Project Details Toward a comprehensive definition of valence

Project No.:
Start Date:
21 January 2019
End Date:


The semantics of valence in chemistry is the ability of an atom to bond. In English, valence appears in several different quantitative connotations, besides composed terms like covalence, bond valence, and valence bond. For a numerical parameter, existence of alternative values due to differing perceptions is undesirable and obscures communication.

This project objective is to find out whether a comprehensive definition of valence can be formulated.


At the Inorganic Chemistry Division (Div II) meeting in 2016, it was decided to start a project to find out whether a comprehensive definition of valence can be formulated, with a task group that would include representatives of Divisions I (Physical and Biophysical), III (Organic and Biomolecular) and of the CCE (Chemistry Education).

Preparing the project, the task group chair conducted in 2016 a survey of numerical valence assignments among members of these IUPAC groups. It revealed that the valence use among chemists can be categorized into several, simple, heuristic, alternative definitions suitable for teaching.

The objective of this project is to test these possible definitions of valence on suitable examples and see whether one of them can be singled out behind the current use of the term while respecting the IUPAC reflectivity principle. That means deciding: A) Whether there is a definition of valence in a substantial current use that approximates the remaining alternative definitions by not grossly contradicting those of them that are unique or not derivative. B) Whether it yields numerical values that cover consistently the widest range of example compounds, while not contradicting other variables in current use that contain the term, e.g., bond valence.
If such a comprehensive definition of valence of an atom in a compound can be formulated and endorsed, heuristic algorithms will be worked out to evaluate its simple numerical value. If not, the analyses and examples will be included in the Technical Report that explains why no comprehensive definition of valence could be endorsed, concluding this Project. Valence will remain a term of different values, somewhat similar to electronegativity. In that analogy, suitable adjectives might discern such alternative valences.

The project of three parts will summarize at first the history and current usage of the term valence in chemistry and teaching, including relevant recent debates. This carries important etymology factor and will require a linguistic feeling. Alternative definitions of valence, implied by the surveys, will be validated against the current use in textbooks, papers and debates, and a list of possible definitions of valence will be set up.
In the second part, these alternative definitions of valence will be scrutinized against a series of practical examples. Each example in writing and drawing, subsequently by each member of the team, in an attempt to achieve a conclusion. After that, an insight should emerge whether or not a generalization is possible into the desired comprehensive definition of valence.
The third part are write-ups:
1. Technical Report or Provisional Recommendation.
If the latter:
2. Suggestion how to revise valence in the Gold Book.
3. Suggestion how to revise the Wikipedia entry.
4. Actual Wikipedia edit with associated discussions.


Jan 2020 update – During the first year of this project, the task group simultaneously initiated reviews on various aspects of the project:

1. Introductory documents on history and current use of valence:
• Valence history (origins and development of the term valence from 19th century to the present; inspired by chemistry-history books and consistently checked on original articles.)
• Current use of “valence” in textbooks (40 textbooks of general, organic, inorganic, physical, and materials chemistry were searched for valence definition and usage context. Referenced with copies of relevant paragraphs.)
• Statistics of valence-related terms in textbooks (33 chemistry textbooks were searched for terms: Covalent(ly), valence electron(s), VSEPR, valence bond, covalency, n-valent (specific n), valence shell(s) not referring to VSEPR, valence energy or level(s), hypervalence and hypervalent, mixed valence and mixed valent, valence orbital, low-valent, high-valent, covalence.)

2. Valence-related terms have three levels of quantitative connotation. A glossary is being compiled that includes the following terms:
• Valence terms of quantitative connotation but no numerical value (hypervalence or hypervalent, hypovalence or hypovalent, polyvalence or polyvalent, aliovalent vs isovalent, heterovalent, sub-valent, semi-valent, expandable valence, saturated valence, intervalence)
• Nouns with “valence” that are countable (valence orbitals, valence electrons, valence pairs)
• Composed terms with “valence” that have a numerical value (mixed valence or mixed valent, bond valence, electrovalence)

3. Results of the 2016 anonymous survey among 28 IUPAC colleagues about valence in about 20 chemical examples convert into respondents’ valence-definition preferences.
Based on these, the task group is testing 8 possible valence definitions against chemical examples.


July 2020 update – The 8 alternative valence definitions are being applied to 37 chemical examples covering mainly binary compounds of various bonding types across the periodic table, including cases with electrons shared versus electron pairs donated. Several visual representations of bonding are suited to derive numerical valences; certainly Lewis formulas for molecules, even if at times coming in alternative forms that include, but are not necessarily limited to, resonance formulas. For extended structures, nearest-neighbor bonding approximations of bond graphs help with more ionic compounds, while the formalism of the 18-plet electron-counting rules suits networks of transition-metal compounds. Task-group members are checking the clarity of arguments as well as the correctness and mutual relations of the numerical values obtained. The merits of the individual valence definitions are not being discussed yet. By now, all active group members have checked 25 of those 37 chemical examples.


April 2021 update – The task group has completed evaluation of nine valence-related quantities in each of 47 sets for 38 chemical entities and several types of their formulas or bonding schemes. These nine alternative quantity values for each and all atoms forming the tested molecule, ion, or formula unit are sensible numbers with specific meaning and mutual relationships that might be useful to formulate and summarize.
The task group analyzes the systematic relationships, correlations and connections among the nine tested valence-related quantities; both as such and versus several types of chemical formulas or bonding schemes on which we evaluated them. Then it will test, in an objective and independent manner, the perception how well these quantities obtained by the possible definitions cover the current use of the term valence in English.

Dec 2021 update – Having derived valence values for 47 telling chemical formulas by 9 alternative valence definitions, the task group focused on “discoveries” how these values are related. Eight general relationships emerged between the alternative valence values and specific groups of formula types or compounds. The simplest of these 8 relationships states for which atoms in which compounds all 9 alternative definitions give the same valence value. Since reflection is a very important principle of the IUPAC normative work, we analyzed how these 9 alternative definitions reflect the current relatively modest use of the term valence in English. For each of 15 examples with high variation of the alternative valences, we asked: Do chemists say that an atom in this compound is m-valent or n-valent or o-valent? The answer was obtained with Google-Scholar (reads the entire text) searching for articles in which we then analyzed the context of the given numerical adjective x-valent and its variants in Greek/Latin, as searched at various geographical locations. The result might be surprisingly simple and general, but we all must first go through the statistics of the answers to produce an agreed and reliable conclusion.

In the next few months, the task group will go through and check the occurrence statistics of the alternative answers to obtain a reliable conclusion. Then the group will draft a Technical Report, in which they will describe how the long and important history of the term valence shaped its current use as a quantity, recapitulating roughly the progress line of this project, presenting the quantitative results and conclusion.

Page last update 15 Dec 2021