Isotopes of the Elements

In the nuclear model of the atom, protons and neutrons are located in the nucleus. The electrons are found in the remainder of the atom (which is mostly empty space because electrons are very tiny). 

A neutral atom contains the same number of protons and electrons. 

The atomic number of an element is the number of protons in the nucleus of an atom of that element. The atomic number determines the identity of an atom. For example, every atom with an atomic number of 1 is a hydrogen atom it contains one proton in its nucleus. Every atom with an atomic number of 6 is a carbon atom it contains 6 protons in its nucleus. Every atom with an atomic number of 92 is a uranium atom it contains 92 protons in its nucleus. The atomic number tells us not only the number of positive charges in the nucleus but also the number of electrons in the neutral atom, since a neutral atom contains the same number of electrons and protons. 

You don t need to memorize the atomic numbers of the elements because a periodic table is usually provided in texts, in laboratories, and on examinations. The atomic numbers of all elements are shown in the periodic table on the inside front cover of this book and are also listed in the table of atomic masses on the inside front endpapers. 

Shortly after Rutherford s conception of the nuclear atom, experiments were performed to determine the masses of individual atoms. These experiments showed that the masses of nearly all atoms were greater than could be accounted for by simply adding up the masses of all the protons and electrons that were known to be present in an atom. This fact led to the concept of the neutron, a particle with no charge but with a mass about the same as that of a proton. Because this particle has no charge, it was very difficult to detect, and the existence of the neutron was not proven experimentally until 1932. All atomic nuclei except that of the simplest hydrogen atom contain neutrons. 

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Isotopic dilution analysis is widely used to determine the amounts of trace elements in a wide range of samples. The technique involves the addition to any sample of a known quantity (a spike) of an isotope of the element to be analyzed. By measuring isotope ratios in the sample before and after addition of the spike, the amount of the trace element can be determined with high accuracy. The method is described more fully in Figure 48.13. 

Isotopic ion. Any ion containing one or more of the less abundant naturally occurring isotopes of the elements that make up its structure. 

NAA involves the bombardment of the sample with neutrons, which interact with the sample to form different isotopes of the elements in the sample (14). Many of these isotopes are radioactive and may be identified by comparing their radioactivity with standards. This technique is not quite as versatile as XRF and requires a neutron source. 

This technique is based upon the detection of corrosion products, in the form of dissolved metal ions, in the process stream. A thin layer of radioactive material is created on the process side of an item of plant. As corrosion occurs, radioactive isotopes of the elements in the construction material of the plant pass into the process stream and are detected. The rate of metal loss is quantified and local rates of corrosion are inferred. This monitoring technique is not yet in widespread use but it has been proven in several industries. 

Atoms that contain the same number of protons but a different number of neutrons are called isotopes. The three kinds of hydrogen atoms just described are isotopes of that element They have masses that are very nearly in the ratio 1 2 3. Among the isotopes of the element uranium are the following ... 

All of the isotopes of the element with atomic number 87 are radioactive. Hence, it is not found in nature. Yet, prior to its preparation by nuclear bombardment, chemists were confident they knew the chemical reactions this element would show. Explain. What predictions about this element would you make ... 

But what would become of Mendeleev s periodic system which now seemed to consist of 300 or so "elements" To some chemists, the discovery of isotopes implied the end of the periodic system as it was known.3 These chemists suggested that it would be necessary to consider the individual new isotopes as the new "elements." But the chemist Paneth adopted a less reductionist approach, arguing that the periodic table of the familiar chemical elements should be retained because it dealt with the "elements" that were of interest to chemists. A justification for this view was provided by the fact that, with a few exceptions, the chemical properties of isotopes of the same element are indistinguishable.4 Moreover, Paneth appealed to Mendeleev s distinction between the two senses of the concept of an "element" in order to provide a philosophical rationale for the retention of the chemist s periodic table. Paneth argued that the discovery of isotopes of the elements represents the discovery of new elements as simple substances, whereas periodic... 

Atoms with the same atomic number (belonging to the same element) but with different mass numbers are called isotopes of the element. All isotopes of an element have exactly the same atomic number hence they have the same number of protons and electrons. An isotope is named by writing its mass number after the... 

From a mass spectrometry perspective, the molecular weight of an analyte is defined as that mass containing the most/more abundant isotope of the elements present, e.g. CeHsCl = 112, based on C = 12, H = 1 and Cl = 35. 

Isotope dilution mass spectrometry (IDMS) can be applied with most of the ionisation methods used in mass spectrometry to determine isotope ratios with greater or lesser accuracy. For calibration by means of isotope dilution, an exactly known amount of a spike solution, enriched in an isotope of the element(s) to be determined, is added to an exactly known amount of sample. After isotopic equilibration, the isotope ratio for the mixture is determined mass spectrometrically. The attraction of IDMS is its potential simplicity it relies only on the measurement of ratios. The... 

