Rows of the periodic table

The period (or row) of the periodic table m which an element appears corresponds to the principal quantum number of the highest numbered occupied orbital (n = 1 m the case of hydrogen and helium) Hydrogen and helium are first row elements lithium in = 2) IS a second row element... 

If IS offen convenienf to speak of the valence electrons of an atom These are the outermost electrons the ones most likely to be involved m chemical bonding and reac tions For second row elements these are the 2s and 2p electrons Because four orbitals (2s 2p 2py 2pf) are involved the maximum number of electrons m the valence shell of any second row element is 8 Neon with all its 2s and 2p orbitals doubly occupied has eight valence electrons and completes the second row of the periodic table... 

Nitrogen and oxygen are in the same row of the periodic table so their relative... 

In Figure 8.1(c) the higher-energy orbitals are indicated as being valence orbitals but, in most applications of AES, they are core orbitals. For this reason the technique is not usually concerned with atoms in the first row of the periodic table. 

Bases of low polarizabiUty such as fluoride and the oxygen donors are termed hard bases. The corresponding class a cations are called hard acids the class b acids and the polarizable bases are termed soft acids and soft bases, respectively. The general rule that hard prefers hard and soft prefers soft prevails. A classification is given in Table 3. Whereas the divisions are arbitrary, the trends are important. Attempts to provide quantitative gradations of "hardness and softness" have appeared (14). Another generaUty is the usual increase in stabiUty constants for divalent 3t5 ions that occurs across the row of the Periodic Table through copper and then decreases for zinc (15). 

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

The connection between basicity and nucleophilicity holds when compaiing atoms in the same row of the periodic table. Thus, HO is more basic and more nucleophilic than F , and H3N is more basic and more nucleophilic than H2O. It does not hold when proceeding down a column in the periodic table. For exanple, I is the least basic of the halide ions but is the most nucleophilic. F is the most basic halide ion but the least nucleophilic. 

Some large basis sets specify different sets of polarization functions for heavy atoms depending upon the row of the periodic table in which they are located. For example, the 6-311+(3df,2df,p) basis set places 3 d functions and 1 f function on heavy atoms in the second and higher rows of the periodic table, and it places 2 d functions and 1 f function on first row heavy atoms and 1 p function on hydrogen atoms. Note that quantum chemists ignore H and Ffe when numbering the rows of the periodic table. 

Basis sets for atoms beyond the third row of the periodic table are handled somewhat differently. For these very large nuclei, electrons near the nucleus are treated in an approximate way, via effective core potentials (ECPs). This treatment includes some relativistic effects, which are important in these atoms. The LANL2DZ basis set is the best known of these. 

A contraction resulting from the filling of the 4f electron shell is of course not exceptional. Similar contractions occur in each row of the periodic table and, in the d block for instance, the ionic radii decrease by 20.5 pm from Sc to Cu , and by 15 pm from Y to Ag . The importance of the lanthanide contraction arises from its consequences ... 

Effective core potentials (ECP) replace the atomic core electrons in valence-only ab initio calculations, and they are often used when dealing with compounds containing elements from the second row of the periodic table and above. 

The first step in reducing the computational problem is to consider only the valence electrons explicitly, the core electrons are accounted for by reducing the nuclear charge or introducing functions to model the combined repulsion due to the nuclei and core electrons. Furthermore, only a minimum basis set (the minimum number of functions necessary for accommodating the electrons in the neutral atom) is used for the valence electrons. Hydrogen thus has one basis function, and all atoms in the second and third rows of the periodic table have four basis functions (one s- and one set of p-orbitals, pj, , Pj, and Pj). The large majority of semi-empirical methods to date use only s- and p-functions, and the basis functions are taken to be Slater type orbitals (see Chapter 5), i.e. exponential functions. 

What cations from the fourth row of the periodic table could be present in a solution with the following behavior. 

This special stability associated with the inert gas electron populations was found to pervade the chemistry of every element of the third row of the periodic table (see Section 6-6.2). Each element forms compounds in which it contrives to reach an inert gas electron population. Elements with a few more electrons than an inert gas are apt to donate one or two electrons to some other more needy atom. Elements with a few less electrons than an inert gas are apt to acquire one or two electrons or to negotiate a... 

Potassium has one valence electron. It is the first member of the fourth row, the row based on the cluster of orbitals with about the same energy as the 45 orbital. There are nine such orbitals, tne 4s orbital, the three 4p orbitals, and the five 3d orbitals. Hence the fourth row of the periodic table will differ from the second and third rows. The fourth row, as seen in the periodic table, consists of eighteen elements. 

Just as the long fourth row of the periodic table arises from filling the 4s, 3d, and Ap orbitals, the fifth row, which also consists of eighteen elements, comes from filling the 5s, Ad, and 5p orbitals. In the sixth row, something new happens. After the 6s and the first one of the 5d electrons have entered, subsequent electrons go into the 4/orbitals. The fact that there are seven 4/ orbitals means that fourteen electrons can be accommodated in this manner. Filling the Af orbitals gives rise to a series of elements with almost identical chemical properties called the rare earth... 

We see that the rows of the periodic table arise from filling orbitals of approximately the same energy. When all orbitals of similar energy are full (two electrons per orbital), the next electron must be placed in an s orbital that has a higher principal quantum number, and a new period of the table starts. We can summarize the relation between the number of elements in each row of the periodic table and the available orbitals of approximately equal energy in Table 15-V. 

In Chapter 6 we saw that the chemical compounds of the third-row elements display a remarkable regularity. Return to Chapter 6 and reread Section 6-6.2. The same simple trend in chemical formulas is discovered in the second row of the periodic table. Now we have a basis for explaining why these trends are found. 

Fluorine, Fs, oxygen, 02, and nitrogen, N2, all form molecular crystals but the next member of this row of the periodic table, carbon, presents another situation. There does not seem to be a small molecule of pure carbon that consumes completely the bonding capacity of each atom. As a result, it is bound in its crystal by a network of interlocking chemical bonds. 

Aluminum, silicon, and sulfur are close together in the same row of the periodic table, yet their electrical conductivities are widely different. Aluminum is a metal silicon has much lower conductivity and is called a semiconductor sulfur has such low conductivity it is called an insulator. Explain these differences in terms of valence orbital occupancy. 

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