Atomic and Mass Numbers

Strategy Use the principle that atomic and mass numbers must balance to find the symbol of the product nucleus. 

A Both the atomic and mass numbers in this representation are counted values and therefore integers. 

However, for the reason already outlined (see Section 1,1.8.) some chemists do not like this system and try to avoid it. As the descriptors are based on the CIP sequence rules, which in turn are based on atomic material properties, i.e., the atomic and mass number, the system cannot in general express genetic relationships within a collection of related compounds and throughout the complete range of reactants undergoing a given reaction. Unfortunately, reconciliation of the conflicting demands is impossible. 

Two very important numbers, the atomic number and the mass number, tell you much of what you need to know about an atom. Chemists tend to memorize these numbers like baseball fans memorize batting averages, but clever chemistry students like you need not resort to memorization. You have the ever-important periodic table of the elements at your disposal. We discuss the logical structure and organization of the periodic table in detail in Chapter 4, so for now, we simply explain what the atomic and mass numbers mean without going into great detail about their consequences. 

To specify the atomic and mass numbers of an element, chemists typically write the symbol of the element in the form where Z is the atomic number, A is the mass number, and X is... 

TABLE I RELATIONSHIP OF ATOMIC AND MASS NUMBERS IN DESIGNATING ISOTOPES... 

The terms radioactive transmutation and radioactive decay arc synonymous-Gencrally, the term decay is preferred in the English literature. As already mentioned in section 2.1, many radionuclides were found after the discovery of radioactivity in 1896, These radionuclides were named UXi, UX2,.., or mesothorium 1, mesothorium 2,., or actinouranium,.., in order to indicate their genesis. Their atomic and mass numbers were determined later, after the concept of isotopes had been established. 

As noted, the symbol for an element is simply either one or two letters. Sometimes it is necessary to show the atomic and mass numbers of the elements as well. When you see the symbol for an element written out with the atomic and mass numbers, like helium shown earlier, you can work out how many of each type of particle there are in a neutral atom of the element. Sodium, for example, is shown in Figure 1.3.3. 

Nuclides (i.e. kinds of atomic nuclei) having the same atomic and mass numbers, but existing in different energy states. One is always unstable with respect to the other, or both may be unstable with respect to a third. In the latter instance the energy of transformation in the two cases will differ. See geometric isomer optical isomer. 

Atomic and Mass Numbers Section 1.2A See problem 1.1 for explanation. 

Isomers. Nuclides having the same atomic and mass numbers but differing in energy and spin of the nuclei. For example, "Tc and "mTc are isomers. 

The standard notation for designating an atom s atomic and mass numbers is to show the atomic number as a subscript and the mass number as a superscript to the left of the letter representing the element. For example, the two naturally occurring isotopes of hydrogen are written jH and2H. 

Note that the electrons gained are written to the left of the reaction arrow (they are reactants), whereas the electrons lost are written as products to the right of the reaction arrow. For simplification the atomic and mass numbers are often omitted, because they do not change during ion formation. For example, the sodium cation would be written as Na+ and the anion of fluorine as F . 

Calculate the number of neutrons in an isotope, given the atomic and mass numbers. 

U neutron, also known as deuterium), and (2 neutrons, also known as tritium). Anuclide is an elemental form distinguished from others by its atomic and mass numbers. Some nuclides, such as U and Cs, are radioactive and spontaneously decay to a different nuclide with the emission of characteristic energy particles or electromagnetic waves isomers of a given nuclide that differ in energy content are metastable (i.e., " Cd) and characterized in part, by the half-life of the isomer. 

Notice that the atomic and mass numbers decreased by 2 and 4 units, respectively. The helium nucleus shown in the equation is an alpha particle. 

Thus the stopping power depends on the charge and velocity of the incident particle as well as on the effective ionization potential and the atomic and mass numbers of the atoms of the medium. 

The high final energy of the ions, in the order of tens of MeV, provides such a good resolution in energy or energy-loss measurements, that every single ion can usually be identified both by atomic and mass numbers. 

For every isotope of every element there is a nuclear ground state with a nuclear spin quantum number I. I must have a value of /2, where n is an integer. Isotopes having atomic and mass numbers that are both even (e.g. Si, Fe) have 1=0, and these nuclei have no magnetic moment and so do not give NMR... 

The isotope that can be produced in the largest quantities is Fm. It is also the nuclide of highest atomic and mass numbers ever isolated from either reactor- or thermonuclear-produced materials. The neutron-production chain essentially terminates at mass 257 owing to the very short half-lives of the heavier isotopes. 

Of the stable nuclides, has the highest atomic number and mass number. All known nuclides beyond it in atomic and mass numbers are radioactive. Naturally occurring is radioactive and disintegrates by the loss of a particles. 

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