Question: Who discovered a better understanding of valence bonding?
About the article "Chemistry", section "Chemistry as science".

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Walter Heinrich Heitler (; 2 January 1904 – 15 November 1981) was a German physicist who made contributions to quantum electrodynamics and quantum field theory.
He brought chemistry under quantum mechanics through his theory of valence bonding. 
Resonant bonding.
Hypervalent bonding.
In hypervalent molecules, there exists bonds which have significant non-bonding ionic quality to them.
This manifests as non-bonding orbital levels in molecular orbital theory, while in valence bond theory it is analyzed as a form of resonant bonding. 
If the structure is not known, the average bond valence, Sa can be calculated from the atomic valence, V, if the coordination number, N, of the atom is known using Eq.
3. formula_6 (Eq.
3) If the coordination number is not known, a typical coordination number for the atom can be used instead.
Some atoms, such as sulfur(VI), are only found with one coordination number with oxygen, in this case 4, but others, such as sodium, are found with a range of coordination numbers, though most lie close to the average, which for sodium is 6.2.
In the absence of any better information, the average coordination number observed with oxygen is a convenient approximation, and when this number is used in Eq.
3, the resulting average bond valence is known as the "bonding strength" of the atom.
Since the bonding strength of an atom is the valence expected for a bond formed by that atom, it follows that the most stable bonds will be formed between atoms with the same bonding strengths.
In practice some tolerance is allowed, but bonds are rarely formed if the ratio of the bonding strengths of the two atoms exceeds two, a condition expressed by the inequality shown in Eq.
4.
This is known and the "valence matching rule".
formula_7 (Eq.
4) Atoms with non-bonding valence electrons, i.e., with lone pairs, have more flexibility in their bonding strength than those without lone pairs depending on whether the lone pairs are stereoactive or not.
If the lone pairs are not stereoactive, they are spread uniformly around the valence shell, if they are stereoactive they are concentrated in one portion of the coordination sphere preventing that portion from forming bonds.
This results in the atom having a smaller coordination number, hence a higher bonding strength, when the lone pair is stereoactive.
Ions with lone pairs have a greater ability to adapt their bonding strength to match that of the counter-ion.
The lone pairs become stereoactive when the bonding strength of the counter-ion exceeds twice the bonding strength of the ion when its lone pairs are inactive.
Compounds that do not satisfy Eq.
4 are difficult, if not impossible, to prepare, and chemical reactions tend to favour the compounds that provide the best valence match.
For example, the aqueous solubility of a compound depends on whether its ions are better matched to water than they are to each other. 
Valence is a commune in the Charente department in southwestern France.
It was the birthplace of William de Valence, who later became Earl of Pembroke. 
Valence shell electron pair repulsion (VSEPR) rules are a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion.
It is also named Gillespie–Nyholm theory after its two main developers.
The acronym "VSEPR" is sometimes pronounced "vesper" for ease of pronunciation; however, the phonetic pronunciation, "vuh-"seh"-per", is technically more correct.
The premise of VSEPR is that the valence electron pairs surrounding an atom mutually repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecular geometry.
The number of atoms bonded to a central atom plus the number of lone pairs of its nonbonding valence electrons is called its steric number.VSEPR theory is usually compared with valence bond theory, which addresses molecular shape through orbitals that are energetically accessible for bonding.
Valence bond theory concerns itself with the formation of sigma and pi bonds.
Molecular orbital theory is another model for understanding how atoms and electrons are assembled into molecules and polyatomic ions.VSEPR theory has long been criticized for not being quantitative, and therefore limited to the generation of "crude", even though structurally accurate, molecular geometries of covalent molecules.
However, molecular mechanics force fields based on VSEPR have also been developed.