Octet generalized

Nonmetal haUdes are generally hydroly2ed to a hydrogen haUde and to an oxy-acid containing the other element. The first row nonmetal haUdes, eg, CCI4, resist hydrolysis because the nonmetal element cannot expand its octet of electrons to form a bond to water before its bond to the haUde is broken. Hydrolysis requires either an energetic water molecule to strike the haUde or ioni2ation of the covalent nonmetal—halide bond, processes that tend to be quite slow (16). 

These examples illustrate the principle that atoms in covalently bonded species tend to have noble-gas electronic structures. This generalization is often referred to as the octet rule. Nonmetals, except for hydrogen, achieve a noble-gas structure by sharing in an octet of electrons (eight). Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two). 

Although most of the molecules and polyatomic ions referred to in general chemistry follow the octet rule, there are some familiar species that do not. Among these are molecules containing an odd number of valence electrons. Nitric oxide, NO, and nitrogen dioxide, N02, fall in this category ... 

For example, nitrogen ( N ) has five valence electrons and needs three more electrons to complete its octet. Chlorine (-CL) has seven valence electrons and needs one more electron to complete its octet. Argon OArO already has a complete octet and has no tendency to share any more electrons. Hydrogen (H-) needs one more electron to reach its helium-like duplet. Because hydrogen completes its duplet by sharing one pair of electrons, we say that it has a valence of 1 in all its compounds. In general, the valence of an element is the number of bonds that its atoms can form. 

Some species have an odd number of valence electrons, and so at least one of their atoms cannot have an octet. Species having electrons with unpaired spins are called radicals. They are generally highly reactive. One example is the methyl radical, CH, which is so reactive that it cannot be stored. It occurs in the flames of burning hydrocarbon fuels. The single unpaired electron is indicated by the dot on the C atom in -CHj. 

Once the number of valence electrons has been ascertained, it is necessary to determine which of them are found in covalent bonds and which are unshared. Unshared electrons (either a single electron or a pair) form part of the outer shell of just one atom, but electrons in a covalent bond are part of the outer shell of both atoms of the bond. First-row atoms (B, C, N, O, F) can have a maximum of eight valence electrons, and usually have this number, although some cases are known where a first-row atom has only six or seven. Where there is a choice between a structure that has six or seven electrons around a first-row atom and one in which all such atoms have an octet, it is the latter that generally has the lower energy and that consequently exists. For example, ethylene is... 

The octet principle can be expressed as a formula by the generalized 8—N rule according to E. Mooser W. B. Pearson. We restrict our considerations to binary compounds, and presuppose the following ... 

Linnett used the concept that an octet of valence shell electrons consists of two sets of four opposite-spin electrons to show that in diatomic and other linear molecules the two tetrahedra are not in general formed into four pairs as we have discussed for F2 and the CC triple bond in C2H2. This idea is the basis of the double-quartet model, which Linnett applied to describe the bonding in a variety of molecules. It is particularly useful for the description of the bonding in radicals, including in particular the oxygen molecule, which has two unpaired electrons and is therefore paramagnetic This unusual property is not explained by the Lewis structure... 

In these examples, it can be seen that the carbon and chlorine atoms can achieve octets of electrons by sharing pairs of electrons with other atoms. Hydrogen atoms attain duets of electrons because the first shell is complete when it contains two electrons. We note from Sec. 5.4 that main group cations generally lose all their valence electrons, and then have none left in their valence shell. 

The concept of valence has been subject to revision over the years. Initially, valence was regarded as the combining power of an element and was derived from the composition of compounds. At the end of the period before the age of quantum chemistry, valence was generally formulated in relation to the octet rule [1—3), a simple relation which still finds useful application in modem chemistry. 

Because of the apparent success frequently achieved by applying the simple octet rule, we took this rule as our starting point. However, the plain formulation of the octet rule is somewhat cumbersome and is conveniently substituted by the generalized (8-N) rule (77—28). The generalized rule has received considerable attention in recent years as a powerful tool for predicting compound semiconductors. 

