1. O n t h e T r a c k o f M o d e r n P h y s i c s „Tria àtoma” is shouting the ski-lift operator in Taigetos, snowy mountains across the Peloponesus, near Sparta, when the next wagon is approaching. The word „a-tomos” has, todays, in Greek, the same meaning as in Democritus times Ταύγετος - view from Sparta Democritus (Abdera, ca. 470-370 BC). Atoms for Democritus had two properties: size and shape. 1. Atoms connect each other in different orders (stoichometry and structural formula?) and create different substances. 2. Atoms are in permanent motion and collide each other (like in Boltzmann’s perfect gas model?), their motion decides about their mass (the mass of proton results not from gluons’ rest mass but from their motion, thanks to Einstein’s m=E/c 2 3. Atoms emit fluids (photons?) which can be observed Can we agree with Democritus now, in XXI century? Without any doubt, Atom in Physics is not undividable: electrons around the nucleus, and protons and neutrons inside the nucleus, and mesons binding hadrons, and quarks and gluons inside both mesons hadrons, and W and Z bosons showing-up from time to time, and neutrino, etc. But „a-tomos” means in English „in-dividual” = un- divisible – a thing in itself – a person. In this sense, in spite of the fact, that a body consists of legs, arms, chemical elements, proteins, water and iron, a „person” remains undividable. Like Democritus’ A-tomos. 1. Quite precisely atoms can be „counted” by X-rays: they are diffracted on crystal planes, like the light on a CD disk. Closer are placed the atoms – more distant are light spots on the screen. 2. There are others ways of counting atoms (i.e. calculation of the Avogadro or Loschmidt number). For example from the sedimentation rate in liquid (J. Perrin) or from Brown’s motion (A. Einstein). 3. Today is possible to count on „fingers” leading it on the crystal surface. But the finger must be quite small and precise: the best is a sharp tip of the tungsten needle – when it touches an atom (or better: approaching it, then needle gets slightly attracted. We call it Atomic Force Microsopy. Perspective color view of graphite surface. http://www.physics.leidenuniv.nl/sections/cm/ip/group/Principle_of_SPM.htm LORENZO ROMANO AMEDEO CARLO AVOGADRO Charicature-donated by William Jensen, University of Cincinnati http://www.woodrow.org/teachers/chemistry/institutes/1992/AVOGADRO.GIF Ringrazio Mario Fedrizzi per suggerimento di questo tema „Dimensions” of atoms can be obtained in many ways but very different answers 1. In gases, the atomic "dimension" is the distance, at which they start to push each other in a rather brutal way – their diameters are estimated with deviations from the perfect-gas equations, when it is impossible to pressurize more the gas and it condensates. Like the dew in the morning. 2. In liquids, dimensions of the particles influence the “neighborhood” effects, like the viscosity. Huge molecules, like this polymer, never stop sipping. 3. In crystals, atomic diameters are defined as dimensions of elementary cells, visible by the X-rays or electron-beam diffraction. Atomic diameters could be also evaluated from the density – if we knew how many atoms were contained in one portion, i.e. in 1 mole of the substance. But we need to know the advocate's Avogadro number, what is also difficult How to measure atoms? Did you ever play with a drop of oil on the surface of the broth? Trying to stretch it to a bigger pond? In 1926, Jean Baptiste Perrin got a Nobel prize for this - he measured atom dimensions. How to count atoms? The best way is on your fingers. But you need a very small finger! Carbon Monoxide Man on Platinum (111) http://www.almaden.ibm.com/vis/stm/images/stm5.jpg 2/1