Examples mass number

Nuclide. Each nuclide is identified by element name and the mass number A, equal to the sum of the numbers of protons Z and neutrons N in the nucleus. The m following the mass number (for example, Zn) indicates a metastable isotope. An asterisk preceding the mass number indicates that the radionuclide occurs in nature. Half-life. The following abbreviations for time units are employed y = years, d = days, h = hours, min = minutes, s = seconds, ms = milliseconds, and ns = nanoseconds. 

Included in the table are all compounds for which information was available through the C, compounds. The mass number for the five most important peaks for each compound are listed, followed in each case by the relative intensity in parentheses. The intensities in all cases are normalized to the w-butane 43 peak taken as 100. Another method for expressing relative intensities is to assign the base peak a value of 100 and express the relative intensities of the other peaks as a ratio to the base peak. Taking ethyl nitrate as an example, the tabulated values would be... 

The primary process of SiH decomposition is electron impact which produces a large number of different neutral and ionic species as shown in Table 1. The density of S1H2 and SiH neutral species produced has been found to be much larger than the density of the ions. For example, mass spectrometric data for silane discharges indicate that the density of ionic species is lower by 10 compared with the density of neutral species. Further, mass spectrometer signals of ionic species, such as SiH SiH 25 SiH", SiH", and Si2H , increase by more than two orders of magnitude as the r-f power is increased, eg, from 2 to 20 W. A rapid rise in the population of ions, with power, implicitly means an increase in electron density. 

Kumar and Ramkrishna (1996a,b) present a solution to population balanee problems of agglomeration proeesses without restrieting the ehoiee of the grid of the diseretized length seale. Two arbitrarily ehosen properties, for example total number and mass of partieles, ean be preserved. 

Allhough (he mass numbers of the proton and neutron are both one, the masses of these fundamental particles are not identical. The mass of one mole of protons is 1.00762 grams and (hai of one mole of neutrons is 1.00893 grams. Furiher invesiigation would show that the experimentally measured mass of the nucleus of any given isotope is not the exact sum of the masses of protons and neutrons confined in ihe nucleus according to our model. For example, the mass of ihe nucleus of the uranium isotope of mass number 233 is less than the exact sum of the masses of 92 protons and 143 neutrons. 

The total number of protons and neutrons in a nucleus is called the mass number, A, of the atom. A nucleus of mass number A is about A times as heavy as a hydrogen atom, which has a nucleus that consists of a single proton. Therefore, if we know that an atom is a certain number of times as heavy as a hydrogen atom, then we can infer the mass number of the atom. For example, because mass spectrometry shows that the three varieties of neon atoms are 20, 21, and 22 times as heavy as a hydrogen atom, we know that the mass numbers of the three types of neon atoms are 20, 21, and 22. Because for each of them Z = 10, these neon atoms must contain 10, 11, and 12 neutrons, respectively (Fig. B.7). 

Fission does not occur in precisely the same way in every instance. For example, more than 200 isotopes of 35 different elements have been identified among the fission products of uranium-235, with most products having mass numbers close to 90 or 130 (Fig. 17.22). 

Figure 2-19 shows the mass spectrum of the element neon. The three peaks in the mass spectrum come from three different isotopes of neon, and the peak heights are proportional to the natural abundances of these isotopes. The most abundant isotope of neon has a mass number of 20, with 10 protons and 10 neutrons in its nucleus, whereas its two minor isotopes have 11 and 12 neutrons. Example illustrates how to read and interpret a mass spectmm. 

The molar mass (MM) of any substance is the mass of one mole of that substance. As described in Section 2-1. each isotope of a particular element has a different mass. Therefore, the mass of one mole of any isotope has a unique value, its isotopic moiar mass. This characteristic molar mass can be found by multiplying the mass of one atom of that isotope by Avogadro s number. For example, mass spectrometry experiments reveal that one atom of carbon-13 has a mass of 2.15928 X 10- g, from which we can calculate the isotopic molar mass of... 

The parent ion usually breaks apart into a collection of smaller pieces, many of which are also positively charged. The masses of these fragments provide a chemical fingerprint that indicates how the atoms in a molecule are connected together. A relatively simple example is the mass spectmm of methane, shown in the figure. There are peaks at mass numbers 16, 15, 14, 13, 12, and 1. Nearly all hydrogen atoms have. <4 = 1, and nearly all carbon atoms have A = 12, so these peaks can be identified as CH4 ,... 

Cu). Alternatively, the name of the element is followed by its mass number, as in copper-63. Example provides some practice in writing the symbols of nuclides. 

Although nuclides with mass numbers around 60 are the most stable, the balance of electrical repulsion and strong nuclear attraction makes many combinations of protons and neutrons stable for indefinite times. Nevertheless, many other combinations decompose spontaneously. For example, all hydrogen nuclides with j4 > 2 are so... 

