Binary-type, extended

Binary-type nomenclature can be extended beyond simple stoichiometric names. Let’s have a look at the compound with empirical formula HKO. If we were trying to come up with purely stoichiometric name, it would be either potassium hydride oxide or hydrogen potassium oxide, but nobody calls it that. Moreover, it is customary to write its formula not like I did (with element symbols ordered alphabetically), but KOH. Why? Because it is known that KOH is an ionic compound which will dissociate in water into cations K⁺ and anions OH⁻. Attention please: we have just zoomed from (macroscopic) compound to (microscopic) molecular entities.

So we’ve got some extra structural information, viz. that the anion is composed of oxygen and hydrogen. The anion OH⁻ is known as hydroxide and thus our compound is named potassium hydroxide.

Likewise, it is known that NH₄NO₃ dissociates into cations NH₄⁺ and anions NO₃⁻. The NH₄⁺ cation is known as ammonium and the NO₃⁻ anion as nitrate so our compound is is named ammonium nitrate.

Stoichiometric names

The Red Book [1, p. 5] uses the term compositional nomenclature

to denote name constructions which are based solely on the composition of the substances or species being named, as opposed to systems involving structural information.

It is the simplest systematic way of naming chemical substances. Compositional nomenclature can be used for both compounds and elementary substances. In case of compounds, is is also known as binary-type nomenclature [2]. Why “binary”? Because the names of compounds named that way always consist of two parts, positive and negative.

What are compounds anyway?

According to Oxford English Dictionary, “compound” (in chemistry) is

a substance formed from two or more elements chemically united in fixed proportions.

(1)

I quite like this definition. There are four statements in it:

• compound is a substance (therefore, it is macroscopic);
• compound contains at least two (different) elements;
• these elements are “chemically united”, i.e. chemically bound;
• they are bound in fixed proportions.