The $d$ block transition metals are metals with an incomplete $d$ subshell in at least one of their ions.

→ This means that zinc and scandium are not transition metals as they each only have one ion which does not contain an incomplete $d$ sub-shell.

The filling of the $d$ orbitals follows the aufbau principle, with the exception of chromium and copper atoms.

→ These exceptions are due to the special stability associated with the $d$ subshell being half-filled or completely filled.

When atoms from the first row of the transition elements form ions, it is the 4$s$ electrons that are lost first rather than the 3$d$ electrons.

An element is said to be in a particular oxidation state when it has a specific oxidation number.

The oxidation number can be determined by the following

• uncombined elements have an oxidation number of $0$

• ions containing single atoms have an oxidation number that is the same as the charge on the ion

• in most of its compounds, oxygen has an oxidation number of $−2$

  → except in peroxides, where it is $-1$

  → except when bonded to fluorine, when it is $+2$

• in most of its compounds, hydrogen has an oxidation number of $+1$

  → except when bonded metals, when it is $-1$

• halogens normally have an oxidation number of $-1$

• the sum of all the oxidation numbers of all the atoms in a neutral compound must add up to zero

• the sum of all the oxidation numbers of all the atoms in a polyatomic ion must be equal to the charge on the ion

A transition metal can have different oxidation states in its compounds.