As an example, consider the decay of free neutrons. A neutron has A — 1, so its decay products must also have A = 1. The stable particle with A = 1 is a proton, and neutron decay results in a proton. The neutron has zero charge, so the sum of the charges of its decay products must also be zero. Because the proton carries a -i-l charge, another particle with a -1 charge is required. This particle must have >1 = 0 to ensure that the mass number is... 

EXAMPLE 3.6. Choose the integer quantities from the following list (a) atomic number, (b) atomic weight, and (c) mass number. 

EXAMPLE 22.1. Show that the mass number and the total charge are both conserved in the natural disintegration of 2g U ... 

The emission of a gamma particle causes no change in the charge or mass number of the original particle. (It does cause a change in its internal energy, however.) For example,... 

EXAMPLE 22.7. When a 2 1 nucleus disintegrates, the following series of alpha and bela particles is emitted alpha, beta, beta, alpha, alpha, alpha, alpha, alpha, beta, alpha, beta, beta, beta, alpha. (Since emission of gamma particles accompanies practically every disintegration and since gamma particles do not change the atomic number or mass number of an isotope, they are not listed.) Show that each isotope produced has a mass number that differs from 238 by some multiple of 4. 

In each case, the final product must differ from the original parent by some multiple of 4 mass numbers. For example, the 208Pb differs in mass number from 232Th by 24 = 4 x 6. There must have been six alpha particles emitted in this decay scries, with a reduction of four mass numbers each. (The beta and gamma particles emitted do not affect the mass number.)... 

Any nuclear species is referred to as a nuclide. Thus, H, 23uNa, 12SC, 23892U are different recognizable species or nuclides. A nuclide is denoted by the symbol for the atom with the mass number written to the upper left, the atomic number written to the lower left, and any charge on the species, q to the upper right. For example,... 

Each type of atom is designated by the atomic number, Z, and a symbol derived from the name of the element. The mass number, A, is the whole number nearest to the mass of that species. For example, the mass number of H is 1, although the actual mass of this isotope is 1.00794 atomic mass units (amu). Because protons and neutrons have masses that are essentially the same (both are approximately 1 atomic mass unit, amu), the mass number of the species minus the atomic number gives the number of neutrons, which is denoted as N. Thus, for 157N, the nucleus contains seven protons and eight neutrons. 

Alpha (a.) decay. As we shall see later, the alpha particle, which is a helium nucleus, is a stable particle. For some unstable heavy nuclei, the emission of this particle occurs. Because the a particle contains a magic number of both protons and neutrons (2), there is a tendency for this particular combination of particles to be the one emitted rather than some other combination such as s3Li. In alpha decay, the mass number decreases by 4 units, the number of protons decreases by 2, and the number of neutrons decreases by 2. An example of alpha decay is the following ... 

An example is shown in figure 1 of the molecular interferences which must be dealt with around mass 87 if one wishes to use a mass spectrometer for rubidium/strontium measurements in a geological sample [22]. The major elements in this lunar sample all have mass numbers less than 48. Thus, the mass 87 region should be completely free of atomic peaks except for the minor components such as rubidium and strontium. This is clearly not the case and at most mass numbers in the rubidium region there are major interferences from molecules. 

Until recently the only satisfactory way to separate these molecular interferences has been on the basis of nuclear mass defects, i.e., the mass of molecules having the same mass number differs from that of the atoms of the same mass number. Figure 2 shows the resolution that is needed to resolve the molecular impurities present in the previous example. Clearly, an unambiguous identification can be made, and all molecular fragments can only be eliminated for an instrument with resolution M/AM approximately 20,000. Once again, the need for high resolution will cause the transmission efficiency to be low. 

B We know that 19 F is stable, with approximately the same number of neutrons and protons 9 protons, and 10 neutrons. Thus, nuclides of light elements with approximately the same number of neutrons and protons will be stable. In Practice Example 26-1 we saw that positron emission has the effect of transforming a proton into a neutron. /T emission has the opposite effect transforming a neutron into a proton. The mass number does not change in either case. Now let us analyze our two nuclides. 

UV Etching. A typical mass spectrum of the vaporized UV etching products is shown in Figure 4, together with a background spectrum obtained without UV irradiation. The comparison clearly shows that UV irradiation causes an increase in intensity for various mass peaks. For example, the intensity of the peaks of m/e=15, 31, 59 and m/e=41, 69 increased drastically by UV irradiation. The former three are due to side-chain scission caused by UV absorption at the C=0 unit, while the latter two are due to main-chain scission initiated by side-chain scission (11). The structure and mass numbers of typical vaporized species are shown in Table I. From here on, we use the spectral intensity after the background is subtracted. 

X represents the element symbol (from the periodic table), Z is the atomic number, the number of protons, and A is the mass number, the sum of the protons plus neutrons. Subtracting the atomic number from the mass number (A— Z) gives the number of neutrons present in this particular isotope. For example, iiNa would contain 11 protons, 11 electrons, and 12 neutrons (23-11). 

Example The mass number of an atom is the sum of the atomic number and ... 

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