Octet rule Many representative elements

Octet rule Many representative elements attain at least a share of eight electrons in their valence shells when they form molecular or ionic compounds there are some limitations. 

Indeed, the great majority of substances which are made of elements of low tomic number, are represented by structural formulae which are in agreement fcith such an octet rule (a doublet rule for H). This is, for example, the case for drocarbons and many other organic molecules, even when their structures described by resonant hybrids. The rule therefore represents an extremely [useful tool for the quick writing of structural formulae. 

For now, we restrict our discussion to compounds of the representative elements. The octet rule alone does not let us write Lewis formulas. We still must decide how to place the electrons around the bonded atoms—that is, how many of the available valence electrons are bonding electrons (shared) and how many are unshared electrons (associated with only one atom). A pair of unshared electrons in the same orbital is called a lone pair. A simple mathematical relationship is helpful here ... 

When an atom has a share of more than eight electrons, as does P in PF5, we say that it exhibits an expanded valence shell. The electronic basis of the octet rule is that one r and three p orbitals in the valence shell of an atom can accommodate a maximum of eight electrons. The valence shell of phosphorus has = 3, so it also has hd orbitals available that can be involved in bonding. It is for this reason that phosphorus (and many other representative elements of Period 3 and beyond) can exhibit an expanded valence shell. By contrast, elements in the second row of the periodic table can never exceed eight electrons in their valence shells, because each atom has only one s and three p orbitals in that shell. Thus, we understand why PF5 can exist but NF5 cannot. 

The symbol of the element represents the nucleus and all of the electrons except the outer valence shell. The valence electrons are represented by dots, and sometimes by crosses or circles. A key concept in building Lewis structural formulas is to satisfy the octet rule which states that many elements achieve stability by forming covalent bonds in order to fill their outer shell with eight electrons. 

---