A Unitary Model for Atomic Structure
| Scientific Paper | |
|---|---|
| Title | A Unitary Model for Atomic Structure |
| Read in full | Link to paper |
| Author(s) | Sithamalli K Balasubramanian |
| Keywords | Atomic Structure, nuclear geometry, chemical bonding, chirality |
| Published | 2010 |
| No. of pages | 27 |
Read the full paper here
Abstract
In the unitary model the atomic nucleus plays a decisive role in atomic structure and atomic interactions including chemical bonding. On the assumption that nuclear structure determines and dictates atomic structure and molecular geometry, structures for Carbon, Silicon, Boron and Chlorine atoms are proposed that explain their covalent bonding patterns. Arguments are presented against the current interpretation of "atomic number".
Overview
Balasubramanian, a chemist writing from Pune, attacks quantum chemistry from the one direction its practitioners rarely defend: the bench chemistry of ordinary molecules. The paper's target is what he calls the "bicameral" model of the atom — the assumption that an atom has two independent parts, a nucleus that is no more than a point charge and a cloud of extranuclear electrons arranged by the aufbau principle. Against this he proposes a unitary model in which the nucleus is a geometrical solid whose corners and faces are the bonding sites, so that "nuclear geometry dictates and determines molecular or atomic geometry."
The consequence he draws is deliberately provocative. If bonding happens at nuclear surfaces, there is no need for a fixed complement of electrons permanently resident around the nucleus; charges instead "rise from and subside into" the nucleus according to context. The atomic number then "gives the position of the element in the periodic table without any implication as to the number of electrons in the ground state of the atom." The document comes in two parts: a short technical article proposing polyhedral structures for carbon, silicon, boron and chlorine, and a longer appendix — marked by the author as "not for publication" — titled Critique of Quantum Mechanics in Chemistry, which supplies the negative case in eight numbered heads.
The unitary model
Premises
Two assumptions do all the work. First, following Linus Pauling's 1965 Science paper, nuclei are built from nucleons of uniform size that "may have masses of one, two or three" — the heavier ones being dineutrons and trineutrons. Second, nuclear geometry determines molecular geometry, which is what makes the model "unitary": nuclear models are atomic models. Bonding sites are classified into primary b-surfaces (drawn red in the paper's figures) and secondary quiescent or q-surfaces (yellow), with b–b bonds most stable, followed by b–q and q–q. Periodicity is explained not by electron shells but by structural similarity — "a common scaffold" — so that carbon and silicon share tetrahedral bonding sites despite different scaffolds, and fluorine, chlorine and even manganese are said to be built on the same scaffold.
Inert gases are held to have no extranuclear electrons at all and no sites where charge can rise and subside. Elements heavier than helium are proposed as composites of helium nuclei and further inert units — an octahedron of mass number 6, an icosahedron of mass number 12 — chemically inert because each nucleon is bound to four or more others. Thus 20Ne is five 4He, and 22Ne is four 4He plus an octahedron.
Carbon, silicon, boron, chlorine
12C is built on an octahedral core with six further nucleons added at triangular sites. The model then permits exactly three ways for carbon to form four single bonds: the ordinary tetrahedral pattern; a square pattern of two b- and two q-surfaces, which the author assigns to carbon monoxide and to an iron bare-carbon complex; and a pattern of three b-surfaces with a q-surface at the centre of a tetrahedral face, which he assigns to propellane. Two distinct double bonds are allowed — the ordinary edge-sharing "van 't Hoff" double bond and a higher-energy tetrahedral one identified with cyclopropene and with Zeise's salt — and two triple bonds, a face-sharing linear one and a b-b, b-b, q-q variety assigned to the benzyne intermediate and made responsible for aromatic stabilisation. The new triple bond is predicted to show cis–trans isomerism.
For 28Si an icosahedral core with sixteen added nucleons yields two structures chiral to one another, which the author uses to explain the solid-state chirality of quartz — dismissing the crystallographic symmetry account as "merely a description of the phenomenon and not an explanation." Because these chiral forms would equilibrate rapidly in solution by bond shifting, tetrasubstituted silicon derivatives SiR4 should show chirality only in the solid state. Structural isomerism within a single isotope is a new prediction, for which he coins the name siomerism. He also notes that decamethylsilicocene exists in the solid state as linear and angular forms in a 1:2 ratio and claims the model reproduces that ratio.
11B is "a truncated Carbon without the tetrahedral apex," giving four pentacovalent bonding patterns and hence the interlinked icosahedra of crystalline boron; diborane B2H6 is drawn with a B=B (q–q) double bond and hydrogens on the red surfaces. Chlorine is built on the fluorine scaffold, with a quadruple bond to oxygen in the chlorate anion and a described bond-switching mechanism, pivoting on a common site as fulcrum, that equilibrates the chiral forms of sodium chlorate without breaking bonds.
The case against the aufbau atom
The empirical core of the argument is a list of compounds whose physical behaviour Balasubramanian says is inconsistent with large numbers of resident non-bonded electrons. Uranium hexafluoride, "the heaviest binary molecule," is a gas above 55 °C although the aufbau atom requires 86 non-bonded electrons on the central atom. BF3, CF4, SiF4 and GeF4 are gases with sub-zero boiling points; the hexafluorides of sulfur, selenium and tellurium are gases and dielectrics, which he reads as showing that "electron effects are totally and measurably absent"; nickel tetracarbonyl is a gas above 30 °C. Phosphorus and arsenic trifluorides, whose parent elements are electron donors, act as acceptor ligands, and the pentafluorides are Lewis acids — a reversal he takes as evidence of a large change in electron count on fluoride formation.
