Answer to Question #91648 in General Chemistry for Ellen Silverberg-Brennan

Question

How do I know if an element has more than one oxidation number like Cu, Fe, and Sn?

Answer 1

To determine the possible degrees of oxidation, it is necessary to consider the position of the element in the Periodic Law and to list its electronic configuration.

To begin, consider Sn. This is an element of group IV of the a subgroup. The external energy level has the following configuration: $5s^25p^2$ We see the presence of two unpaired electrons at the р sublevel, which means that the oxidation state will be +2. But, the presence of one empty р orbital makes it possible to have an excited state, an electron from a s sublevel can get into this vacant orbital, which makes it possible to have an oxidation state of +4 (particle capable of giving away 4 electrons). Copper Cu and Iron Fe are not, in contrast to Tin Sn, elements of the a subgroup. They are respectively assigned to b subgrops of groups I and VIII.

Electronic configuration of the external level Cu: $3d^{10}4s^1$ . Moreover, in this case we observe the failure of electrons (similar to Silver Ag and Gold Au). The presence of an failure of electron and a similar structure of the outer valence level implies the presence of several degrees of oxidation. So, for Copper Cu ⁺¹ and Cu ⁺².

Electronic configuration of the external level Fe: $3d^64s^2$ . Due to this structure of the external group, various configurations for iron ions are possible, for example: Fe⁺² ($3d^64s^0$) and Fe⁺³ ($3d^54s^0$).

IMPORTANT: all represented elements - metals, possess positive degrees of oxidation.

Answer: in order to assess the possible degrees of oxidation, it is necessary to evaluate the electronic configuration of the element (specifically, its external level).