p-Block Elements: Group 17
Oxidation states
The general electron configuration of halogens is ns2np5. Hence, in most cases gain of one electron makes the octet in the valence shell. Generally, halogens have a universal oxidation state of -1. However, if the halogen is bonded to oxygen or to another halogen in higher electronegativity, then it can adopt different states. The -2 rule for oxygen takes over this rule in the case of two different halogens bonded together. The more electronegative element takes precedence and adopts the -1 oxidation state. This is possible because halogens with a higher atomic number than fluorine have n-d orbitals in a vacant state. When electrons are excited and promoted to d orbital, then the resultant ion can obtain other oxidation states such as +1, +3, +5 and +7. This higher oxidation state of chlorine, bromine, and iodine can occur when these halogens are in a reaction with small and highly electronegative atoms of fluorine and oxygen.The oxides and oxoacids of chlorine and bromine have oxidation states of +4 and +6 respectively. Alternatively, there are no valence shell d orbitals in fluorine atom and it cannot expand its octet.
Fluorine always has a fixed oxidation state of-1 in its compounds owing to the absence of d orbital. Recall that electronegativity increases across a period, while it decreases as we go down a group. This means that fluorine has the electronegativity of 4.0 which is the highest of all the elements. Its position on the periodic table and reactivity correspond to one another. The electron configuration is 1s2 2s2 2p5 and the 2p shell does not contain d orbital.When fluorine gains one more electron, the outermost p orbital is completely filled resulting in a full octet. Also, due to high electronegativity, it can easily remove the desired electron from a nearby atom without external excitation. Accordingly, Fluorine is isoelectronic with a noble gas and has eight valence electrons where all the outermost orbitals are filled. Fluorine is considered much more stable in this state. An example of an electronic configuration in the excited states of an iodine atom is shown below:
As seen from figures above, the electrons in higher-order halogens except for fluorine jump from p orbital to d orbital in an excited state to obtain variant oxidation states.
Likewise, in the case of Iodine chloride ICl, Chlorine has an oxidation state of -1, while iodine attains oxidation state of +1. This indicates that chlorine is more electronegative than iodinegiving it the preferable -1 oxidation state.As for perbromic acid, HBrO4, oxygen has a total oxidation state of -8 (-2 charge x 4 atoms= -8 total charge) while hydrogen has a total oxidation state of +1. When these values are added together then the total oxidation state of the compound so far is -7. Because the final oxidation state of the compound must be 0, the oxidation state of bromineis +7.
Fig : Various oxidation states of halogens and their compounds.