Octet rule, exceptions

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. 

In most of their covalent compounds, the representative elements follow the octet rule, except that hydrogen always shares only two electrons. 

The octet rule is a useful guide for most molecules with Period 2 central atoms, but not for every one. Also, many molecules have central atoms from higher periods. As you ll see, some central atoms have fewer than eight electrons around them, and others have more. The most significant octet rule exceptions are for molecules containing electron-deficient atoms, odd-electron atoms, and especially atoms with expanded valence shells. 

SAMPLE PROBLEM 10.5 Writing Lewis Structures for Octet-Rule Exceptions Problem Write Lewis structures for (a) H3PO4 (pick the most likely structure) (b) BFCI2. Plan We write each Lewis structure and examine it for exceptions to the octet rule. In (a), the central atom is P, which is in Period 3, so it can use d orbitals to have more than an octet. Therefore, we can write more than one Lewis structure. We use formal charges to decide if one resonance form is more important. In (b), the central atom is B, which can have fewer than an octet of electrons. 

These species do not obey the octet rule. Draw a Lewis structure for each, and state the type of octet-rule exception ... 

He and H cannot serve as central atoms in a Lewis structure. Both can have no more than two valence electrons. Fluorine needs only one electron to complete its valence level, and it does not have d orbitals available to expand its valence level. Thus, it can bond to only one other atom. 10.3 All the structures obey the octet rule except c and g. 

SAMPLE PROBLEM 10.5 Writing Lewis Structures for Octet-Rule Exceptions... 

The periodicity of valence of the s- and p-block elements w as described and rationalized in terms of the octet rule. Exceptions from the octet rule were discussed. These include compounds exhibiting hypervalence as a result of the expansion of the valence shell of the central atom, and compounds in which the inert pair effect is apparent, the valency of the central element being two units lower than expected for the group valency. 

Interactivity Octet Rule Exceptions ARIS, Interactives... 

Lewis electron-dot formulas are simple representations of the valence-shell electrons of atoms in molecules and ions. You can apply simple rules to draw these formulas. In molecules with delocalized bonding, it is not possible to describe accurately the electron distribution with a single Lewis formula. For these molecules, you must use resonance. Although the atoms in Lewis formulas often satisfy the octet rule, exceptions to the octet rule are not uncommon. You can obtain the Lewis formulas for these exceptions by following the rales for writing Lewis formulas. The concept of formal charge will often help you decide which of several Lewis formulas gives the best description of a molecule or ion. 

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