Abstract
A lively philosophical debate has lately arisen over the nature of elementhood in chemistry. Two different senses in which the technical term ELEMENT is currently in use by chemists have been identified, leaving chemistry open to the logical fallacy of equivocation. This paper introduces a third, more elemental candidate: the high-enthalpy short-lived unbonded atom. An enthalpy index based on free-atoms-as-elements is established, whereby one can monitor the degree to which an atom’s spin-based attractive force is implemented exo-enthalpically when the atom binds chemically to others. Enthalpy indexing shows that the strength of an atom’s attractive force is proportional to its spin angular momentum. Vibrational spectroscopy shows that the force varies inversely as the fourth power of the inter-atom distance. Both features differentiate the chemical force from the stronger electromagnetic force and from the weaker Van der Waals force.
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Notes
Short-lived under standardized thermodynamic conditions. The enthalpy, entropy, and stability of chemical entities vary widely with temperature and pressure. All entities cited in this article are deemed to be at the standard or benchmark thermodynamic conditions of T = 25o C (298 K) and P = 1 atmosphere (101.3 kPa) as signified by the superscript o in ΔH o and S o parameters. This allows all entities to be compared on a level playing field.
From the Greek homos, like; meros, part.
Thermodynamic data, normalized to 298 K and 101.3 kPa, are taken from Wagman et al. The NBS Tables of Chemical Thermodynamic Properties. J. Phys. Chem. Ref. Data, 11, Supplement 2, (1982). Most of the data is more conveniently available in the CRC Handbook of Chemistry and Physics.
The asterisk is widely used by chemists to designate a short-lived high-enthalpy reactive intermediate.
At 298 K and 101.3 kPa the unbonded inter-particle distance is 3,500 pm.
1 Avogadro Unit = 6.02 × 1023 atoms. The SI mole unit of chemical amount identifies 6.02 × 1023 molecules; regardless of part-count. This new unit enables the normalization of thermodynamic values across molecules with different part-counts. It is Avogadro Units and not mole units that are conserved in chemical reactions. Hence the requirement that chemical reactions be balanced.
Most chemists today believe that chemical bonding is electrical in origin, arising from electrical interactions between the components of electrically-neutral atoms when atoms are in close proximity. Nobody doubts that the electromagnetic force binds electrons exo-enthalpically to an atom in numbers sufficient to achieve charge neutrality (Fig. 5). But where is the experimental evidence indicating this same force extends beyond charge neutrality, supporting the claim that chemical bonding arises from electrical interactions? The Heitler-London calculation showing that chemical bonding arises from interactions between the components of electrically-neutral atoms (e.g. overlap integrals, exchange interactions) is teleological in that it always starts with the atoms sufficiently close that their components must interact. But what force brings the atoms close enough that their components can interact? Spin angular momentum is a quantifiable attribute of widely separated electrically-neutral atoms and experimental evidence shows that the strength of the chemical force correlates with the free atom’s spin (Fig. 2). The Morse (1929) experiments indicate a 1/r4 separation dependence for the chemical force (Fig. 4). Neither piece of evidence is consistent with a charge-based force. It is at least plausible that a spin-dependent force of attraction brings the atoms close enough that they can neutralize their paramagnetic spins through spin pairing, in exactly the same way the electromagnetic force brings oppositely-charged particles together in the atom so they can neutralize their charges. Since the force generating the experimentally-observed Morse potential does not go as 1/r2, and since at large distances the atoms attracting each other have zero charge, I do not find the charge explanation for bond formation very persuasive. Eyring et al. (1944) and Feynman et al. (1964) didn’t find it very persuasive either. Does the charge-based theory allow one to predict that the bonding in Ca(s) will be twice as strong as that in K(s), as does the spin-based theory? Does the charge-based theory allow one to predict the strength of bonding in metals at all?
In the rare earths, spin–orbit coupling is so strong that spin S is no longer a good quantum number.
