6 - transition metals & oxidation numbers

• a transition metal is defined as a metal that, in at least one of its stable ions has a partly filled d subshell
• all d block transition metals are elements with an incomplete d subshell in at least one of their ions
• transition metal general properties

  they have atoms or ions with an incomplete d subshell, they can form complexes, they have variable oxidation states, they show catalytic activity and they form coloured ions
• the filling of the d orbitals follows the aufbau principle, except for chromium and copper, which is due to a special stability associated with the d orbitals being half filled or completely filled as its a lower energy arrangement than filling the 4s subshell
• when a transition metal forms an ion, the electrons from the 4s shell are lost first, rather from the 3d subshell
• the oxidation number is equal to the charge on the ion
• the oxidation number and oxidation state are interchangeable, so it can be said an element is in a particular oxidation state when it has a specific oxidation number
• the varying oxidation state gives a numerical value for the degree of reduction or oxidation in an element
• oxidation can be described as an increase of oxidation number
• reduction can be seen as a decrease of oxidation number
• rules for assigning and using oxidation

  1- the oxidation number for a free or uncombined element is 0

  2- for ions consisting of single atoms, the oxidation number is the same as the charge on the ion

  3- the oxidation number for hydrogen is +1 and oxygen is -2

  4- in compounds fluorine has an oxidation number of -1

  5- the sum of all the oxidation number in a molecule must equal 0

  6- the sum of all oxidation numbers in a polyatomic ion must be equal to the charge of the ion
• compounds containing metals in a high oxidation state tend to be oxidising agents
• compounds with metals in a low oxidation state tend to be reducing agents
• compounds of the same transition metal but in different oxidation states may have different colours
• transition metals exhibit variable oxidation states of differing stability
• oxidation state of vanadium = +5

  yellow
• oxidation state of vanadium = +4

  blue
• oxidation state of vanadium = +3

  green
• oxidation state of vanadium = +2

  violet
• as oxidation state changes so too does the colour, compounds where this occurs are good self-indicating reagents