More about the atom

On page 24 you saw that all atoms consist of a nucleus and a cloud of electrons that move round the nucleus. The nucleus is itself a cluster of two sorts of particles, protons and neutrons. 

All the particles in an atom are very light. Their mass is measured in atomic mass units, rather than grams. Protons and electrons also have an electric charge  

The sodium atom is a good one to start with. It has 11 protons, 11 electrons and 12 neutrons. They are arranged like this  

The different levels for the electrons are called electron shells. Each shell can hold only a limited number of electrons  

The nucleus is very tiny compared with the rest of the atom. If the atom was the size of a football stadium, the nucleus (sitting on the centre spot) would be the size of a pea  

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We will discuss the ways in which the first three of Dalton s postulates have had to be amended after we learn more about the atom. 

These three common examples show how strongly the physical properties of a polymer are influenced by its chemical composition. To begin to explore the world of polymers systematically, we will need to know a little more about the atoms that are the simple building blocks of these giant molecules. 

Early models of the atom had electrons spinning around the nucleus in a random fashion. But as scientists learned more about the atom, they found that this representation probably wasn t accurate. Today, two models of atomic structure are used the Bohr model and the quantum mechanical model. The Bohr model is simple and relatively easy to understand the quantum mechanical model is based on mathematics and is more difficult to understand. Both, though, are helpful in understanding the atom, so I explain each in the following sections (without resorting to a lot of math). 

Both the adjacency and distance matrices provide information about the connections in the molceular structure, but no additional information such as atom type or bond order. One type of matrix which includes more information, the Atom Connectivity Matrix (ACM), was introduced by Spialtcr and is discussed in Ref, [38]. This approach was eventually abandoned but is listed here because it was quite a unique approach. 

We can learn more about the effect of structure on acidity by considering the oxoacids. These acids form structurally related families, and so we can examine the effect of different central atoms with the same number of O atoms (as in HC103 and HBr03). Alternatively, we can look at the influence of different numbers of O atoms attached to the same central atom (as in HC103 and HC104). 

By repeating the experiment with molecules having different speeds and different states of rotational or vibrational excitation, chemists can learn more about the collision itself. For example, experimenters have found that, in the reaction between a Cl atom and an HI molecule, the best direction of attack is within a cone of half-angle 30° surrounding the H atom. 

Butadiene, used in the chemical industiy as a precursor of synthetic mbber, is a hydrocarbon with the formula C4 Hg. (See Box 13-1 to learn more about the mbber industry.) The Lewis stmcture of butadiene contains two double bonds on sequential pairs of carbon atoms. The chemistry of butadiene, including its ability to form mbber, can be traced to the delocalized electrons in the tt system of the molecule. 

Every space group listed in the family tree corresponds to a structure. Since the space group symbol itself states only symmetry, and gives no information about the atomic positions, additional information concerning these is necessary for every member of the family tree (Wyckoff symbol, site symmetry, atomic coordinates). The value of information of a tree is rather restricted without these data. In simple cases the data can be included in the family tree in more complicated cases an additional table is convenient. The following examples show how specifications can be made for the site occupations. Because they are more informative, it is advisable to label the space groups with their full Hermann-Mauguin symbols. 

A structural classification of 8 is difficult due to the fact that an arrangement of metal atoms as in 8 is uncommon in the whole field of molecular metal clusters. For this reason, detailed understanding of the bonding properties in 8 requires quantum chemical calculations. Theoretical analysis seems to be especially applicable to learning more about the bond between the two tetrahedra, which appears at first to be an isolated metal-metal bond between two metal atoms in the formal oxidation state zero. 

Another possibility to find out more about the structure of these dendrimers was chosen by incorporating fluorine atoms. The use of 19F-NMR spectroscopy offered an additional tool to study the conformation of the dendrimer, especially with the fluorines attached close to the stereogenic centers [91 ]. Following our previously developed methods [92], fluorine-containing 1st- and 2nd-genera-tion chiral dendrimers such as 76 were synthesized (Fig. 24). 

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

When atoms get closer to each other, they may become held together by forces of attraction called chemical bonds. To explain why this happens, we need to understand more about the electron configurations of atoms. 

In order to learn more about the Rouse model and its limits a detailed quantitative comparison was recently performed of molecular dynamics (MD) computer simulations on a 100 C-atom PE chain with NSE experiments on PE chains of similar molecular weight [52]. Both the experiment and the simulation were carried out at T=509 K. Simulations were imdertaken,both for an explicit (EA) as well as for an united (l/A) atom model. In the latter the H-atoms are not explicitly taken into account but reinserted when calculating the dynamic structure factor. The potential parameters for the MD-simulation were either based on quantum chemical calculations or taken from literature. No adjusting... 

The richness of the chemistry of carbon largely reflects the facts that, first, each atom of carbon forms several chemical bonds, usually four, simultaneously and, secondly, carbon, uniquely, can form stable, endlessly long chains with itself Much more about the wealth of molecules resulting from these simple facts follows later. 

Rutherford s discovery of the atomic nucleus was his greatest contribution to physics and it established him as the leading experimental physicist of his day. However, it was only a beginning, and many questions about the atom remained unanswered. As yet nothing was known about electron orbits or about the relationship between the structure of the atom and the periodic table. Before Rutherford performed his experiments, it was thought that the atom was understood. Now it was apparent that much remained to be learned. But then great discoveries in physics seem always to suggest new questions and open up new lines of research. The more that is known, the better the picture scientists have of what remains unknown. 

The global thermodynamic approach used in the above sections is insensitive to details at the atomic level and can only yield a gross characterization of the surface. Properties such as the specific surface area and the presence or absence of pores can be determined using the above approach since only the average surface —not atomic details —is involved. The existence of a distribution of surface energy sites can also be inferred from adsorption data, but the method falls short when it comes to specifics about this distribution. Observations on an atomic scale are needed to learn more about the details of the surface structure. Such observations comprise the subject matter of the last two sections of the chapter. 

Although the HRTEM movies provide important information about the atomic-scale surface dynamics, they capture only the result of the atomic diffusion events and not directly the individual atomic displacements. To understand the origin of the transport mechanisms, the observations have been complemented with results based on DFT calculations. The calculations show that step edges facilitate methane dissociation and that carbon atoms bind more strongly to step edges than... 

The Laue and the Bragg condition give us information about the angular distribution of the diffraction peaks. To calculate the peak intensities, we have to know more about the scattering properties of the atoms or molecules in the crystal. In the case of X-rays and electrons the scattering probability is proportional to the electron density ne(r) within the crystal. Since n,(r) has to have the same periodicity as the crystal lattice, we can write it as a three-dimensional Fourier series (using the notation eikx = cos kx + i sin kx) ... 

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