Molecules stable octet

Carbocations react rapidly with Lewis bases (molecules or ions that can donate electron pair) to achieve a stable octet of electrons. 

Lewis and many other chemists had recognized the shortcomings of the ionic bond. When diatomic molecules, such as or Cl, were considered, there was no reason why one atom should lose an electron and an identical atom should gain an electron. There had to be another explanation for how diatomic molecules formed. We have seen how the octet rule applies to the formation of ionic compounds by the transfer of electrons. This rule also helps explain the formation of covalent bonds when molecules (covalent compounds) form. Covalent bonds result when atoms share electrons. Using fluorine, F, as a representative halogen, we can see how the octet rule applies to the formation of the molecule. Each fluorine atom has seven valence electrons and needs one more electron to achieve the stable octet valence configuration. If two fluorines share a pair of electrons, then the stable octet configuration is achieved ... 

The ammonia molecule donates a pair of electrons to the electron-deficient aluminum atom in the above reaction, thus completing a stable octet of electrons. This example is a pertinent one, since adsorption of ammonia has been used as a way of measuring the acidity of solid catalyst surfaces. 

Examples of neutral Lewis acids are halides of group 3A elements, such as BF3. Boron trifluoride, a colorless gas, is an excellent Lewis acid because the boron atom in the trigonal planar BF3 molecule is surrounded by only six valence electrons (Figure 15.12). The boron atom uses three sp2 hybrid orbitals to bond to the three F atoms and has a vacant 2p valence orbital that can accept a share in a pair of electrons from a Lewis base, such as NH3. The Lewis acid and base sites are evident in electrostatic potential maps, which show the electron poor B atom (blue) and the electron rich N atom (red). In the product, called an acid-base adduct, the boron atom has acquired a stable octet of electrons. 

The hydrogen molecule (H2) is particularly attractive to a few elements, including fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). These four elements are each highly electronegative, meaning they readily attract electrons to form a stable octet. 

You have seen how Lewis structures can help you draw models of ionic, covalent, and polar covalent compounds. When you draw a Lewis structure, you can count how many electrons are needed by each atom to achieve a stable octet. Thus, you can find out the ratio in which the atoms combine. Once you know the ratio of the atoms, you can write the chemical formula of the compound. Drawing Lewis structures can become overwhelming, however, when you are dealing with large molecules. Is there a faster and easier method for writing chemical formulas ... 

Carbon, oxygen, and nitrogen atoms often form double bonds by sharing two pairs of electrons. Carbon and nitrogen atoms may even share three pairs of electrons to form a triple bond. Think about the molecule N2. With five valence electrons, each N atom needs three more electrons for a stable octet. Each N atom contributes three electrons to form three bonding pairs. The two N atoms form a triple bond by sharing these three pairs of electrons, or a total of six electrons. Because the two N atoms share the electrons equally, the triple bond is a nonpolar covalent bond. 

Sulfuranes with hypervalent sulfur atoms possess expanded valence shells, and consequently the molecules are relatively unstable. The central sulfur atom can, however, attain the normal stable octet of electrons by extruding a ligand or a pair of ligands in an elimination process. The former results in ligand coupling (Figure 8 and Schemes 14a and... 

Carbon has four electrons in its valence shell and needs four more electrons to reach the stable octet of electrons. It may do this by combining with four hydrogen atoms to form a methane molecule. Write down a dot and cross representation of this molecule and calculate the charge that is present on each of the five atoms. 

It has already been shown that the F2 molecule involves a single covalent bond. If the two O atoms of O, are treated in the same way they have insufficient valence shell electrons to form stable octets by sharing just one electron pair, but if two electron pairs are shared, stable octets are formed on both O atoms, with the sharing of the two electron pairs. This situation is referred to as a double bond ... 

Now, cormt all the electrons arormd each atom, including the ones that are shared. You ll see that each atom has a stable octet. Study the result of this electron sharing in Figure 4.17. By sharing electrons, the three atoms achieve a more stable arrangement than they had as three separate atoms. A molecule of carbon dioxide is more stable than one carbon atom and two oxygen atoms. As with water, the molecule of carbon dioxide is different from the sum of its parts. The macroscopic properties of carbon dioxide are a result of the unique properties of carbon dioxide molecules, not the properties of carbon or oxygen atoms. 

The chlorine atoms are extremely reactive because of their strong tendency to acquire a stable octet of electrons. The following reactions occur when a chlorine atom reacts with an ozone molecule (O3). First, chlorine pulls an oxygen atom away from ozone ... 

The term stable octet describes the fact that many atoms in molecules are most stable when the valence shell contains effectively eight electrons. This counts both non-... 

In this way they are like BF3 and AICI3. Most carbocations are also short-lived and highly reactive. They occur as intermediates in some organic reactions. Carbocations react rapidly with Lewis bases—with molecules or ions that can donate the electron pair that they need to achieve a stable octet of electrons (i.e., the electronic configuration... 

Arrange the atoms to form a skeleton structure for the molecule, and place electron pairs between atoms to represent covalent bonds. You can predict the arrangement of atoms by figuring out how many covalent bonds each atom must form in order to achieve a stable octet. Each chlorine atom, which has 7 valence electrons, must form a single covalent bond. Sulfur, which has 6 valence electrons, must form two covalent bonds. The only possible structure is Cl—S—Cl. 

Borane is very reactive because the boron atom has only six electrons in its valence shell. In tetrahydrofuran solution, BH3 accepts an electron pair from a solvent molecule in a Lewis acid-base reaction to complete its octet and form a stable BH3-THF complex. 

There are four types of organic species in which a carbon atom has a valence of only 2 or 3/ They are usually very short lived, and most exist only as intermediates that are quickly converted to more stable molecules. However, some are more stable than others and fairly stable examples have been prepared of three of the four types. The four types of species are carhocations (A), free radicals (B), carbanions (C), and carbenes (D). Of the four, only carbanions have a complete octet around the carbon. There are many other organic ions and radicals with charges and unpaired electrons on atoms other than carbon, but we will discuss only nitrenes (E), the nitrogen analogs of carbenes. 

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 BH3 molecule is not stable as a separate entity. This molecule can be stabilized by combining it with another molecule that can donate a pair of electrons (indicated as ) to the boron atom to complete the octet (see Chapter 9). For example, the reaction between pyridine and B2H6 produces C5H5N BH3. Another stable adduct is carbonyl borane, OC BH3 in which a pair of electrons is donated from carbon monoxide, which stabilizes borane. In CO, the carbon atom has a negative formal charge, so it is the "electron-rich" end of the molecule. Because the stable compound is B2H6 rather than BH3, the bonding in that molecule should be explained. 

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