• The physical construction of the The physical construction of the chemical atom: Is there a way chemical atom: Is there a way

back?back?

Mercè Izquierdo, UAB merce.izquierdo@uab.cat

Agustin Aduriz-Bravo, UBA

Science Science needs didactics of scienceneeds didactics of science to to survive survive

Science without the Science without the history of sciencehistory of science does not imagine the futuredoes not imagine the future

Didactics of science Didactics of science looks at the futurelooks at the future and needs a and needs a science with historyscience with history

Science teaching and HS

HS STS

Resources

Histories that help learningHistories that help learning

Sciencemodel

In order to identify epistemological obstaclesin the classroom

There have been many contexts of scientific activity...

According to the audiences, to economy,to politics...

We will deal now with teachers’ chemistry.

Disagreeing? How do representations (formulas, tables) and experiments interact?

We have studied university textbooks from the first four decades of the 20th century

We have studied some of the books written by the creators of the physical atom (1900-1930)

We have studies the heuristic function of the periodic table in the construction of the physical atom

The construction of the chemical atom was very difficult:Dalton formulates his ideas with ‘maximum simplicity’Berzelius expresses the mass relations between elements in terms of atomsOne-, two- or four-volume formulas?

Mendeleev presents a system of chemistry based on elements… and we arrive at the end of the century with a very developed

chemistry, without a physical atom

The birth of physical chemistry

• Cannizzaro uses physical data to calculate atomic mass and to organise organic chemistry

• Physical and chemical properties are related Mendeleev did it.

• New instruments are constructed: spectroscopes, polarimeters… new amazing phenomena appear!

Radiations

Particles

Energy, electromagnetism

Chemistry

A diversity of styles of doing science: pragmatism, in the UK, the ‘look and see method’ criticised by Duhem (France)

Cambridge: Maxwell, J.J. Thomson (1856-1940)

Manchester: E.Rutherford (1871-1937), Moseley (1887-1915)

Munich: A. Sommerfeld (1868- 1951),Heisenberg (1901-1976)

Zürich: E.Schrödinger (1887-1961)

Copenhagen: N. Bohr (1885-1928)

Vienna: Boltzmann (1844-1906)

Paris: M. Curie(1867- 1934),J. Perrin (1870-1942)

New questions… new evidences

• What is ether? Is electricity matter or movement? A new science of electricity is emerging: electromagnetism, new ideas about energy

• Atoms are considered ‘old’ (Mach, Duhem, Le Châtelier, Urbain)… but young people (Perrin, Langevin, Curie…) are enthusiastic about particles!

• Perrin’s book, Les atomes,(1913) is a landmark.

Idealism – positivism - conventionalismpragmatism…

A physical atom starts to be developed

Textbooks begin to change

Chemists: new ideas about electricity ‘atoms of electricity’ (Helmholz), ions

(Arrhenius) Physicists start to build the atom

Zeeman (1896) calculates e/m (1897)J.J. Thomson compares this relation with that in electrolysis: the hydrogen-atom mass has to

be 1000 larger than the electron mass

J.J. Thomson(1856-1940)Atom representations – scale models

He proposes a positive shell with electrons inside, in concentrical spheres

A lot of radiations

• Mme. Curie identifies radioactive elements (1896-1903)

• Rutherford, 1898- 1907, relates Helium with alfa particles

• Mendeleev rejects these new ideas about (chemical) atoms

• Is it that atoms have grown an “incurable suicidal mania”? (Henry E. Armstrong, chemist)

• It is difficult to explain atoms’ stability, but it is needed… because it has to be also a chemical atom

The periodic table, a ‘heuristic’

• H. Moseley (1887-1915), with Rutherford (Manchester), identifies an ‘atomic number’. What did it mean?

• Thomson’s proposal (a mechanical model for the atom) is accepted both by chemists and physicists, because it tries to explain periodicity

A new model… among others

• Rutherford: nuclear atom (1911)

• Bohr chooses to work with him due his interest in the Periodic Table

• Bohr-Sommerfeld’s physical model represents chemical atom (anschaulich)

• Textbooks start to speak about n, m, l (after the First World War, the 1920s), and evidences about physical atoms begin to become accepted

The wars: Heisenberg, Schrodinger… and the chemists

• Bohr, Heisenberg (1922): The language of quantum mechanics should be use as in poetry… elicit images in conscience and establish symbolic links. Could we some time understand the structure of atoms?

• Is it necessary to substitute mechanical models with mathematical models?

• De Broglie invents a nice theory about the electron (doctoral thesis, 1923), and Schrödinger develops it (1926)

• Bohr looks for correspondences between classical and quantum mechanics… complementarity

Confrontation

• Indeterminacy/uncertainty in the object or in the way of knowing it?

• Heisenberg considers ‘revolting’ the wave equation• Bohr & Heisenberg do not agree with Einstein during

the rest of their lives. They consider that knowledge refers to a real uncertainty in the nature of things.

• Chemists go on doing chemistry…

C C

HH

H

H

H

H

C C

HH

H

HH

H1919 Langmuir, from Lewis’s ideas, establishes covalent and electrovalent bonding

1903 J.J. Thomson’s ideas: internal polarities created by ‘force-tubes’

Valence, kinetics, bonds… Chemistry was giving criteria

Pauling, Professor

• General Chemistry (1947): new textbook, chemistry is reconstructed, there is finally a chemical and a physical atom

• A new way of considering the Periodical Table, interpreting periodicity with electronic configurations

Linus Pauling explaning C tetra-valence: Chemistry has an atom, students can understand chemistry

What is ‘understanding’?

• ‘Having rules well related to experimental data

• Having a mathematical structure well related to data

• Having a ‘microstructure’ (batygen explanation)…

but it is necessary to know what is done and what the question is

Today, our students talk about the electronic structure of atoms, Lewis structures of compunds… but what is the meaning of all this for them?

What kind of evidences about atoms, in chemical phenomena?

• Now, we cannot study chemistry without the PT… but electronic configurations is what is usually studies!

•With this, do we teach how to DO chemistry??

Conclusions:Textbooks nowadays do not differentiate the physical and the chemical atoms… Teh physical atom is given priority… but this would not have existed without chemistry

But if there is no more chemistry, we will lose for sure this amazing adventure on how the physical atom was constructed from a chemical atom

From Mendeleev to Pauling (two greatteachers)

From a system of chemistry… to a system of electrons

But current students cannot walkthe way back