Atoms more than three electrons

However, electrooxidation of 1-bromoada-mantane involves the transfer of more than three electrons to afford Af-(1-adamantyl)acetamide. In a comparable study of the oxidation of 2-haloadamantanes and their methyl-substituted analogues [13], it was observed that the products depend upon the identity of the halogen atom as well as the number of bridgehead methyl groups. 

Similarly, non-metals whose atoms have one, two or three electrons less than an inert gas structure form monovalent (e.g. bromine, Br ), divalent (e.g. sulphur, S2 ) and trivalent (e.g. nitrogen, N3-) anions. In general, the addition or loss of more than three electrons is energetically unfavourable, and atoms requiring such transfers generally bond covalently (Section 2.3.1). 

In general, metals contain up to three electrons in the outer shells of their atoms and non-metals contain more than three electrons. 

When an atom gains or loses an electron, it develops a charge and becomes an ion. In general, the loss or gain of one, two, or sometimes even three electrons can occur, but an element doesn t lose or gain more than three electrons. 

Atoms with More Than Three Electrons... 

Singly, doubly, triply, etc. charged ion. An atom, molecule, or molecular moiety that has gained or lost one, two, three, or more electrons. The term multiply charged ion is used to refer to ions that have gained or lost more than one electron where the number of electrons lost or gained is not designated. 

Bonded atoms can share more than one electron pair. A double bond occurs when bonded atoms share two electron pairs in a triple bond, three pairs of electrons are shared. In ethylene (Q2H4) and acetylene (QHJ, the carbon atoms are linked by a double bond and triple bond, respectively. Using two parallel lines to represent a double bond and three for a triple bond, we write the structures of these molecules as... 

The next step in our journey takes us from hydrogen and its single electron to the atoms of all the other elements in the periodic table. A neutral atom other than a hydrogen atom has more than one electron and is known as a many-electron atom. In the next three sections, we build on what we have learned about the hydrogen atom to see how the presence of more than one electron affects the energies of... 

A multielectron atom can lose more than one electron, but ionization becomes more difficult as cationic charge increases. The first three ionization energies for a magnesium atom in the gas phase provide an illustration. (Ionization energies are measured on gaseous elements to ensure that the atoms are isolated from one another.)... 

The concept of an octet of electrons is one of the foundations of chemical bonding. In fact, C, N, and O, the three elements that occur most frequently in organic and biological molecules, rarely stray from the pattern of octets. Nevertheless, an octet of electrons does not guarantee that an inner atom is in its most stable configuration. In particular, elements that occupy the third and higher rows of the periodic table and have more than four valence electrons may be most stable with more than an octet of electrons. Atoms of these elements have valence d orbitals, which allow them to accommodate more than eight electrons. In the third row, phosphoms, with five valence electrons, can form as many as five bonds. Sulfur, with six valence electrons, can form six bonds, and chlorine, with seven valence electrons, can form as many as seven bonds. 

The location of electrons linking more than three atoms cannot be illustrated as easily. The simple, descriptive models must give way to the theoretical treatment by molecular orbital theory. With its aid, however, certain electron counting rules have been deduced for cluster compounds that set up relations between the structure and the number of valence electrons. A bridge between molecular-orbital theory and vividness is offered by the electron-localization function (cf p. 89). 

There are also molecules that are exceptions to the octet rule because one of the atoms has fewer, rather than more than, eight electrons in its valence shell in the Lewis structure (Figure 1.19). These molecules are formed by the elements on the left-hand side of the periodic table that have only one, two, or three electrons in their valence shells and cannot therefore attain an octet by using each of their electrons to form a covalent bond. The molecules LiF, BeCl2, BF3, and AIC13 would be examples. However, as we have seen and as we will discuss in detail in Chapters 8 and 9, these molecules are predominately ionic. In terms of a fully ionic model, each atom has a completed shell, and the anions obey the octet rule. Only if they are regarded as covalent can they be considered to be exceptions to the octet rule. Covalent descriptions of the bonding in BF3 and related molecules have therefore... 

We have so far considered valence shells containing four pairs of electrons, but we can extend the same arguments to other numbers of valence shell electron pairs. The most probable arrangements of pairs of opposite spin electrons in the valence shell of an atom in a molecule are two pairs, collinear three pairs, equilateral triangular four pairs, tetrahedral five pairs, trigonal bipyramidal six pairs, octahedral. This is because, as we will now see, these are the arrangements that keep the electron pairs as far apart as possible. We discuss valence shells with more than six electron pairs in Chapter 9. 

The use of resonance structures such as 7 and 8 to describe bond polarity led to a subtle change in the meaning of the octet rule, namely, that an atom obeys the octet rule if it does not have more than eight electrons in its valence shell. As a result, resonance structures such as 7 and 8 are considered to be consistent with the octet rule. However, this is not the sense in which Lewis used the octet rule. According to Lewis, a structure such as 7 would not obey the octet rule because there are only three pairs of electrons in the valence shell of carbon, just as BF3 does not obey the octet rule for the same reason. Clearly the octet rule as defined by Lewis is not valid for hypervalent molecules, which do, indeed, have more than four pairs of shared electrons in the valence shell of the central atom. 

Consider a molecule consisting of more than three atoms, with an even number of valence electrons, 2n (n > 2). The basic assumption of the model is that the... 

We have just explained that the wave equation for the helium atom cannot be solved exacdy because of the term involving l/r12. If the repulsion between two electrons prevents a wave equation from being solved, it should be clear that when there are more than two electrons the situation is worse. If there are three electrons present (as in the lithium atom) there will be repulsion terms involving l/r12, l/r13, and l/r23. Although there are a number of types of calculations that can be performed (particularly the self-consistent field calculations), they will not be described here. Fortunately, for some situations, it is not necessary to have an exact wave function that is obtained from the exact solution of a wave equation. In many cases, an approximate wave function is sufficient. The most commonly used approximate wave functions for one electron are those given by J. C. Slater, and they are known as Slater wave functions or Slater-type orbitals (usually referred to as STO orbitals). 

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