Electron configuration of molecules

Molecules. The electronic configurations of molecules can be built up by direct addition of atomic orbitals (LCAO method) or by considering molecular orbitals which occupy all of the space around the atoms of the molecule (molecular orbital method). 

Electron configurations of transition metal complexes are governed by the principles described in Chapters. The Pauli exclusion principle states that no two electrons can have identical descriptions, and Hund s rule requires that all unpaired electrons have the same spin orientation. These concepts are used in Chapter 8 for atomic configurations and in Chapters 9 and 10 to describe the electron configurations of molecules. They also determine the electron configurations of transition metal complexes. 

In general, the octet rule works for representative metals (Groups lA, IIA) and nonmetals, but not for the transition, inner-transition or post-transition elements. These elements seek additional stability by having filled half-filled or filled orbitals d or/subshell orbitals. The octet rale does not, however, accurately predict the electron configurations of all molecules and compounds. Not all nonmetals, nor metals, can form compounds that satisfy the octet rale. As a result, the octet rale must be used with caution when predicting the electron configurations of molecules and compounds. For example, some atoms violate the octet rale and are surrounded with more than four electron pairs. 

The electron configurations of isolated atoms are found in atomic orbitals the configurations of atoms about to bond are represented by atomic and hybridized atomic orbitals and the electron configurations of molecules are represented by molecular orbitals. Molecular orbital theory is an advanced topic, but it may be simplified to representing the bonds between atoms as overlapping electron density shapes from atomic orbitals. There are two typical locations for molecular orbitals. 

We are now ready to study the ground-state electron configuration of molecules containing second-period elements. We will consider only the simplest case, that of homonuclear diatomic molecules, or diatomic molecules containing atoms of the same elements. 

Since the MO approach is a theoretical model, what experimental evidence is there for this tr-Tr crossover The actual electronic configurations of molecules are nearly always determined spectroscopically, particularly by... 

Since the MO approach is a theoretical model, what experimental evidence is there for this a-n crossover The actual electronic configurations of molecules are nearly always determined spectroscopically, particularly by photoelectron spectroscopy, a technique in which electrons in different orbitals are distinguished by their ionization energies (see Box 5.1). Experimental data support the orbital orderings shown in Figure 2.9. Table 2.1... 

Aufbau principle In building up the electronic configuration of an atom or a molecule in its ground state, the electrons are placed in the orbitals in order of increasing energy. 

Pure anhydrous aluminium chloride is a white solid at room temperature. It is composed of double molecules in which a chlorine atom attached to one aluminium atom donates a pair of electrons to the neighbouring aluminium atom thus giving each aluminium the electronic configuration of a noble gas. By doing so each aluminium takes up an approximately tetrahedral arrangement (p. 41). It is not surprising that electron pair donors are able to split the dimer to form adducts, and ether, for example, forms the adduct. 

The molecular orbital approach to chemical bonding rests on the notion that as elec trons m atoms occupy atomic orbitals electrons m molecules occupy molecular orbitals Just as our first task m writing the electron configuration of an atom is to identify the atomic orbitals that are available to it so too must we first describe the orbitals avail able to a molecule In the molecular orbital method this is done by representing molec ular orbitals as combinations of atomic orbitals the linear combination of atomic orbitals molecular orbital (LCAO MO) method... 

Use Configuration Interaction to predict the electronic spectra of molecules. The Configuration Interaction wave function computes a ground state plus low lying excited states. You can obtain electronic absorption frequencies from the differences between the energies of the ground state and the excited states. 

Polyatomic molecules cover such a wide range of different types that it is not possible here to discuss the MOs and electron configurations of more than a very few. The molecules that we shall discuss are those of the general type AFI2, where A is a first-row element, formaldehyde (FI2CO), benzene and some regular octahedral transition metal complexes. 

Chemical bonds are strong forces of attraction which hold atoms together in a molecule. There are two main types of chemical bonds, viz. covalent and ionic bonds. In both cases there is a shift in the distribution of electrons such that the atoms in the molecule adopt the electronic configuration of inert gases. 

In those few cases where hydration and pseudobase formation parallel each other, the agreement can be traced to the fortuitous circumstance that the structure and electronic configuration of the molecule permit both phenomena to occur simultaneously. Quin-azoline-3-methochloride, one of these rare examples, is discussed in Section III,C, 1. 

Ground state (Section 1.3) The most stable, lowest-cnergv electron configuration of a molecule or atom. 

When N valence atomic orbitals overlap, they form N molecular orbitals. The ground-state electron configuration of a molecule is deduced by using the building-up principle to accommodate all the valence electrons in the available molecular orbitals. The bond order is the net number of bonds that hold the molecule together. 

EXAMPLE 3.7 Sample exercise Deducing the ground-state electron configuration of a diatomic molecule... 

The ground-state electron configurations of diatomic molecules are deduced by forming molecular orbitals from all the valence-sbell atomic orbitals of the two atoms and adding the valence electrons to the molecular orbitals in order of increasing energy, in accord ivith the building-up principle. 

Deduce the ground-state electron configurations of Period 2 diatomic molecules (Toolbox 3.2 and Example 3.7). 

BF can be obtained by the reaction between BF, and B at a high temperature and low pressure, (a) Determine the electron configuration of the molecule in terms of the occupied molecular orbitals and calculate the bond order, (b) CO is isoelectronic with BF. How do the molecular orbitals in the two molecules differ ... 

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