Exceptions to The Octet Rule and Variable Valence
Exceptions to the octet rule
** For a time
it was believed that all compounds obeyed the Octet rule or the Rule of eight. However,
it gradually became apparent that quite a few molecules had non-octet structures.
Atoms in these molecules could have number of electrons in the valence shell
short of the octet or in excess of the octet.
** Some
important examples are:
(1) Four or six electrons around the central atom
** A stable molecule
as of beryllium chloride, BeCl2, contains an atom with four
electrons in its outer shell.
** The compound
boron trifluoride, BF3, has the Lewis structure :
The boron
atom has only six electrons in its outer shell.
Beryllium
chloride and boron trifluoride are referred to as electron-deficient compounds
(2) Seven electrons around the central atom
** There are a
number of relatively stable compounds in which the central atom has seven electrons
in the valence shell. A simple example is chlorine dioxide, ClO2.
The chlorine
atom in ClO2 has seven electrons in its outer shell.
** Methyl
radical (CH3) has an odd electron and is very short lived. When two
methyl free radicals collide, they form an ethane molecule (C2H6)
to satisfy the octet of each carbon atom.
** Any species
with an unpaired electron is called a free radical.
(3) Ten or more electrons around the central atom
** Non-metallic
elements of the third and higher periods can react with electronegative elements
to form structures in which the central atom has 10, 12 or even more electrons.
The typical examples are PCl5 and SF6.
** The
molecules with more than an octet of electrons are called superoctet
structures.
** In
elements C, N, O and F the octet rule is strictly obeyed because only four
orbitals are available (one 2s and three 2p) for bonding. In the elements P and
S, however, 3s, 3p, and 3d orbitals of their atoms may be involved in the
covalent bonds they form. Whenever an atom in a molecule has more than eight
electrons in its valence shell, it is said to have an expanded octet.
Variable Valence
** Some
elements can display two or more valences in their compounds.
** The
transition metals belong to this class of elements. The Electronic Structure of
some of these metals is given below:
** Most of the
transition metals have one or two outer-shell electrons and they form monovalent
or bivalent positive ions. But because some of the d electrons are close in
energy to the outermost electrons, these can also participate in chemical bond
formation. Thus transition metals can form ions with variable valence.
** For example,
copper can form Cu1+ and Cu2+ ions and iron can form Fe2+
and Fe3+ ions.
** The complete
electronic configuration of an iron atom is:
It can form Fe2+
by losing two 4s electrons,
When iron loses
two 4s electrons and one of the three 3d electrons, if forms Fe3+
ion
** Copper form
Cu1+ and Cu2+ ions by losing one 4s electron, and one 4s
and 3d electron respectively
** It may be
noted that the structures of Fe2+, Fe3+, Cu1+,
Cu2+, Cr3+, etc., are not isoelectronic with any of the
noble gases, and hence the d electrons being unstable are available for bond
formation.
** The atoms
and ions that have the same number of electrons are said to be Isoelectronic.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
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