Electron duet

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). 

The unfused cyclobutadiene system is stable in complexes with metals (see Chapter 3), but in these cases electron density is withdrawn from the ring by the metal and there is no aromatic quartet. In fact, these cyclobutadiene-metal complexes can be looked upon as systems containing an aromatic duet. The ring is square planar, the compounds undergo aromatic substitution, and NMR spectra of monosubstituted derivatives show that the C-2 and C-4 protons are equivalent. ... 

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. 

Add the remainder of the available electrons to complete the octets (or duets) of all the atoms. There should be just enough if the molecule or ion follows the octet rule. 

It must be emphasized that the octet rule does not describe the electron configuration of all compounds. The very existence of any compounds of the noble gases is evidence that the octet rule does not apply in all cases. Other examples of compounds that do not obey the octet rule are BF,. PF5, and SF6. But the octet rule does summarize, systematize, and explain the bonding in so many compounds that it is well worth learning and understanding. Compounds in which atoms attain the configuration of helium (the duet) are considered to obey the octet rule, despite the fact that they achieve only the duet characteristic of the complete first shell of electrons. 

In the Lewis structure for C2FL, each carbon atom shares four pairs of electrons with three hydrogen atoms and the other carbon atom. With this arrangement, the carbon atoms complete their octets while the hydrogen atoms complete their duets. 

Hydrogen never has an octet of electrons in any of its compounds, but rather a pair (or duet, if you prefer). An example is the Lewis structure of H20 (below). In many compounds in which the central atom is from the second period or higher, there are more than eight electrons around the central atom an example of a compound with such an expanded octet is IC13 (below). Finally, in some compounds, there are less than eight electrons around the central atom one such electron deficient compound is BF3. 

Atoms containing only Is electrons simply don t have eight slots to fill, so hydrogen and helium obey the duet (two) rule. They re perfectly happy with only two electrons. 

When atoms combine to produce molecules, they often do so in accord with the octet rule. Your text undoubtedly contains a fairly detailed discussion of the octet rule. In essence, it may be described as the tendency for an atom to lose, gain, or share electrons in order to achieve an s2p6 configurationin 1 e 0uter most shell. The simplest atoms (H, Li, Be, and so on) tend to achieve a Is2 configuration, according to what might be called the duet rule. 

Arrange the remaining electrons around the atoms to satisfy the duet rule for hydrogen and the octet rule for other atoms. 

In drawing Lewis structures for covalently bonded molecules, the octet rule is used as a guide (remember that for hydrogen the octet is changed to a duet). Shared electrons count toward the electron total for both atoms in the bond. For example, the Lewis structure for SiH4 looks like this ... 

Arrange the remaining electrons as lone pairs or create double or triple bonds to satisfy the octet rule. Exceptions Hydrogen satisfies the duet (two) rule, and boron and aluminum satisfy the six-electron rule. 

Because the maximum number of electrons in the first shell of an atom is 2, helium is stable with 2 electrons in its only occupied shell. The other very light elements—hydrogen, lithium, and beryllium—tend to form stable states by achieving the 2-electron configuration of helium. Having 2 electrons in the first shell, when that is the only shell and therefore the outermost shell, is a stable state, and the 2 electrons are sometimes called a duet. When there is only one shell, 2 electrons in that shell act like 8 electrons in any other outermost shell. Therefore, an atom with 2 electrons in its outermost first shell is often said to obey the octet rule, although duet rule would be more precise. 

Electrons shared between hydrogen atoms are counted toward the duets of both atoms. In the hydrogen molecule, each hydrogen atom has a total of two electrons in its first shell and, thus, a stable configuration. Electrons shared between other nonmetal atoms are counted toward the octets of both. 

Be sure to check that all the atoms have the proper octets (duets for hydrogen atoms) and that all valence electrons and no other electrons are shown. 

An atom is electron-deficient if it lacks an octet of electrons in its valence shell (or, for H, a duet of electrons). 

An atom in a molecule is most stable if it can achieve the electronic configuration of the nearest noble gas, thus having a completely filled valence shell. Hydrogen with two electrons around it, a duet, achieves the configuration of helium. Second-row elements achieve the configuration of neon with an octet of valence electrons. Third-row elements achieve an octet but may also expand their valence shell for example SFg is a stable molecule with six single bonds to sulfur (12 bonding electrons total). 

Hydrogen atoms attain duets of electrons because the first shell is complete when it contains two electrons. 

Hydrogen forms stable molecules where it shares two electrons. That is, it follows a duet rule. For example, when two hydrogen atoms, each with one electron, combine to form fhe H2 molecule, we have... 

Explain tbe "duet" and "octet" rules and bow they are used to describe the arrangement of electrons in a molecule. 

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