Frost diagram

A Latimer diagram lists the numbers; a Frost diagram makes them into a landscape you can read at a glance. You plot, for each oxidation state of an element, a kind of relative energy on the vertical axis against the oxidation state on the horizontal axis. The lower a point sits, the more stable that oxidation state is — so the most stable species of an element is simply the lowest point on the curve, like the bottom of a valley where a ball would come to rest.

The quantity plotted up the y-axis is nE°, the oxidation state n times the potential needed to reach it from the element (this is proportional to the free energy of forming that state from the element). The genius of the plot is that the slope of the line between any two points equals the reduction potential between those two states. That turns reading redox chemistry into reading a graph: a steep downhill slope means a strong, eager reduction; an uphill slope means the reduction is unfavourable. A species sitting above the line joining its two neighbours is a convex bump and will disproportionate (it can roll down to lower-energy states on either side); a species sitting below that line is in a concave dip and is stable, so its neighbours will instead comproportionate to form it.

Frost diagrams are the most visual tool in descriptive redox chemistry, letting you compare a whole element's states, or even compare elements across a group, at a single look. The same honest caveats apply as for Latimer diagrams: the curve is drawn for a specific pH, and the vertical axis is a derived bookkeeping energy (proportional to free energy), not a directly measured one. It shows thermodynamic stability, never reaction rate.