Second-period elements

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- ... 

Predicted and Observed Properties of Diatomic Molecules of Second Period Elements... 

Chapter 4 describes the VSEPR model for the main group second-period elements. 

Ans. Onlv hvdrogen. Lithium and beryllium arc metals, which tend to lose electrons (and form ionic bonds) rather than share. The resulting configuration of two electrons in the first shell, with no other shells occupied, is stable, and therefore is also said to satisfy the octet rule. Second-period elements of higher atomic number tend to acquire the electron configuration of neon. If the outermost shell of an atom is the first shell, the maximum number of electrons in the atom is 2. 

Homonuclear diatomic molecules of second-period elements B2, C2, and N2 ... 

Second period elements can never exceed an octet of electrons. Also keep the following rules in mind ... 

In general, first bond the multicovalent atoms to each other and then, to achieve their normal covalences, bond them to the univalent atoms (H, Cl, Br, I, and F). If the number of univalent atoms is insufficient for this purpose, use multiple bonds or form rings. In their bonded state, the second-period elements (C, N, O, and F) should have eight (an octet) electrons but not more. Furthermore, the number of electrons shown in the Lewis structure should equal the sum of all the valence electrons of the individual atoms in the molecule. Each bond represents a shared pair of electrons. 

Structures with the greatest number of covalent bonds are most stable. However, for second-period elements (C, O, N) the octet rule must be observed. 

CI,C is less basic than F,C . F can disperse charge only by an inductive effect [see Fig. 3-5( )j. In addition to an inductive effect. Cl uses its empty 3d orbitals to disperse charge by p-d tr bonding, see Fig. 3-5(fc). F is a second-period element with no 2d orbitals. 

Changes in the radii of atoms of the second period elements according to their atomic numbers. 

These rules do not apply strictly, but provide useful guidelines for synthesis design. The rules are generally not applicable to electrocydic reactions or to substrates containing non-second-period elements (e.g. P or S), because their longer bond lengths imply different geometric constraints. 

Other than oxygen, O2, which homonuclear diatomic molecule(s) of second period elements should be paramagnetic Ans. Boron, B2... 

Table 1-1. Covalences of H and Second-Period Elements in Groups 2 through 7...

In an electron dot diagram, the symbol of the element represents the nucleus of the atom plus its inner shells of electrons, and dots around the symbol stand for the valence electrons. The dots are placed arbitrarily to the left or right or above or below the symbol. In unbonded atoms, two dots, at most, are located in each position. For example, atoms of the second period elements may be represented as follows ... 

The horizontal rows of elements in the periodic table are called periods. Horizontal row one is called the first period (it contains H and He) row two is called the second period (elements Li through Ne) and so on. 

The process of the completion of the electronic shell is repeated as lor ihc second period elements and is shown by the electronic formulae ... 

Both the b and k values were estimated by regression analysis from the Taft steric constant Es and the Charton steric constant in an iterative procedure using 96 substituents. The k values are equal to 1.0 for the second period elements, except for the fluorine atom (k = 0.8) k = 1.2,1.3, and 1.7 for the third, fourth, and fifth period elements, respectively. 

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