Neon atomic mass

As happens so often in science, a new and more precise technique of measurement led to a major discovery. When scientists first used mass spectrometers they found—much to their surprise—that not all the atoms of a single element have the same mass. In a sample of perfectly pure neon, for example, most of the atoms have mass 3.32 X 10-26 kg, which is about 20 times as great as the mass of a hydrogen atom. Some neon atoms, however, are found to be about 22 times as heavy as hydrogen. Others are about 21 times as heavy (Fig. B.6). All three types of atoms have the same atomic number so they are definitely atoms of neon. 

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). 

Isotopes of an element all have the same number of protons in their nuclei (and of electrons orbiting them), but differ in their number of neutrons. Neon-20 has ten protons (an atomic number of 10) and ten neutrons neon-22 has ten protons and twelve neutrons. The atomic mass of an isotope is the total number of protons and neutrons in its nucleus here 20 and 22 respectively. Chemists denote a particular isotope of an element by writing its atomic mass as a superscript before the elemental symbol °Ne, Ne. 

Helium, He, has an atomic mass of4.003 atomic mass units, and neon, Ne, has an atomic mass of 20.180 atomic mass units. 

The atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 isotope. The relative atomic mass of an element is the weighted average of the isotopes relative to I/12(of jhe carbon-12 isotope. For example, the atomic mass of neon is 20.17 amu and is calculated from the following data neon-19 (amu of 19.99245, natural abundance of 90.92%), neon-20 (amu of 20.99396, natural abu dan c of 0,260%) and ncon-21 (amu of 21.99139, natural abundance ofc 82%) ... 

Note Some samples of gases may have equal values for these attributes. Assume the larger containers have a volume twice the volume of the smaller containers and assume the mass of an argon atom is twice the mass of a neon atom. 

The basic principle underlying mass spectrometry was formulated by J. J. Thomson (the discoverer of the electron) early in the century. Working with cathode ray tubes, he was able to separate two types of particles, each with a slightly different mass, from a beam of neon ions, thereby proving the existence of isotopes. (Isotopes are atoms of the same element that have slightly different atomic masses due to the presence of differing numbers of neutrons in the nucleus.) The first mass spectrometers were built in 1919 by F. W. Aston and A. J. Dempster. 

Every atomic nucleus can be described not only by its atomic number but also by its mass number. The mass number is equal to the total number of particles of the nucleus—that is, the total number of protons and neutrons. For example, a particular atom of neon has a mass number of 20, as shown in Figure 12. Therefore, the nucleus of this atom has a total of 20 protons and neutrons. Because the atomic number for an atom of neon is 10, neon has 10 protons. You can calculate the number of neutrons in a neon atom by subtracting neon s atomic number (the number of protons) from neon s mass number (the number of protons and neutrons). 

The neon atom has 10 protons, 10 neutrons, and 10 electrons. This atom s mass number is 20, or the sum of the numbers of protons and neutrons in the atom. 

Figure 4 displays the result of the HBr pick-up on a cluster with 130 neon atoms. The simulation can be visualized in two ways. On Fig. 4a the density of the dopant atom as a function of the distance from the center of mass of the cluster is depicted. For comparison also the density of neon atoms is added in the graph. The majority of the dopant atoms stays in the surface mea of the cluster, i.e., in the third shell. There is also a peak in the very central position of the cluster. Note, however, that the depicted (lumitity is a density, which should be multiplied by a factor 47rr to obtain the number of dopant atoms. Thus, only... 

Element neon Atomic number = 10 Mass number = 22... 

