Recent debates regarding the concept of 'element' - Eric Scerri by hcj


									Structural realism and essentialism
            in chemistry

 A Lecture given at the International Society for the
              Philosophy of Chemistry.

                   Eric Scerri
             Department of Chemistry

             Structural Realism
• An intermediate position between entity realism and

• What survives across theory change is mathematical
  structure not scientific entities

• Originally due to Henri Poincaré, more recently
  discussed by John Worrall.

• More recently, Every Thing Must Go, Ladyman and
           Advantages of S.R.
• Avoids the criticism against realism that many
  entities (phlogiston, ether) have turned out to
  be non-referring.

• Avoids going as far as anti-realism that denies
  any form of realism.

• SR recommends that we should be realists
  about just ‘structure’
              SR in chemistry?

• So far SR has been discussed only in physics. Many
  examples of mathematical structures that persist
  after theory change.

• What about chemistry, the next most mathematical
  The Periodic Table as ‘structure’
           in chemistry
• May not be a manifestly mathematical structure but
  ‘structure’ nevertheless. It’s literally a structure!
• Although there is also a mathematical structure
  underlying the PT (group theory, information theory,
  similarity studies etc.)
• It is the structural relationship between the elements,
  embodied in the periodic table, that has survived
  major scientific revolutions like QM and relativity
      Rule of 8 - Octet Rule as an
        element of ‘structure’
One structural aspect of the P.T. that goes beyond entities and
  even theories.

• Mendeleev and Newlands 1860s (rule of 8, law of octaves)
  Kossell and Abegg (rule of 8)
  Lewis and Langmuir (octet rule)
  Bohr, Schrödinger etc. atomic shell structure ( ns2np6 )

• The rule of 8 precedes any entities such as electrons.
  It was discovered before the electron.
  It survives to this day. Admittedly, there are exceptions to
  the octet rule.
• Concentrating on structural aspects such as
  the rule of 8, in the periodic table and
  chemical bonding, is consistent with the
  appeal to elements as basic substances
  rather than as concrete entities, such as
  atoms or bonded atoms.

• Basic substances operate at an abstract level,
  as does mathematical structure. Neither
  involves entities.
       Aspects of the octet rule
• The octet rule is of course broken by many
  compounds but this does not diminish it’s
  importance as a form of ‘magic number’ in

• One might object by saying that many periods
  in the modern periodic table consist of more
  than 8 elements. This is perfectly correct and
  yet all the noble gases, except He, have eight
  outer electrons
             18 and 32-electron rules
•   If one insists on the fact that other periods contain 18 or 32 elements one
    may appeal to other magic numbers in chemistry, namely 18 and 32 and
    the associated 18-electron and 32-electron rules which are very useful in
    rationalizing transition metal and f block compounds respectively.

•   The fact remains that the periodic table, and the chemistry of the
    elements is governed by a few simple magic numbers, namely 2, 8, 18 and
• Essentialism
Now out of favor in biological classification
Most authors think it still has a place in chemistry,

      • Mendeleev’s periodic table if often cited as a model for
        essentialism. All and only members of a particular element share a
        common essence – their unique and common atomic structure. And
        knowledge of that structure enables us to predict and explain the
        behavior of instances of that element (Ereshefsky, 2007. p. 1)

• Of course there are problems.
• It works for elements, but as soon as we move to
  molecules Z fails as a criterion for identification.

• Structure must be added to distinguish
•     e.g. CH3OCH3 from CH3CH2OH
• Also what Ereshefsky says is not strictly true when applied to
  classification of elements.

• Atomic structure gives primary classification or ordering of
  elements but is sometimes ambiguous on the question of secondary
  classification or placement into groups.

• Consider H and He

• (a) For H we usually consider the number of electrons present
      – one. So we place H in group 1.
• (b) For He we usually consider the number of electrons missing to
      obtain a full shell (zero). So we place He in the noble gases.

• If we applied (a) to He we would place it in group 2
• If we applied (b) to H we would place it in group 17 (halogens)
  Why the inconsistent criterion for group membership?
What really matters in the debate about ‘element’ is to
 clearly distinguish between

• 1. Combined element,
• 2. Element as simple substance,
• 3. Element as basic substance.

• Structural realism about the P.T. is consistent with
  recognizing the importance of elements as ‘basic
  substances’ abstractly not as entities. It is just the
  rule of 8 that survives across theory changes.
Chemical essentialism via atomic number,
and/or atomic structure, is decisive for
ordering the elements but not for putting
them into groups (classification in literal

The nature of elements as basic
substances is inferred from the nature of
the other two senses of ‘element’
(combined elements and elements as
simple substances) maybe in equal
measure. To be explored further.

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