It has been observed that almost all the compounds of non-transition elements contain aneven number of bonding electrons. Very few exceptions like `NO, NO_2` etc. possess odd numberof bonding electrons. Such type of compounds are called Odd Molecules. The odd molecules are classified into following two categories

(i) `text(One electron bond molecules and ions)` : `H_2^(+)` ion is an example in which one electron bond is formed. `H_2^(+)` ion is considered to be resonance hybrid of the two resonating structures (a) and (b).

(a) `text()^(+)H � H` (b) `H � H^(+)`

`H_2^(+)` ion has its bond energy equal to `61` `kcal`/`mol`. These two forms are of equal stability. `H_2` molecules has its bond energy equal to `109` `kcal`/`mol`. Internuclear distance between the `H`-atoms in `H_2^(+)` is `1.06``A^o` and in `H_2` molecule it is `0.74` `overset(o)(A)`.

Other examples having `1` electron bond are `Li_2^(+)`, `Na_2^+`, `K_2^(+)` ions and `B_2H_6` (diborone) molecule.

(ii) `text(Three electron bond molecules and ions)` : Examples of some molecules and ions are given below.

`He_2^(+)` `text(ion)` : This ion is represented as a resonance hybrid of (a) and (b).

`He_(text x)^(text x)` `� He^(+)` `->` `text()^+He �` `text()_(text x)^(text x)He`

Resonance between the two forms (a) and (b) leads to the formation of a three-electron bond between `He` and `He^(+)`. It is only one electron that plays part in resonance. Thus, `1`- and `3`-electron bonds have approximately the same energy. The binding energy of `He_2^(+)` is `58` `kcal`/`mol`.

`O_2` `text(molecule)` : Normal VBT structure for `O_2` molecules

`: underset(. .)O = undersettext(x x)oversettext(x x)O`

This structure however, does not represent the paramagnetic property of `O_2` molecule. It is therefore assumed that two `O`-atoms in `O_2` molecule are linked by a normal covalent bond and two `3`-electron bonds. Thus `O_2` molecule is `O overset(. . .)underset(. . .)(-) O`