When a pure elemental gas, such as neon, was analyzed by a mass spectrometer, multiple peaks (two in the case of neon) were observed (see Fig. 1.11). Apparently, several kinds of atoms of the same element exist, differing only by their relative masses. Experiments on radioactive decay showed no differences in the chemical properties of these different forms of each element, so they all occupy the same place in the periodic table of the elements (see Chapter 3). Thus the different forms were named isotopes. Isotopes are identified by the chemical symbol for the element with a numerical superscript on the left side to specify the measured relative mass, for example °Ne and Ne. Although the existence of isotopes of the elements had been inferred from studies of the radioactive decay paths of uranium and other heavy elements, mass spectrometry provided confirmation of their existence and their physical characterization. Later, we discuss the properties of the elementary particles that account for the mass differences of isotopes. Here, we discuss mass spectrometry as a tool for measuring atomic and molecular masses and the development of the modern atomic mass scale. 

Calculate the atomic mass of naturally occurring neon. 

Do you notice anything strange about this configuration It is exactly the same electron configuration as the neutral atom of neon Does that mean that the Mg2+ ion is the same as neon The answer is yes and no. The nucleus of the magnesium ion contains 2 more protons than the neon atom, so they have different masses and different atomic numbers and are certainly not the same element. However, having the same configuration as... 

We will need the molecular mass of the oxygen, which is diatomic, and the atomic mass of the neon, which exists as monatomic particles. We can get this information from the periodic table. 

The subject of ISS has been considered in some detail by Baun. The technique is a powerful one for identification of atoms adsorbed on surfaces. It is best illustrated by an example. In catalysts where ionic solids are doped with foreign ions it must be of importance to know the distribution of the foreign ions. Though not used here in a catalytic study, ISS has been used to establish the way in which Pb concentrates in the surface of AgBr crystals. In the experiment Ne" ions were used as projectiles with energy and scattered (through 90°) with energy E2. E2/E1 = (M2 — Mi)/(M2 Mi), where M2 = mass of substrate atom and Mi = mass of neon atom. 

Initially, diffraetion experiments were performed on monoatomie liquids such as argon and neon, or on liquid metals sueh as mereury. The analysis of scattering data from moleeular liquids is more difficult. Nuclei with different atomic masses are present, and the ineident radiation is seattered by different amounts by each... 

For example, the atomic mass of the element neon listed in the periodic table is 20.1797, giving a molar mass of 20.1797 g/mol. This measurement provides the following conversion factors for converting between grams and moles of neon. 

What is the weighted average mass in atomic mass units (u) of each atom of the elements (a) calcium and (b) neon ... 

The first mass spectrometer, developed in the 1920s by the English physicist F. W. Aston,1 was crude by today s standards. Nevertheless, it provided indisputable evidence of the existence of isotopes—neon-20 (atomic mass 19.9924 amu and natural abundance 90.92 percent) and neon-22 (atomic mass 21.9914 amu and natural abundance 8.82 percent). When more sophisticated and sensitive mass spectrometers became available, scientists were surprised to discover that neon has a third stable isotope with an atomic mass of 20.9940 amu and natural abundance 0.257 percent (Figure 3.4). This example illustrates how very important experimental accuracy is to a quantitative science like chemistry. Early experiments failed to detect neon-21 because its natural abundance is just 0.257 percent. In other words, only 26 in 10,000 Ne atoms are neon-21. The masses of molecules can be determined in a similar manner by the mass spectrometer. 

The element neon has three naturally occurring isotopes. One of these has a mass of 19.99 amu and a natural abundance of 90.48%. A second isotope has a mass of 20.99 amu and a natural abundance of 0.27%. A third has a mass of 21.99 amu and a natural abundance of 9.25%. Calculate the atomic mass of neon. 

As we have seen in Giapter 1, an elemrat may be conqx>sed of atoms that, while having the same number of protons in the nuclei, have different mass numbers and, therefore, different numbers of neutrons. Neon, for exanq>le, has an atomic number of 10, which means that the number of protons in the nuclei of all neon atoms is 10 however, 90% of the neon atoms in nature have 10 neutrons present in their nuclei while 10% of die atoms have 12 neutrons. Such atoms of constant Z but differrat A are called isotopes. The heavy hydrogen isotopes and are used so oftm in nuclear science that they have been given special names and symbols, deuterium (D) and tritium (T), respectively. 

Using the data in the table above, calculate the average atomic mass of neon. 

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