Mass number, atomic

Atoms with the same number of protons but a different number of neutrons are called isotopes. To identify an isotope we use the symbol E, where E is the element s atomic symbol, Z is the element s atomic number (which is the number of protons), and A is the element s atomic mass number (which is the sum of the number of protons and neutrons). Although isotopes of a given element have the same chemical properties, their nuclear properties are different. The most important difference between isotopes is their stability. The nuclear configuration of a stable isotope remains constant with time. Unstable isotopes, however, spontaneously disintegrate, emitting radioactive particles as they transform into a more stable form. 

The most important types of radioactive particles are alpha particles, beta particles, gamma rays, and X-rays. An alpha particle, which is symbolized as a, is equivalent to a helium nucleus, fHe. Thus, emission of an alpha particle results in a new isotope whose atomic number and atomic mass number are, respectively, 2 and 4 less than that for the unstable parent isotope. 

An alplia p uticle is an energetic helium nucleus. The alplia particle is released from a radioactive element witli a neutron to proton ratio tliat is too low. The helium nucleus consists of two protons and two neutrons. The alplia particle differs from a helimn atom in that it is emitted witliout any electrons. The resulting daughter product from tliis tj pe of transformation lias an atomic number Uiat is two less tluin its parent and an atomic mass number tliat is four less. Below is an e. aiiiple of alpha decay using polonium (Po) polonium has an atomic mass number of 210 (protons and neutrons) and atomic number of 84. 

Proton capture processes by heavy nuclei have already been briefly mentioned in several of the preceding sections. The (p,y) reaction can also be invoked to explain the presence of a number of proton-rich isotopes of lower abundance than those of nearby normal and neutron-rich isotopes (Fig. 1.5). Such isotopes would also result from expulsion of a neutron by a y-ray, i.e. (y,n). Such processes may again be associated with supernovae activity on a very short time scale. With the exceptions of " ln and " Sn, all of the 36 isotopes thought to be produced in this way have even atomic mass numbers the lightest is Se... 

Our present views on the electronic structure of atoms are based on a variety of experimental results and theoretical models which are fully discussed in many elementary texts. In summary, an atom comprises a central, massive, positively charged nucleus surrounded by a more tenuous envelope of negative electrons. The nucleus is composed of neutrons ( n) and protons ([p, i.e. H ) of approximately equal mass tightly bound by the force field of mesons. The number of protons (2) is called the atomic number and this, together with the number of neutrons (A ), gives the atomic mass number of the nuclide (A = N + Z). An element consists of atoms all of which have the same number of protons (2) and this number determines the position of the element in the periodic table (H. G. J. Moseley, 191.3). Isotopes of an element all have the same value of 2 but differ in the number of neutrons in their nuclei. The charge on the electron (e ) is equal in size but opposite in sign to that of the proton and the ratio of their masses is 1/1836.1527. 

Element has no stable nuclides the value given in parentheses is the atomic mass number of the isotope of longest known half-life. However, three such elements (Th, Pa and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. 

Numbers in parentheses are atomic mass numbers of most stable isotopes. 

The substances we call elements are composed of atoms. Atoms in turn are made up of neutrons, protons and electrons neutrons and protons in the nucleus and electrons in a cloud of orbits around the nucleus. Nuclide is the general term referring to any nucleus along with its orbital electrons. The nuclide is characterized by the composition of its nucleus and hence by the number of protons and neutrons in the nucleus. All atoms of an element have the same number of protons (this is given by the atomic number) but may have different numbers of neutrons (this is reflected by the atomic mass numbers or atomic weight of the element). Atoms with different atomic mass but the same atomic numbers are referred to as isotopes of an element. 

Glenn Theodore Seaborg (1912-1999), together with Stanley Gerald Thompson (1912-1967) and Albert Ghiorso ( 1915). The bombardment of americium-241 with alpha particles led to element 97 with atomic mass number 243. The enrichment involved chemical methods, as the properties of the element were assumed to be analogous to those of the lanthanides. 

Elastic scattering is the most likely interaction between fast neutrons and low atomic mass number absorbers. The interaction is sometimes referred to as the "billiard ball effect." The neutron shares its kinetic energy with the target nucleus without exciting the nucleus. 

