Weight, atomic

Atoms are small pieces of matter, so they have mass. In this section we discuss the mass scale used for atoms and introduce the concept of atomic weights. 

Scientists of the nineteenth century were aware that atoms of different elements have different masses. They found, for example, that each 100.0 g of water contains 11.1 g 

Today we can determine the masses of individual atoms with a high degree of accuracy. For example, we know that the H atom has a mass of 1.6735 X 10 g and the atom has a mass of 2.6560 X 10 g. As we noted in Section 2.3, it is convenient to use the atomic mass unit when dealing with these extremely small masses  

The atomic mass unit is presently defined by assigning a mass of exactly 12 amu to a chemically unbound atom of the isotope of carbon. In these units, an H atom has a 

Most elements occur in nature as mixtures of isotopes. We can determine the average atomic mass of an element, usually called the element s atomic weight, by summing (indicated by the Greek sigma, 2) over the masses of its isotopes multiplied by their relative abundances  

Symbol Number of Protons Number of Electrons Number of Neutrons 

Atoms with identical atomic numbers but different mass numbers (that is, same number of protons but different numbers of neutrons) are called isotopes of one another. Several isotopes of carbon are listed in A TABLE 2.2. We will generally use the notation with superscripts only when referring to a particular isotope of an element. 

How many protons, neutrons, and electrons are in (a) an atom of Au, (b) an atom of strontium-90  

Answer (a) 56 protons, 56 electrons, and 82 neutrons, (b) 15 protons, 15 electrons, and 16 neutrons 

The relative abundances of the various isotopes of the light elements Li, Be and B therefore depend to some extent on which detailed model of the big bang is adopted, and experimentally determined abundances may in time permit conclusions to be drawn as to the relative importance of these processes as compared to x-process spallation reactions. 

Because of their central importance in chemistry, atomic weights have been continually refined and improved since the first tabulations by Dalton (1803 -5). By 1808 Dalton had included 20 elements in his list and these results were substantially extended and improved by Berzelius during the following decades. An illustration of the dramatic and continuing improvement in accuracy and precision during the past 100 y is given in Table 1.3. In 1874 no atomic weight was quoted to better than one part in 200, but by 1903 33 elements had values quoted to one part in 10 and 2 of these (silver and 

Accurate atomic weight values do not automatically follow from precise measurements of relative atomic masses, however, since the relative abundance of the various isotopes must also be determined. That this can be a limiting factor is readily seen from Table 1.3 the value for praseodymium (which has only 1 stable naturally occurring isotope) has two more significant figures than the value for the neighbouring element cerium which has 4 such isotopes. In the twelve years since the first edition of this book was published the atomic weight values of no fewer than 55 elements have been improved, sometimes spectacularly, e.g. Ni from 58.69( 1) to 58.6934(2). 

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The atomic weight of a particular sample depends on the origin of the sample. 

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

Dulong and Pedt s law The product of the atomic weight and the specific heat of a metal is constant of value approximately 6-2. Although not true for all metals at ordinary temperatures, these metals and several non-metals approximate to the law at high temperatures. 

It is a ratio. The molecular weight is equal to the sum of the atomic weights of the constituent nuclei. The molecular weight expressed in grams is known as the gram molecular weight. 

By 1850. values of atomic weights (now called relative atomic masses) had been ascertained for many elements, and a knowledge of these enabled Newlands in 1864 to postulate a law of octaves. When the elements were arranged in order ol increasing atomic weight, each... 

Lead has only one form, a cubic metallic lattice. Thus we can see the change from non-metal to metal in the physical structure of these elements, occurring with increasing atomic weight of the elements carbon, silicon, germanium, tin and lead. 

All Group IV elements form both a monoxide, MO, and a dioxide, MO2. The stability of the monoxide increases with atomic weight of the Group IV elements from silicon to lead, and lead(II) oxide, PbO, is the most stable oxide of lead. The monoxide becomes more basic as the atomic mass of the Group IV elements increases, but no oxide in this Group is truly basic and even lead(II) oxide is amphoteric. Carbon monoxide has unusual properties and emphasises the different properties of the group head element and its compounds. 

All Group IV elements form tetrachlorides, MX4, which are predominantly tetrahedral and covalent. Germanium, tin and lead also form dichlorides, these becoming increasingly ionic in character as the atomic weight of the Group IV element increases and the element becomes more metallic. Carbon and silicon form catenated halides which have properties similar to their tetrahalides. 

The Molecular weight of the acid itself is clearly equal to that of the silver salt minus the atomic weight of sih er plus the weight of hydrogen displaced by G X 107-9 ... 

Then, since Atomic Weight of platinum = 195 2, if g g. platinum is obtained from G g. chloroplatinate,... 

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