Octet rule The observation that atoms

Octet rule the observation that atoms of nonmetals tend to form the most stable molecules when they are surrounded by eight electrons (to fill their valence orbitals). (13.10) Optical isomerism isomerism in which the isomers have opposite effects on plane-polarized light. (20.4)... 

Octet rule the observation that atoms of nonmetals form the most stable molecules when they are surrounded by eight electrons (to fill their valence orbitals). 

Note that the sodium ion has the same electron configuration as neon (ls 2s 2p ), a noble gas. This observation leads to one of the most important principles in chemistry, the octet rule. The octet rule states that atoms tend to gain, lose, or share electrons in order to acquire a full set of eight valence electrons. This reinforces what you learned earlier that the electron configuration of filled s and p orbitals of the same energy level (consisting of eight valence electrons) is unusually stable. Note that the first period elements are an exception to the rule, as they are complete with only two valence electrons. 

Lewis structures are drawn to represent the arrangement of the valence electrons in a molecule. The rules for drawing Lewis structures are based on the observation that nonmetal atoms tend to achieve noble gas electron configurations by sharing electrons. This leads to a duet rule for hydrogen and to an octet rule for many other atoms. 

The octet rule applies quite well to the first full row of the periodic table (Li through F), but beyond this it is generally applicable only to the non-transition elements, and even in many of these it cannot explain many of the bonding patterns that are observed. The principal difficulty is that a central atom that is bonded to more than four peripheral atoms must have more than eight electrons around it if each bond is assumed to consist of an electron pair. In these cases, we hedge the rule a bit, and euphemistically refer to the larger number of electrons as an expanded octet . 

Sulfur hexafluoride (4.5) provides an example of a so-called hypervalent molecule, i.e. one in which the central atom appears to expand its octet of valence electrons. However, a valence bond picture of the bonding in SFg involving resonance structures such as 4.6 shows that the S atom obeys the octet rule. A set of resonance structures is needed to rationalize the observed equivalence of the six... 

Section 8.7 The octet rule is not obeyed in all cases. The exceptions occur when (a) a molecule has an odd number of electrons, (b) it is not possible to complete an octet aroimd an atom without forcing an unfavorable distribution of electrons, or (c) a large atom is surrounded by so many small electronegative atoms that it must have more than an octet of electrons aroimd it. In this last case we envision using the unfilled d orbitals of the large atom to "expand" the valence shell of the atom. Expanded octets are observed for atoms in the third row and beyond in the periodic table, for which low-energy d orbitals are available. 

---