For a general closed-shell AX , species, the Lewis-type assumption of a shared A X electron-pair bond for each coordinated monovalent atom X nominally requires m orbitals on A to accommodate the 2m bonding electrons, plus additional orbitals for any nonbonded pairs. Thus, for m bonds and t lone pairs, apparent octet-rule violations occur whenever... 

We know that the electronegativity difference between atoms must be greater than 1.9 to form an ionic bond. But if the electronegativity values of the atoms are similar, the tendency of the atoms to take or give electrons will also be similar. The transfer of electrons is not possible between such atoms, so the atoms must share electrons to gain a stable octet. The bond that is formed as a result of electron sharing is called a covalent bond. Covalent bonds are generally formed between two nonmetals. 

Quantum chemists have developed considerable experience over the years in inventing new molecules by quantum chemical methods, which in some cases have been subsequently characterized by experimentalists (see, for example, Refs. 3 and 4). The general philosophy is to explore the Periodic Table and to attempt to understand the analogies between the behavior of different elements. It is known that for first row atoms chemical bonding usually follows the octet rule. In transition metals, this rule is replaced by the 18-electron rule. Upon going to lanthanides and actinides, the valence f shells are expected to play a role. In lanthanide chemistry, the 4f shell is contracted and usually does not directly participate in the chemical bonding. In actinide chemistry, on the other hand, the 5f shell is more diffuse and participates actively in the bonding. 

Niels Bohr s 1913 hydrogen atom paper demonstrates the traditional interest of some physicists in placing the facts and laws of chemistry within a broader framework of foundational principles laid out by physicists. During the course of the next two decades, a number of physicists who became known as quantum physicists developed physical theories and mathematical techniques that they claimed would create a mathematical and theoretical chemistry. However, few of them had much chemical knowledge beyond a general understanding of the periodic table of the elements and familiarity with the Lewis-Langmuir theory of the electron duplet and octet. 

Atoms are generally most stable when they have a complete octet (eight electrons). 

Tetrahedral structures . In a more limited field than that of the previously considered general octet rule, it may be useful to mention the tetrahedral structures which form a subset of the general valence compounds. According to Parthe (1963, 1964, 1991), if each atom in a structure is surrounded by four nearest neighbours at the corner of a tetrahedron, the structure is called normal tetrahedral structure . 

For a general formulation of the Zintl-Klemm concept, consider an intermetallic AmX phase, where A is the more electropositive element, t3 pically an alkali or an alkaline earth metal. Both A and X, viewed as individual atoms, are assumed to follow the octet rule leading to transfer of electrons from A to X, i.e., A AF, X —> X , so that mp = nq. The anionic unit X arising from this electron transfer is considered to be a pseudoatom, which exhibits a structural chemistry closely related to that of the isoelectronic elements [11]. Since bonding also is possible in the cationic units, the numbers of electrons involved in A-A and X-X bonds of various types (caa and exx> respectively) as well as the number of electrons e not involved in localized bonds can be generated from the numbers of valence electrons on A and X, namely and ex, respectively, by the following equations of balance ... 

In summary, ionic bonds form when there is a transfer of electrons between atoms of different elements. The result of this transfer produces oppositely charged ions. The ions produced generally obtain the valence electron configuration of noble gases, that is, conform to the octet rule. The oppositely charged ions produced are held together by electrostatic attraction. This attractive force is the ionic bond. 

The Lewis dot structure for BF3 shows the central boron atom being surrounded by only six electrons, which violates the octet rule. This illustrates that the octet rule, while providing general guidelines, has exceptions. 

Nucleon a proton or a neutron, the number of nucleons in an atom equals the sum of protons and neutrons in the nucleus Octet Rule general rule that states that the most stable electron configuration occurs when an atom surrounds itself with eight valence electrons... 

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