Kind of Atomic Mass Number Isotope Number of Number of Number of... 

Fig. 3.42. Depletion below solar abundances of elements in the H I gas towards f Ophiuchi plotted against atomic mass number in (a) and condensation temperature in (b), based in part on the curve of growth shown in Fig. 3.11. Vertical boxes indicate error bars. The dotted curve in the left panel represents an A-1/2 dependence expected for non-equilibrium accretion of gas on to grains in the ISM. The condensation temperature gives a somewhat better, though not perfect, fit, suggesting condensation under near-equilibrium conditions at a variety of temperatures either in stellar ejecta or in some nebular environment. Note the extreme depletion of Ca ( Calcium in the plane stays mainly in the grain ). After Spitzer and Jenkins (1975). Copyright by Annual Reviews, Inc.

Thus, as long as t0 o, is large, aN declines slowly with increasing atomic mass number k, but when o becomes very small at the magic neutron numbers, there is a sudden drop (Fig. 6.3). (Such behaviour is characteristic of any distribution... 

Atomic mass Number equal to the number of protons plus the number of neutrons present in the nucleus of an atom. 

Mass of an atom mass of hydrogen atom mass Number of atoms in unit cell Avogadro number... 

Only a few relevant points about the atomic structures are summarized in the following. Table 4.1 collects basic data about the fundamental physical constants of the atomic constituents. Neutrons (Jn) and protons (ip), tightly bound in the nucleus, have nearly equal masses. The number of protons, that is the atomic number (Z), defines the electric charge of the nucleus. The number of neutrons (N), together with that of protons (A = N + Z) represents the atomic mass number of the species (of the nuclide). An element consists of all the atoms having the same value of Z, that is, the same position in the Periodic Table (Moseley 1913). The different isotopes of an element have the same value of Z but differ in the number of neutrons in their nuclei and therefore in their atomic masses. In a neutral atom the electronic envelope contains Z electrons. The charge of an electron (e ) is equal in size but of opposite sign to that of a proton (the mass ratio, mfmp) is about 1/1836.1527). 

However, Mendeleev received credit for devising the modern periodic table of the elements, even though his table was based on atomic mass numbers rather than the atomic proton numbers of the elements. In 1871 he arranged the elements not only by their atomic mass in horizontal rows (periods), but also in vertical columns (groups, also called families) by their valences as well as other chemical and physical characteristics. 

Examples of isotopes are abundant. The major form of hydrogen is represented as H (or H-1), with one proton H, known as the isotope deuterium or heavy hydrogen, consists of one proton and one neutron (thus an amu of 2) and is the isotope of hydrogen called tritium with an amu of 3. Carbon-12 ( C or C-12) is the most abundant form of carbon, though carbon has several isotopes. One is the C isotope, a radioactive isotope of carbon that is used as a tracer and to determine dates of organic artifacts. Uranium-238 is the radioactive isotope (Note The atomic number is placed as a subscript prefix to the element s symbol—for example, —and the atomic mass number can be written either as a dash and number fol-... 

Heavy metah—a general term for metals with an atomic mass number greater than 200. Several heavy metals are extremely toxic. 

Not only is ununoctium expected to be a gas, but it should also be a nonmetal when discovered. It is located at the bottom of group 18 (VIIIA) in the periodic table and could be expected to have some of the characteristics of it neighbors above it in this group. When first and erroneously reported as being discovered, it was said to have 118 protons and 175 neutrons in its nucleus for an atomic mass number (amu) of 293, which would make it the heaviest of the yet-to-be discovered elements. 

Work carried out in the Kellogg Laboratory confirmed the existence of a gap at atomic mass number A = 8. Earlier, in the same laboratory, in 1939, the absence of a stable nuclei at A = 5 had been confirmed. Gamow was convinced that all the research relevant to atomic mass 5 was mistaken. 

Let us note the following key point nuclei for which Z and N are even are more abundant than their immediate neighbours. Nuclei with even atomic mass number A are favoured to the detriment of those with odd A. We must move down to the fifteenth position on the list of nuclei in order to find the most abundant one with odd mass number after hydrogen. This is magnesium-25. Note also the sudden drop in abundances in the region of A = 5-11 and around A = 45. 

Higher atomic mass number precedes lower... 

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