Because xenon does not form Xe–Xe bonds it should not be classed with the atomic elements, all of which are homonuclear bonded when manifest macroscopically. Short of ionization, Xe(g) molecules do have vacant 5d and 6s orbitals into which electrons can be oxidatively excited when they are reacted with a sufficiently strong oxidizing agent. Both fluorine and oxygen are strong enough, and xenon will form Xe–F bonds and Xe–O bonds under these conditions.
Technical sources (e.g. the CRC Handbook of Chemistry and Physics) and non-technical sources (e.g. the Oxford English Reference Dictionary) are agreed in defining the word ATOM as “the smallest particle of an element”. Yet when “of an element” refers to N2 or O2 or Cl2 or I2 the smallest particle to reflect the properties of these materials is not an atom but a molecule, highlighting once again the ambiguity infecting the technical term “element”.
The Van der Waals radius is, on average, 60% greater than the covalent radius because the chemical force pressing atom to atom is stronger than the Van der Waals force pressing molecule to molecule.
References
Dalton J.: A New System of Chemical Philosophy, plate 4, p. 219. Bickerstaff, Manchester (1808). Through mistaking molecular elements for atomic elements, 9 of Dalton’s 19 ‘atomic’ weights were in error by at least 80%; thereby creating huge problems for analytical chemists trying to establish empirical formulas in the 19th century
Eyring H., et al.: Quantum Chemistry, p. 214. Wiley, (1944)
Feynman R., et al.: The Feynman Lectures on Physics, vol. II, pp. 37-1 and 37-2. Addison-Wesley, Reading (1964)
Gaydon A.G.: Dissociation Energies and Spectra of Diatomic Molecules. Chapman & Hall, London (1968)
Harré, R.: Chemical kinds and essences revisited. Found. Chem. 7, 7–30 (2005)
Hendry, R.F.: Lavoisier and Mendeleev on the elements. Found. Chem. 7, 31–48 (2005)
Lavoisier A.: Traité élémentaire de Chimie, présenté dans un ordre nouveau et d’aprés les découvertes modernes [Elementary Treatise on Chemistry], pp. 5–6, Cuchet, Paris (1789). The Kerr translation of 1790 is unreliable. Lavoisier’s French “molécule”, used 36 times in Chapter 1, becomes either “atom” or “particle” throughout; the English molecule never appears
Lewis, G.N., Randall, M.: Thermodynamics and the Free Energy of Chemical Substances, pp. 596–608. McGraw-Hill, New York (1923)
Morse, P.M.: Diatomic molecules according to the wave mechanics. II. Vibrational levels. Phys Rev 34, 57–64 (1929)
Paneth, F.: The epistemological status of the chemical concept of element. Br. J. Philos. Sci. 13, 1–14, 144–160 (1962). [reprinted in Found. Chem. 5, 113–145, (2003)]
Parsegian, V.A.: Van der Waals Forces. Cambridge University Press, Cambridge (2006)
Scerri, E.R.: Some aspects of the metaphysics of chemistry and the nature of the elements. HYLE 11, 127–145 (2005)
Sharlow, M.F.: Chemical elements and the problem of universals. Found. Chem. 8, 225–242 (2006)
Wagman, D.D., et al.: The NBS tables of chemical thermodynamic properties. J. Phys. Chem. Ref. Data. 11(Suppl 2), 1–392 (1982)
Wallerstein, G., et al.: Synthesis of the elements in stars: forty years of progress. Rev Mod Phys 69, 995–1084 (1999)
Acknowledgments
The author wishes to thank Reviewer #2 for suggesting a more focused treatment of the chemical elements and for pointing out several errors of fact in an earlier draft.
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Kidd, R.G. Elements of the third kind and the spin-dependent chemical force. Found Chem 13, 109–119 (2011). https://doi.org/10.1007/s10698-010-9100-y
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DOI: https://doi.org/10.1007/s10698-010-9100-y