Outermost electrons paired

SOLUTION The smaller member of a pair of isoelectronic ions in the same period will be an ion of an element that lies farther to the right in a period, because that ion has the greater effective nuclear charge. If the two ions are in the same group, the smaller ion will be the one that lies higher in the group, because its outermost electrons are closer to the nucleus. Check your answer against the values in Appendix 2C. 

Now that we know how to determine hybridization states, we need to know the geometry of each of the three hybridization states. One simple theory explains it all. This theory is called the valence shell electron pair repulsion theory (VSEPR). Stated simply, all orbitals containing electrons in the outermost shell (the valence shell) want to get as far apart from each other as possible. This one simple idea is all you need to predict the geometry around an atom. First, let s apply the theory to the three types of hybridized orbitals. 

The heavier nonmetals may be expected to make some use of the less stable orbitals of the outermost shell (3d for P, S, Cl 4d for As, Se, Br etc-), as is indicated by the existence of compounds such as PCI and SFe, in which the central atom forms more bonds than permitted by the use of orbitals occupied by electron pairs in the adjacent noble gas. In our earlier discussion of the structure of PCI it was pointed out that a rough quantum-mechanical treatment leads to the conclusion that the structure in which the phosphorus atom forms five covalent bonds, with use of one 3d orbital in addition to the 3 and three 3d orbitals, makes a significant contribution to the normal state of the molecule (about 8 percent). 

In order of increasing energy, the outermost orbitals associated only with the C=S chromophore are (cr), ( )> (71), (n), (tc ), and (cr ), where n is the thiocar-bonyl nonbonding orbital, and n a second nonbonding orbital, (chlorine orbitals are excluded). The electronic configuration of the ground state can be written (a)2(nr)2(n)2(n)2(n )0(a )0, where each of the inner orbitals are doubly occupied by an electron pair and the outer orbitals (virtual) are empty. 

Present-day shared electron-pair theory is based on the premise that the s2p6 octet in the outermost shells of the noble gas elements above helium represents a particularly favorable configuration. This is not because of any mysterious properties of octets (or of noble gas elements) by allowing each nucleus to claim half-ownership of a shared electron, more electrons are effectively seeing more nuclei, leading to increased electro-... 

The shared electron-pair is such a fundamental concept of chemical bonding that it is important to have a simple way of writing out a formula that shows the disposition of the shared pairs between the different atoms. This is commonly accomplished by depicting the valence (outermost) electrons of an atom as dots that are written around the atom symbols. Sometimes it is convenient to represent the electrons that are contributed by different atoms by different symbols. For example, the formation of H2 can be depicted as... 

Nonmetal atoms can share electrons with other non-metal atoms, forming covalent bonds. In electron dot diagrams, the shared electrons are counted as being In the outermost shell of each of the bonded atoms, A single bond consists of one shared electron pair a double bond consists of two shared electron pairs a triple bond consists of three shared electron pairs. Macromolecules result from covalent bonding of millions of atoms or more Into giant molecules. 

Next, we consider BF3. The B atom has three outermost electrons to contribute. The three fluorine atoms each contribute their 1 unpaired electron, which makes a total of 6 electrons available. These are distributed in three pairs symmetrically about the B atom ... 

In the case of ammonia, the five valence electrons surrounding nitrogen— two electrons occupying the outermost 2s orbital and three electrons in three 2p orbitals— hybridize to form four sp orbitals that, if equal, would separate themselves in three dimensional space by pointing at the comers of a tetrahedron. One of these orbitals, however, contains the lone electron pair, and the three remaining sp orbitals make additional space available by pointing at the comers of a pyramid with a triangular base. 

Chemical bonding usually involves only the outermost electrons of atoms, also called valence electrons. In Lewis dot representations, only the electrons in the outermost occupied r and p orbitals are shown as dots. Paired and unpaired electrons are also indicated. Table 7-1 shows Lewis dot formulas for the representative elements. All elements in a given group have the same outer-shell electron configuration. It is somewhat arbitrary on which side of the atom symbol we write the electron dots. We do, however, represent an electron pair as a pair of dots and an unpaired electron as a single dot. 

The carbon atom has four electrons in its outermost shell. In order to complete its octet, each atom must share a total of four electron pairs. The order of a bond is the nuiuber of electron pairs... 

Atoms within organic molecules are held together by covalent bonds, which are formed by adjacent atoms sharing pairs of electrons (usually each atom donates one of its outermost, or valency, electrons to the bond). Single, double and even triple bonds can be formed, in which one, two and three electron pairs are shared, respectively, although triple bonds are rare among natural... 

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