Problem: For the following set of atoms and ions, arrange the members in order of decreasing size: Rank items from largest to smallest. To rank items as equivalent, overlap them.Co3+ , Fe3+, Fe2+

Recall that ionic radius is the size of an ion. The trend for ionic radius is as follows: it increases from right to left and down a period in the periodic table. Ionic radius also increases with increasing number of electrons.

Co³⁺:

Co: atomic number = 27

neutral Co = 27 electrons

Co³⁺ = 27 e^- - 3 e^- 

Co³⁺ = 24  e^- 

Fe³⁺:

Fe: atomic number = 26

neutral Fe = 26 electrons

Fe³⁺ = 26 e^- - 3 e^- 

Fe³⁺ = 23  e^- 

Fe²⁺

Fe: atomic number = 26

neutral Fe = 26 electrons

Fe²⁺ = 26 e^- - 2 e^- 

Fe³⁺ = 24  e^-

Fe³⁺: → least # of electrons → smallest size

Co³⁺ and Fe³⁺ → same number of electrons → isoelectronic