Octet rule

If one atom has a very weak attraction for its valence electron(s) (e.g., sodium, Na), and another atom has a very strong attraction for its valence electrons (e.g., fluorine, F), then the weakly held electron is likely to be stripped away from the first atom by the second atom (fluorine will strip sodium s valence electron). This will usually result in the formation of an ionic bond. If an atom with a weak attraction for its valence electron(s) (e.g., sodium, Na) is around another atom that also has a weak attraction on its valence electron(s) (e.g., Mg), then neither has enough attractive force to take the other s valence electron away. This means that under normal circumstances, these atoms won t react. But if both atoms have strong attractions for electrons, they will attract each other s electrons. The result of this type of atomic tug-of-war generally is that the two atoms will hold on to each other and share electrons. This type of interaction is the basis for a covalent (or molecular) bond. 

Ionic bonds are formed by electrostatic attractions between oppositely charged ions. These ions are formed when atoms of low ionization energy (weak attraction for valence electrons) lose one or more electrons to atoms with high electron affinity (strong attraction for electrons). At this point, we can use the octet rule to guide us through the process. 

The same pattern is followed when other combinations of ions produce larger compounds, like A1C13. 

Transition metals do not follow the octet rule simply because their outer electron structures involve both s and d orbitals. The transition metals form cations, so they are all losing electrons. The typical pattern for the loss of electrons in these elements is for the first electron(s) to come out of the s orbital and then for all remaining electrons to come out of the d orbitals. In addition, most of the transition metals can form more than one type of ion. An example would be copper, which can form ions of either 1+ or 2+ charge. 

Remember, in ionic bonding, electrons from one atom are stripped away by electrons of another atom. This requires a large difference in the amount of attraction each atom has for its own electrons—one has to have a very strong attraction and the other a relatively weak attraction. Typically, ionic bonds form between metals and nonmetals. However, when both atoms have relatively strong attractions for electrons, neither atom can strip electrons away from the other. Instead, the electrons from one atom are attracted to the other and vice-versa. The orbitals of each atom will actually overlap, and the electrons will be shared between the atoms. This process will be described in much greater detail in Chapter 7. 

Consider the opposite case, the addition of an electron. Adding an electron to a noble gas would mean adding an electron to a higher energy level 

Let s begin with a very typical and simple example—the formation of sodium chloride (NaCI), ordinary table salt. Sodium is a group 1A element 

Chlorine ls 2s 2p 3s 3p + electron ls 2s 2p 3s 3p = stable, octet Using dot structures to represent this equation, the reaction looks like this  

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In the example in Figure 2-19, the oxygen atom 3 has 2- 4 (row) + 2 + 4 (column) - 4 (diagonal element) = 8 electrons. This shows that the ox> gen atom obeys the octet rule. 

Figure 2-19. The BE-matriK of ethanal allows one to determine tine number of valence electrons (the sum of each row) on the atoms and to validate the octet rule,...

Protonated methane (CH ) does not violate the octet rule of carbon. A bonding electron pair (responsible for covalent bonding between C and H atoms) is forced into sharing with the proton, resulting in 2 electron-3 center bonding (2e-3c) (see Chapter 10). Higher alkanes are protonated similarly. 

Lewis s concept of shared electron parr bonds allows for four electron double bonds and SIX electron triple bonds Carbon dioxide (CO2) has two carbon-oxygen double bonds and the octet rule is satisfied for both carbon and oxygen Similarly the most stable Lewis structure for hydrogen cyanide (HCN) has a carbon-nitrogen triple bond... 

Multiple bonds are very common m organic chemistry Ethylene (C2H4) contains a carbon-carbon double bond m its most stable Lewis structure and each carbon has a completed octet The most stable Lewis structure for acetylene (C2H2) contains a carbon-carbon triple bond Here again the octet rule is satisfied... 

It will always be true that a nitrogen with four covalent bonds has a formal charge of + 1 (A nitrogen with four co valent bonds cannot have unshared pairs because of the octet rule)... 

Lewis structures in which second row elements own or share more than eight valence electrons are especially unstable and make no contribution to the true structure (The octet rule may be ex ceeded for elements beyond the second row)... 

When two or more structures satisfy the octet rule the most stable one is the one with the smallest separation of oppositely charged atoms... 

The two Lewis structures D and E of methyl nitrite satisfy the octet rule... 

Among structural formulas in which the octet rule IS satisfied for all atoms and one or more of these atoms bears a formal charge the most stable reso nance form is the one in which negative charge re sides on the most electronegative atom... 

Section 1 3 The most common kind of bonding involving carbon is covalent bond ing A covalent bond is the sharing of a pair of electrons between two atoms Lewis structures are written on the basis of the octet rule, which limits second row elements to no more than eight electrons m their valence shells In most of its compounds carbon has four bonds... 

There is ample evidence from a variety of sources that carbocations are mterme diates m some chemical reactions but they are almost always too unstable to isolate The simplest reason for the instability of carbocations is that the positively charged car bon has only six electrons m its valence shell—the octet rule is not satisfied for the pos itively charged carbon... 

Free radicals are species that contain unpaired electrons The octet rule notwithstand mg not all compounds have all of their electrons paired Oxygen (O2) is the most famil lar example of a compound with unpaired electrons it has two of them Compounds that have an odd number of electrons such as nitrogen dioxide (NO2) must have at least one unpaired electron... 

Of the two resonance forms A and B A has only six electrons around its positively charged carbon B satisfies the octet rule for both carbon and oxygen It is more stable than A and more stable than a carbocation formed by protonation of a typical alkene... 

Many transition metal complexes including Ni(CO)4 obey the 18 electron rule, which IS to transition metal complexes as the octet rule is to mam group elements like carbon and oxygen It states that... 

Isomtriles are stable often naturally occumng compounds that contain a divalent carbon An example is axisonitnle 3 which can be isolated from a species of sponge and possesses anti malanal activity Write a resonance form for axisonitnle 3 that satisfies the octet rule Don t for get to include formal charges... 

Lewis structure (Section 1 3) A chemical formula in which electrons are represented by dots Two dots (or a line) be tween two atoms represent a covalent bond in a Lewis structure Unshared electrons are explicitly shown and sta ble Lewis structures are those in which the octet rule is sat isfied... 

Octet rule (Section 1 3) When forming compounds atoms gain lose or share electrons so that the number of their va lence electrons is the same as that of the nearest noble gas For the elements carbon nitrogen oxygen and the halo gens this number is 8... 

Note that these compounds are covalently bonded compounds containing only hydrogen and carbon. The differences in their strucmral formulas are apparent the alkanes have only single bonds in their structural formulas, while the alkenes have one (and only one) double bond in their structural formulas. There are different numbers of hydrogen atoms in the two analogous series. This difference is due to the octet rule that carbon must satisfy. Since one pair of carbon atoms shares a double bond, this fact reduces the number of electrons the carbons need (collectively) by two, so there are two fewer hydrogen atoms in the alkene than in the corresponding alkane. 

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