He also disputes the interpretation of Moseley's X-ray data. The Bohr model predicted Z2 proportional to characteristic frequency; the straight line was obtained instead from (Z−1)2, explained before 1980 by an electron dropping into the nucleus, and after 1980 by plotting Z against the square root of frequency. This he calls "obfuscation of an excellent experimental result": the change from Z2 to Z "is in effect a compression of the ordinate," and "any set of results may be made to give a straight-line graph by suitable compression of the ordinates." Moseley's law, on his reading, correlates X-ray frequency with mass-number ordering, leaving the identification of atomic number with nuclear charge "open to question."
The critique of quantum chemistry
The appendix runs through eight complaints. Chirality is the first and, he says, the most important: citing Woolley's 1978 paper and a 1984 discussion, he takes it that quantum mechanics does not permit an isolated molecule to be chiral, so "the most fundamental phenomenon of Chemistry is not accountable on the QM." Hybridisation he calls "equating inequalities," since 2s and 2p orbitals differ in energy and orientation. Beryllium versus argon is offered as an internal inconsistency: beryllium has closed shells yet reacts, explained by a vacant 2p shell, while argon with a vacant 3d shell is inert; and the assignment of 4s below 3d in potassium he calls "an assumption treated as an explanation." Orbital shapes beyond p are "bizarre mathematical constructs" because space has only three dimensions.
Three bonding devices are singled out as ad hoc — hybridisation, the σ-donation/π-back-donation bond of Zeise's salt, and the two-electron three-centre bond of diborane — with the σ/π scheme accused of demanding an energy gain around a closed A→B→A path, which "should make perpetual motion close to realization." Propellane's inverted axial bond he treats as an outright breakdown rather than a patchable case. Oxygen's paramagnetism requires, on his account, a chiral plane and therefore atomic-level structure in oxygen, which he extends to the antiferromagnetism of FeO and MnO and to a two-chiral-halves model of 56Fe explaining the two magnetic poles. Electric current is reinterpreted as a helical charge-carrying vector field rather than moving electrons, on the grounds that current speed is independent of voltage and that current penetrates only a few atoms below a conductor's surface. Superfluid helium is explained as atoms that have simply "settled under gravity" with no inter-atomic attraction, with the creeping film a monatomic layer siphoning upward; superconductivity and the Meissner effect become complete charge subsidence. Cold Fusion — specifically Taleyarkhan's sonofusion — is explained by charge subsidence converting deuterium atoms into dineutrons, sidestepping the Coulomb barrier, which he calls "an over-rated obstruction for fusion." Finally he invokes Cushing's account of the Copenhagen interpretation as a "social contingency."
Assessment
The paper's real strength is that it argues from chemistry rather than at it. The observations it assembles are genuine and the questions are sharp ones that quantum-chemistry textbooks tend to answer by naming a mechanism rather than deriving it: hybridisation, the three-centre bond and the σ-donation/π-back-donation scheme really are introduced case by case, and the inverted bond of [1.1.1]propellane really did surprise the theory. Insisting that a chemical model must be a structure — that "an unstructured or an abstract mathematical formula has no relevance in Chemistry" — is a defensible methodological stance, and the Moseley criticism is technically correct as a point about graph presentation: fitting Z against √ν is indeed a rescaled version of the same relation, and presenting the straight line as independent proof is circular.
But the difficulties are severe, and most of them are of the same kind. The polyhedral structures are asserted, not derived: no interaction, potential or force law is given that would make an octahedral 12C core stable, select the b- and q-surfaces, or predict a bond energy or bond length. The model reproduces known geometries by construction — carbon is given a tetrahedral pattern because carbon is tetrahedral — and so cannot be tested by them. Where quantitative predictions could be made, none are: nothing in the paper computes an ionisation energy, a spectral line, a bond angle or a heat of formation, all of which quantum chemistry now produces to within experimental accuracy.
Several arguments rest on inferences that do not follow. That UF6 is a gas at 55 °C bears on intermolecular forces, not on electron count: volatility tracks molecular symmetry and weak dispersion between closed-shell fluorine surfaces, and SF6 being a good dielectric shows that its electrons are tightly bound, not that they are absent. The claim that inert gases have no extranuclear electrons is directly contradicted by measurement — helium's two-electron photoelectron and X-ray spectra, its measured first ionisation energy of 24.59 eV, and the existence of He+ and He2+ ions — and the paper concedes only that "the spectra are observed under high-energy excitation." Discarding the identification of atomic number with nuclear charge leaves unexplained the Rutherford scattering cross-section, which measures Ze directly, and the ionisation series of the elements. The chirality argument depends on reading Woolley's well-known observation about the Born–Oppenheimer origin of molecular structure as a claim that quantum mechanics forbids chirality, which is not what that literature says; chiral molecules are routinely computed, and the tunnelling-splitting timescale for heavy-atom inversion is calculable and enormous. Superfluid helium is not adequately described as atoms settled under gravity: the lambda transition is a measured, sharp specific-heat anomaly, and He-II exhibits quantised vortices and second sound, neither of which follows from a gravitational picture.
The internal consistency also wavers. Charges are said to "rise and subside" — mediated, the author speculates, by the weak force — yet the process is explicitly distinguished from beta decay and is admitted to be of unknown nature: "we are also not sure if electrons are involved in the process." A mechanism this undefined cannot both carry the explanatory load of variable electron count and be exempt from the energetics that would make it detectable. Similarly, the sonofusion explanation converts deuterium into dineutrons at will, but a free dineutron is not a bound system, and the specific claim about Taleyarkhan's results has since been undercut by the failure of that work to replicate.
Balasubramanian is right that these are chemistry's questions and that they deserve an answer in chemistry's terms. He asks explicitly for "a wider debate." What the paper offers, however, is a descriptive geometry with the same fitted quality it objects to in hybridisation, and without the compensating power to calculate.