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Chapter 1
Main Group Elements and Heteroatoms: Scope and Characteristics
1.1 Aufbau Principle and Sign of Orbitals
All the atoms consist of a dense nucleus surrounded by electrons. The nucleus consists of protons, which are positively charged, and neutrons, which are neutral. The number of electrons is equal to the number of protons; hence, the atom bears no charge and is neutral. The atomic number of an element is equal to the number of protons in the nucleus, and the mass number is the sum of the number of protons and the number of neutrons. The atomic weight is approximately the same as mass number, because an electron is very light in comparison (the ratio of mass, electron:proton = 1:1840; the mass of proton = 1.6726 × 10−24 g).
Electrons are concentrated in specific regions of space around the nucleus called orbitals. The orbitals are designated as s, p, d, and f according to their shape and energy (s orbital is spherical and the lowest in energy), and each orbital designated as above has 1, 3, 5, and 7 orbitals of equal energy. Furthermore, each electron rotates like the earth and there are two kinds (plus and minus) of rotations called spin. Each orbital can contain a maximum of two electrons with opposite spin (the Pauli exclusion principle).
Electrons are placed in orbitals starting from the lowest energy one (s<p<d<f). In case there are more than two (3, 5, or 7) orbitals of the same energy, electrons are first placed sequentially, one by one in different orbitals with parallel spin. Then, extra electrons are placed as the second electron with anti-parallel spin in the orbitals mentioned above. Thus, according to the atomic number, the electronic configuration of an atom can be determined. This is called aufbau principle. For example, the atomic orbital of elements of first period (H and He) consists of 1s, the atomic orbitals of elements of second period consist of 2s and 2p in addition to 1s, and for those of third period, 3s and 3p are added to 2s, 2p, and 1s. For fourth period elements, 4s and 4p orbitals are added, in which orbitals of third period are filled and 3d orbitals are also filled from zinc (group 12) to krypton (group 18). For fifth period, [Kr] 4d10 are filled as kernel and for sixth period, [Xe] 4f14 5d10 are filled as kernel (Table 1.1). The aufbau principle should be familiar to readers and, just to make sure, the shapes and signs of 1s, 2p, and 3d are shown in Fig. 1.1 (3–5).
Table 1.1 Periodic Table and Essential Characteristics of Main Group Elements
The orbitals of electrons are expressed by wave function (Φ) according to quantum mechanics. The probability of finding electrons is expressed as a square of wave function (Φ2) and is thus positive, although wave function has a plus or minus sign according to symmetry. The orbital shapes in common text books of chemistry exhibit specific regions as contour to find electrons with 0.95 (or 0.90) probability, and the sign of wave function (Φ) is added to the contour.
The wave function of 1s orbital is spherical and thus has only one sign, either plus or minus. Usually, plus sign is used. The 2s orbital also has spherical symmetry but has one node. The 2p orbitals are dumbbell shaped and separated by an orthogonal plane (node) bisecting the dumbbell. The two lobes (parts of an orbital) of an orbital have opposite signs and this is shown by + or − or by different colors. The 3d orbitals consist of three cross shaped orbitals of xy, yz, and zx planes, in which two orthogonal axes are tilted by 45°; and the fourth orbital is aligned to the xy axis (3dx2 − y2) and the fifth is aligned to the z axis (3dz2). Hence, there are five orbitals in total (Fig. 1.1) (6, 7).
1.2 Electronic Configuration of an Atom: Main Group Elements and Heteroatoms
The electronic configuration of an atom is made up according to aufbau principle, and all the atoms are arranged in the periodic table based on the electronic configuration. The elements—substances made up of atoms with the same atomic number including isotopes—are classified into main group elements (i.e., typical elements or sp elements) and transition elements (i.e., transition metals or df elements). Main group elements consist of elements of groups 1, 2, and 12–18, and transition elements consist of elements of groups 3–11, descending from fourth period, including lanthanides and actinides.
The electronic configuration of a main group element is made up by filling electrons sequentially in ns and np orbitals using the electronic configuration of inert gas of the (n − 1) period as kernel. Elements of groups 12–18 of the fourth or fifth period contain 10 electrons of 3d or 4d orbitals in the corresponding kernel. Hence, in the fourth period, the atomic number of calcium (group 2) is 20 and that of zinc (group 12) is 30 and that of gallium (group 13) is 31. In the fifth period, atomic number of strontium (group 2) is 38, that of cadmium (group 12) is 48, and that of indium (group 13) is 49. In the sixth period, the kernel involves 10 and 14 electrons of 5d and 4f orbitals; therefore, electrons are filled in 6s and 6p orbitals sequentially. Hence, the atomic number of barium (group 2) is 56, that of mercury (group 12) is 80, and that of thallium (group 13) is 81 (6, 7).
Electrons of inert gases usually do not contribute to the formation of chemical bonds. It is convenient to show kernel, which consists of a nucleus and electrons of filled inner shells, as the symbol of the element. Residual electrons are contained in the outermost shell and contribute to the formation of chemical bonds; hence, they are valence electrons. It is clear that valence electrons of main group elements are accommodated in ns and np orbitals; thus, it is realized that main group elements are also called sp elements. The group 12 elements such as zinc, cadmium, and mercury are sometimes classified as transition elements; however, they are mainly considered as main group elements. This is because their outer shell electronic configurations are (n − 1)d10ns2 and the ions bearing two positive charges are stable containing electrons of (n − 1)d10.
Organo-main-group chemistry (organic chemistry of main group elements) is a branch of organic chemistry dealing with structures, syntheses, and reactions of compounds bearing a carbon—main group element bond. This branch resides between traditional organic chemistry and organo-transition-metal (organometallic) chemistry and bridges the gap between them.
In Table 1.1, four fundamental characteristics of main group elements are summarized, that is, (i) co...
