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.

Page last update 27 July 2020