where 12C is a neutral atom in its nuclear and electronic ground state. 

Scaled to the relative atomic mass C in sp valence states radius... 

To calculate the atomic mass of any element—the average mass of its atoms on a relative scale—two ways (1) from the ratios of masses of equal numbers of atoms, as was done historically, and... 

Atoms are so tiny that, until recently, the masses of individual atoms could not be measured directly (Figure 3.7). However, because mass was so important in Dalton s theory, some measure of atomic masses was necessary. Therefore, a relative scale—the atomic mass scale—is used. This scale is sometimes called the atomic weight scale. On this scale, an average of the masses of all the atoms of the naturally occurring mixture of isotopes of a given element is measured relative to the mass of an atom of a standard. 

The early pioneers of chemistry, trying to verify Dalton s atomic theory, could not measure the mass of individual atoms. The best they could do was to measure the masses of equal numbers of atoms (or other known ratios of atoms) of two (or more) elements at a time, to determine their relative masses. They established one element as a standard, gave it an arbitrary value of atomic mass, and used that value to establish the atomic mass scale. The last naturally occurring mixture of isotopes that was used as a standard was oxygen, defined as having an atomic mass of exactly 16 atomic mass units (amu). That standard has been replaced see the next subsection. The atomic mass unit is tiny it takes... 

A great deal of difficulty was encountered at first, because Dalton s fifth postulate gave an incorrect ratio of numbers of atoms in many cases. Such a large number of incorrect results were obtained that it soon became apparent that the fifth postulate was not correct. It was not until some 50 years later than an experimental method was devised to determine the atomic ratios in compounds, at which time the scale of relative atomic masses was determined in almost the present form. These relative masses are called atomic masses, or sometimes atomic weights. 

Metal Electronic configuration Atomic number Relative atomic mass Metal radius (A) (kJmoL ) Electron affinity (kJmoL ) Electronegativity (Panting scale)... 

From the Table of Weights 1989 by ihe Commissioo on Atomic Weights, lUPAC (Ft/ef, Appl. Chem. 1991.63.97S-10023. Weights are scaled to the relative atomic mass A/ Q 12. 

It is customary to describe the decrease in mass by means of a quantity called the "packing fraction." This is the difference in mass, per fundamental particle (proton or neutron) in the nucleus, relative to as standard. An isotope which has atomic mass equal exactly to its mass number on the scale is said to have zero packing fraction. 

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