The element was generated by bombardment of californium with boron in a linear accelerator. The priority is debated. Isotopes of the elements were observed both by the group of Glenn T. Seaborg and by that of G. N. Flerov in Dubna. IUPAC proposed that the priority be shared. The longest-lived isotope has a half-life of 200 minutes. Lawrencium ends the series of actinides, as the 5f level is fully occupied with 14 electrons. 

The atomic weight of an element is the weighted average of the masses of the individual isotopes of the element. 

When we look at a mass spectrum we can observe that a single peak is never isolated but there is one or more small peaks accompanying it to its right (Figure 2.16). These peaks are due to the isotopes of the elements. Isotopes are atoms of the same element that have the same number of protons and electrons but a different number of neutrons. The total number of protons and neutrons constitutes the mass number and it is indicated as a superscript preceding the atomic symbol, i.e. 1H, 12C, 23Na. 

A nuclide with a neutron/proton ratio which is smaller than that for a stable isotope of the element can increase its ratio by undergoing ... 

The bond energy of a compound will be affected by change in masses of atoms that are bonded. When an element in a compound is replaced by an isotope of the element, the rate of reaction of that compound is altered and the extent of alteration depends on the closeness of bond to the reaction taking place. The effect on the rate will be more when the bond involved in the substitution itself is cleaved in the reaction. The extent to which the rate is affected will depend on the extent to which the bond strength is affected and this in turn will depend on the relative differences in masses of the element and its isotope. 

Secondary ion mass spectra were measured using a Perkin-Elmer+PHI 3500 instrument. Experiments were carried out with 4 kV Ar ions at beam currents of 3 and 300 nanoamps. Spectra were measured to at least 500 daltons (d). Samples were prepared in the manner used for the XPS studies. For measurements on the pure complexes, sample charging occurred, as evidenced by the inability to record secondary ion mass spectra. To reduce charging, a low energy electron beam (50-400 eV) was rastered across the sample during SIMS analysis. Positive and negative ion SIMS spectra were recorded however, only positive ion spectra are of interest for this discussion. In the spectra only unipositive ions were detected, so that the mass numbers detected correspond to combinations of the various isotopes of the elements in the ion. Thus an ion at m/z 17 d is assigned to... 

It involves the simultaneous irradiation of the sample and a standard known mass of the same element to produce a radioactive isotope of the element. The activities of both the sample and the standard are then determined and, because their specific activities will be the same, it is possible to calculate the mass of the unknown sample. 

Average atomic mass data in brackets indicate atomic mass of most stable isotope of the element. 

Mass spectrometry is based upon the separation of charged ionic species by their mass-to-charge ratio, m/z. Within the general chemical context however, we are not used to taking into concern the isotopes of the elemental species involved in a reaction. The molecular mass of tribromomethane, CHBrs, would therefore be calculated to 252.73 g mol using the relative atomic masses of the elements as listed in most periodic tables. In mass spectrometry we have to leave this custom behind. Because the mass spectrometer does not separate by elements but by isotopic mass, there is no signal at m/z 252.73 in the mass spectmm of tribromomethane. Instead, major peaks are present at m/z 250, 252, 254 and 256 accompanied by some minor others. 

ISOTOPES Sodium has 14 isotopes. The only stable isotope of sodium has an average atomic weight of 23 ( Na) and makes up about 100% of all the isotopes of the element sodium found on Earth. All the other 13 isotopes (from Na to Na) are radioactive with relatively short half-lives and thus are unstable. 

ISOTOPES There are 26 isotopes of the element chromium four are stable and found in nature, and the rest are artificially produced with half-lives from a few microseconds to a few days. The four stable isotopes and their percentage of contribution to the total amount of chromium on Earth are as follows °Cr = 4.345%, Cr = 83.789%,... 

Because lawrencium does not exist in nature, it had to be produced artificially. This was done in 1961 by the team of scientists at Berkeley, using an ion accelerator to bombard three different isotopes of the element californium with heavy ions of the elements boron and "boron along with some neutrons that produced the isotope jj,jLr-258. The resulting product weighed only about two millionths of a gram and had a half-life of only 4.1 seconds, fissioning spontaneously. 

ISOTOPES There are 5 known isotopes of the element ununpentium, ranging from Uup-287 to Uup-291. The first two isotopes synthetically produced and confirmed were Uup-287 and Uup-288. Ununpentium s most stable isotope is Uup-288, which has a half-life of 87 milliseconds. It decays by alpha emission into ununtrium (element 113). 

The interpretation of the curious weight of chlorine awaited the discovery of the neutron in 1932. Although all chlorine atoms have 17 protons, different isotopes of the element have different numbers of neutrons. In Table 3-3, the mass numbers of the chlorine isotopes are denoted by superscripts to the upper left of the chemical symbol. 

The element may be determined from its radioactivity using tracer techniques. Isotopes of the element may be identified by mass spectrometry. 

All isotopes of the element are synthesized in the nuclear reactor. The first isotope synthesized had the mass 241, produced by irradiation of milligram quantities of americium-241 with alpha particles of 35 MeV in a cyclotron ... 

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