Electronic configurations of atoms and ions

What Do We Need to Know Already This chapter draws on many of the principles introduced in the preceding chapters. In particular, it makes use of the electron configurations of atoms and ions (Sections 1.13 and 2.1) and the classification of species as Lewis acids and bases (Section 10.2). Molecular orbital theory (Sections 3.8 through 3.12) plays an important role in Section 16.12. 

To summarise, magnetic measurements confirm and supplement spectroscopic data on the electronic configuration of atoms and ions, particularly in regard to the occujiation of the outermost orbitals upon which valency depends. 

Use the aufbau principle to predict electron configurations of atoms and ions and to acconnt for the structure of the periodic table (Section 5.3, Problems 15-24). 

TABLE 2.1 Some Typical Electron Configurations of Atoms and the Ions They Form ... 

Aufbau principle (9) The principle followed to construct ground-state electron configurations of atoms and monatomic ions. 

IV. Ground state electronic configurations of atoms and positive ions at arbitrary field strengths... 

IV GROUND STATE ELECTRONIC CONFIGURATIONS OF ATOMS AND POSITIVE IONS AT ARBITRARY FIELD STRENGTHS... 

Applying of the Aufbau principle, Hund s rule and the Pauli exclusion principle to write electron configurations for atoms and ions up to 2 = 36... 

The electron configuration or orbital diagram of an atom of an element can be deduced from its position in the periodic table. Beyond that, position in the table can be used to predict (Section 6.8) the relative sizes of atoms and ions (atomic radius, ionic radius) and the relative tendencies of atoms to give up or acquire electrons (ionization energy, electronegativity). 

By gaining one electron, the bromine atom attains the electron configuration of krypton and also attains a charge of 1-. The two ions expected are therefore Ca + and Br. Since calcium bromide as a whole cannot have any net charge, there must be two bromide ions for each calcium ion hence, the formula is CaBr2. 

An incorrect dissociation limit is a common failure of SCF MO wavefunctions (as we already noted for H2O). Thus for H2 the SCF MO wavefunction (n ) leads to a dissociation limit which is an equal mixture of atoms and ions because there is no correlation between the two electrons (there is an equal chance of finding the two electrons on the same atom and on different atoms). The addition of a configuration (cTj ), where is the lowest energy unfilled molecular orbital, removes this error, and in the dissociation limit the wavefunction has to be an equal mixture of and Thus a wavefunction that stops at this limit is called an optimum double configuration (ODC) function. 

SAMPLE SOLUTION (a) Potassium has atomic number 19, and so a potassium atom has 19 electrons. The ion K+, therefore, has 18 electrons, the same as the noble gas argon. The electron configurations of K+ and Ar are the same ... 

The non-relativistic wave function (1.14) or its relativistic analogue (2.15), corresponds to a one-electron system. Having in mind the elements of the angular momentum theory and of irreducible tensors, described in Part 2, we are ready to start constructing the wave functions of many-electron configurations. Let us consider a shell of equivalent electrons. As we shall see later on, the pecularities of the spectra of atoms and ions are conditioned by the structure of their electronic shells, and by the relative role of existing intra-atomic interactions. 

As we have seen in Chapter 11, the energy levels of atoms and ions, depending on the relative role of various intra-atomic interactions, are classified with the quantum numbers of different coupling schemes (11.2)— (11.5) or their combinations. Therefore, when calculating electron transition quantities, the accuracy of the coupling scheme must be accounted for. The latter in some cases may be different for initial and final configurations. Then the selection rules for electronic transitions are also different. That is why in Part 6 we presented expressions for matrix elements of electric multipole (Ek) transitions for various coupling schemes. 

Extensive studies of energy spectra and other characteristics of atoms and ions allow one to reveal general regularities in their structure and properties [255-257]. For example, by considering the lowest electronic configurations of neutral atoms, we can explain not only the structure of the Periodical Table of elements, but also the anomalies. The behaviour of the ionization energy of the outer electrons of an atom illustrates a shell structure of electronic configurations. 

The properties of the liquid lanthanide trihalides depend strongly on the atomic number of the halide. The variation in the heat capacity of the lanthanide fluorides indicates a strongly ionic behaviour of the melts with a concomittent irregular trend related to the electronic configuration of the lanthanide ions. In the lanthanide chlorides, bromides and iodides the trend becomes systematically more constant, indicating an increasing molecular nature of the melts. 

Hybridization can also help explain the existence and structure of many inorganic molecular ions. Consider, for example, the zinc compounds shown here. At the top is shown the electron configuration of atomic zinc, and just below it, of the divalent zinc ion. Notice that this ion has no electrons at all in its 4-shell. In zinc chloride, shown in the third row, there are two equivalent chlorine atoms bonded to the zinc. The bonding orbitals are of sp character that is, they are hybrids of the 4s and one 4p orbital of the zinc atom. Since these orbitals are empty in the isolated zinc ion, the bonding electrons themselves are all contributed by the chlorine atoms, or rather, the chlor ide ions, for it is these that are the bonded species here. Each chloride ion possesses a complete octet of electrons, and two of these electrons occupy each sp bond orbital in the zinc chloride complex ion. This is an example of a coordinate covalent bond, in which the bonded atom contributes both of the electrons that make up the shared pair. 

In each multiplet structure, the data are separated according to the spin degeneracy spin quartet state on the right hand and spin doublet state on the left. The electron configuration of the Cr + ion is usually expressed as tP under atomic notation, and the quartet and doublet state correspond to the high spin state and low spin state, respectively. Since the ground. state Mz is a quartet state, the transitions to the quartet states are the spin-allowed transitions which have large intensities and broad bands in the absorption spectra, on the other hand, the transitions to the doublet states are the spin-restricted transitions which have more than one order smaller intensities and narrow line peaks which are not obvious in the experimental spectra in Fig. 3. 

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