Portrait of an atom

by @ulaulaman about #hydrogen #atom #orbitals #Bohr #Rutherford #quantum_mechanics

The study of the structure of the atom is long story, and it begins with Democritus, or from the point of view of the modern science, with John Dalton in 1808: indeed he tried to fix in scientific terms the ideas of the greek philosopher and naturalist.
Dalton's theory was based on five fundamental points:

• matter is made of tiny building blocks called atoms, which are indivisible and indestructible;
• atoms of the same element are all equal to each other;
• the atoms of different elements combine with each other (via chemical reactions) in ratios of whole numbers and generally small, thus giving rise to compounds;
• atoms can be neither created nor destroyed;
• atoms of an element can not be converted into atoms of other elements.

As you can see, there are some other ideas correct and incorrect. We do, however, a jump of a century and we go to 1902 with Mr. Thomson, the first to propose an atomic model: he assumed that the atom was made up as a sort of cake, a positively charged sphere in which were scattered, like raisins, the electrons with a negative charge distribution such as to make the object as a whole neutral.
A few years later, however, in 1911, Rutherford devised and conducted an important experiment⁽¹⁾ in which he sent a beam of alpha particles against gold nuclei. The cross section observed, i.e. the surface on which the scattered particles resulting bump, it was too large to be compatible with the Thomson's hypothesis, but was compatible with that of Rutherford, namely that the atom was made up of a positive nucleus and by a number of electrons that revolved around the core itself at a large distance (compared to nuclear ones, of course).

However, this is not the last step: in 1913 Niels Bohr refined the Rutherford's model⁽²⁾. Accepted the planetary structure of the atom, Bohr suggested that the electrons in their rotational motion, could not occupy orbits at their leisure, but they must themselves at very specific distances from the nucleus: this is the dawn of quantum mechanics, that further refine the atomic model thanks to the famous Schroedinger's equation.

The atom, now, was intended as a postive nucleus consists of protons and neutrons, with a little cloud of electrons that moved around, and not on an orbital but in a sort of spherical cap. And these caps at different energy was recently directly observed by Aneta Stodolna's team^{(3, 4)}:

(...) an experimental method was proposed about thirty years ago, when it was suggested that experiments ought to be performed projecting low-energy photoelectrons resulting from the ionization of hydrogen atoms onto a position-sensitive two-dimensional detector placed perpendicularly to the static electric field, thereby allowing the experimental measurement of interference patterns directly reflecting the nodal structure of the quasibound atomic wave function.⁽³⁾

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via phys.org, io9

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(1) Rutherfor E. (1911). The scattering of α and β particles by matter and the structure of the atom, Philosophical Magazine Series 6, 21 (125) 669-688. DOI: 10.1080/14786440508637080
(2) Bohr N. (1913). On the constitution of atoms and molecules, Philosophical Magazine Series 6, 26 (151) 1-25. DOI: 10.1080/14786441308634955
(3) Stodolna, A., Rouzée, A., Lépine, F., Cohen, S., Robicheaux, F., Gijsbertsen, A., Jungmann, J., Bordas, C., & Vrakking, M. (2013). Hydrogen Atoms under Magnification: Direct Observation of the Nodal Structure of Stark States Physical Review Letters, 110 (21) DOI: 10.1103/PhysRevLett.110.213001
(4) Smeenk, C. (2013). A New Look at the Hydrogen Wave Function Physics, 6 (58) DOI: 10.1103/Physics.6.58