The Second Period

Nobody will argue the importance of the idea of the electron pair bond, introduced by Lewis [67], in chemistry. Together with the Bohr theory of the electronic structure of the atom [68] and its connection with the periodic system [69], one has the ingredients for a true chemical theory. The octet model introduced by Langmuir [70] soon demonstrated its immense explanative power for organic and inorganic structure alike. 

In the second period the electronic structure of benzene - but not naphthalene - was 

In between, the relationship of the two main quantum-chemical methods was established in the general sense by Slater [86] and later by Longuet-Higgins [87]. The fact that molecular orbital and valence bond methods must, if used with the same basis set and the 

It is also possible to find relations between the MO and VB approaches on an intermediate level, as shown by Heilbronner [96]. His rather extreme view was that resonance theory expressed molecular orbital results in a different language. The classical valence bond model would emerge again quite recently in applications by Durand and Malrieu [97] using the Heisenberg Hamiltonian, and Bemardi, Olivucci and Robb [98], modelling photochemical reactions. 

Without the assistance of computers the application of quantum mechanics in chemistry could only progress slowly and, as foreseen by Dirac [107] in the second - never quoted - part of his famous pronouncement, subject to approximations. Two contributions stand out clearly, one is the famous Goeppert-Mayer/Sklar calculation on benzene 

---

The horizontal rows in the table are referred to as periods. The first period consists of the two elements hydrogen (H) and helium (He). The second period starts with lithium (Li) and ends with neon (Ne). 

The elements in Groups 1 and 2aref3Bng an ssubkveL Thus Li and Be in the second period fill the 2s subleveL Na and Mg in the third period fill the 3s sublevel, and so on. 

The elements in Groups 13 through 18 (six dements in each period) fill p sublevels, which have a capacity of six electrons. In the second period, the 2p sublevel starts to fill with B (Z = 5) and is completed with Ne (Z = 10). In the third period, the elements A1 (Z = 13) through Ar (Z = 18) fill the 3p subleveL... 

Among the diatomic molecules of the second period elements are three familiar ones, N2,02, and F2. The molecules Li2, B2, and C2 are less common but have been observed and studied in the gas phase. In contrast, the molecules Be2 and Ne2 are either highly unstable or nonexistent. Let us see what molecular orbital theory predicts about the structure and stability of these molecules. We start by considering how the atomic orbitals containing the valence electrons (2s and 2p) are used to form molecular orbitals. 

The relative energies of the molecular orbitals available for occupancy by the valence electrons of the second period elements are shown in Figure 3. This order applies at least through N2- ... 

To obtain the MO structure of the diatomic molecules of the elements in the second period, we fill the available molecular orbitals in order of increasing energy. The results are... 

However, Pauli s Nobel Prize-winning work did not provide a solution to the question which I shall call the closing of the periods —that is why the periods end, in the sense of achieving a full-shell configuration, at atomic numbers 2,10, 18, 36, 54, and so forth. This is a separate question from the closing of the shells. For example, if the shells were to fill sequentially, Pauli s scheme would predict that the second period should end with element number 28 or nickel, which of course it does not. Now, this feature is important in chemical education since it implies that quantum mechanics cannot strictly predict where chemical properties recur in the periodic table. It would seem that quantum mechanics does not fully explain the single most important aspect of the periodic table as far as general chemistry is concerned. 

The reason for this behavior is that Ihe periodic table shows a repetition in the length of all periods (with Ihc exception of the first veiy shorl period which consists of just the elements hydrogen and helium). The second period consists of eight elements (lithium to noon) followed by another... 

The second period in the development of inorganic peroxide compounds can be considered to extend from the discovery of the Periodic Table to, the application of physical chemistiy... 

During the second period, the cake grows because of the absence of flow. It may grow to a point at which it locally but completely fills the annulus Bridging takes place and the hydrostatic pressure is no longer transmitted to the deeper zones. From the typical mudcake resistance it can be estimated that under both dynamic and static conditions, the fluid loss could require reduction to an American Petroleum Institutue (API) value lower than what is generally considered a fair control of fluid loss. 

Usually a molecule consists of atoms with different electronegativities, and the more electronegative atoms have smaller coordination numbers (we only count covalently bonded atoms as belonging to the coordination sphere of an atom). The more electronegative atoms normally fulfill the 8 —N rule in many cases they are terminal atoms , i.e. they have coordination number 1. Elements of the second period of the periodic table almost never surpass the coordination number 4 in molecules. However, for elements of higher periods this is quite common, the 8 - N rule being violated in this case. 

In addition, as a rule, the principle of maximal connectivity holds for elements of the third and higher periods the 8 — N bonds usually are bonds to 8 - IV different atoms, and multiple bonds are avoided. For carbon, however, being an element of the second period, the less connected graphite is more stable than diamond at normal conditions. At higher pressures the importance of the principle of maximal connectivity increases then, diamond becomes more stable. 

First, let s look at some trends. The binary fluorides of the second period exhibit a consistent increase in shielding, as reflected by the significant decrease in their 19F chemical shifts (more negative) as the difference in electronegativity of the bound atom and fluorine... 

Since O is to the right of C in the second period of the periodic table, O is more electronegative, and we assign control of all eight shared electrons to the two O atoms. (It does not really have complete control of the electrons if it did, the compound would be ionic.) Thus, the oxidation number of each atom is calculated as follows ... 

Starting at the first electron added to an atom, (a) what is the number of the first electron in the second shell of the atom (b) What is the atomic number of the first element of the second period (c) What is the number of the first electron in the third shell of the atom (d) What is the atomic number of the first element of the third period (e) What is the number of the first electron in the fourth shell of the atom (/) What is the atomic number of the first element of the fourth period ... 

In addition to the homonudear molecules, the elements of the second period form numerous important and interesting heteronudear species, both neutral molecules and diatomic ions. The molecular orbital diagrams for several of these species are shown in Figure 3.9. Keep in mind that the energies of the molecular orbitals having the same designations are not equal for these species. The diagrams are only qualitatively correct. 

Semi-empirical methods (e.g. CNDO, MNDO or PM3), which had been successfully applied to elements of the second period, failed for the heavier elements. This is probably due to the determination of the parameters for the d-atomic orbitals which turned... 

Hydrogen never has an octet of electrons in any of its compounds, but rather a pair (or duet, if you prefer). An example is the Lewis structure of H20 (below). In many compounds in which the central atom is from the second period or higher, there are more than eight electrons around the central atom an example of a compound with such an expanded octet is IC13 (below). Finally, in some compounds, there are less than eight electrons around the central atom one such electron deficient compound is BF3. 

III. Note further that beryllium and neon are in the second period. 

Usually the electronic structure of diatomic molecules is discussed in terms of the canonical molecular orbitals. In the case of homonuclear diatomics formed from atoms of the second period, these are the symmetry orbitals 1 og, 1 ou, 2ag,... 

The second period, from 1890 to around 1920, was characterized by the idea of ionic dissociation and the equilibrium between neutral and ionic species. This model was used by Arrhenius to account for the concentration dependence of electrical conductivity and certain other properties of aqueous electrolytes. It was reinforced by the research of Van t Hoff on the colligative properties of solutions. However, the inability of ionic dissociation to explain quantitatively the properties of electrolyte solutions was soon recognized. 

---