Atoms

1. Atoms • Leukippos, Demokritos (500 BC): • thought experiment leads to atom hypothesis: • all matter is made of tiny particles, too small to be seen; (Greek atomos = indivisible) • different shape and size of atoms  differences between materials. • note: Greek atomic picture formulated without experimental evidence (pure speculation). • modern concept of atom: • atom = the smallest unit of an element, the smallest unit of matter that has the chemical character of the element; • can exist alone or combined with other atoms to form a molecule. • modern “atomism” developed during late 18th/early 19th century (Dalton, Lavoisier,..): • chemistry identified many different substances; most substances can be decomposed into simpler substances (“chemical decomposition”) • simpler substances cannot be further decomposed (by chemical means): “chemical elements” • molecule = smallest unit of substance (compound) that still has properties of chemical compound; • atom = smallest unit of element

2. Atoms, cont’d • main findings during early (“chemical”) period: • atoms are neither created nor destroyed (in chemical reactions) • atoms of a given element are identical in character • atoms of different elements are different in character • chemical compounds are formed when atoms of different elements join together to make identical units • law of definite proportions: the different kinds of atoms in a compound are present in simple numerical ratios (1;1, 1:2, 2:3, 1:3,...). • law of multiple proportions: atoms of two or more elements combine in different ratios to produce more than one compound. • Avogadro's law: under identical conditions of temperature and pressure, equal volumes of gases of any kind contain the same number of molecules.

3. PERIODIC TABLE OF ELEMENTS • order elements by weight, find repetitive pattern of properties • arrange into columns such that elements with similar properties are in same column • periodic table, column = group, row = period • note: later, ordering by atomic • number rather than weight • developed (independently) by Lothar Meyer and Dimitri Mendeleev, based on work by Döbereiner and Newlands • regular pattern allowed prediction about as yet undiscovered elements

4. THERMALENERGY,HEAT,TEMPERATURE • observation of “Brownian motion” (1827): • small seeds (e.g. burlap) suspended in liquid show erratic motion (random motion'') • kinetic theory of heat: (Boltzmann, Maxwell,...) • heat is a form of energy; • internal energy = thermal energy of material bodies is related to random motions of molecules or atoms • temperature is a measure of this internal energy . • explanation of Brownian motion: Albert Einstein (1905): calculated speed of “diffusion” from kinetic theory of heat - found in agreement with experimental measurements strong support for atomic picture of matter

5. PHASES OF MATTER • Main phases (“states”) of matter: • solid, liquid, gas, plasma; • there are materials which can exist in several different solid or liquid phases; • transition from denser to less dense phase (e.g. solid to liquid, liquid to gaseous) needs energy (heat), to break bonds, overcome cohesive forces,… • e.g. “heat of fusion”, “latent heat of evaporation”; • phase (“state”) in which given material is depends on temperature and pressure; • solid  liquid  gas  plasma: • random motion increasing, less interaction between molecules/atoms/constituents • solid: • has definite size and shape; • molecules locked in place into fixed arrangement (“lattice of crystals”), densely packed • difficult to compress • chemical bonds, intermolecular forces sufficiently strong and directional to preserve large-scale external form; • kinds of solids: • crystalline, amorphous (glasses), polymers (plastics), and newer kinds of materials that don't quite fit into scheme: • liquid crystals, fullerines, aerogels, quasicrystals

6. Phases, cont’d • liquid: • has definite size, but no definite shape - assumes shape of container; • molecules close to each other, but not locked into lattice; • held together by “Van der Waals” forces (forces between electric dipoles); • in general, liquids a little less dense than solids (but difference is small) • water: solid less dense than liquid • gas: • has no definite size or shape - assumes size and shape of container; • molecules much farther apart than in liquids or solids molecules in random thermal motion; • gas pressure; • very little interaction between molecules (“ideal gas”: no interaction) • plasma: • ionized gas, mixture of charged particles (positive and negative), thermal motion violent enough to overcome electric attraction between charged particles; • 99.9% of visible mass in universe is plasma; • conducts electricity.

7. SOLIDS: • crystalline solids: • atoms or molecules arranged in orderly, repeated fashion -- “lattice”; • short- and long-range order; • examples: grains of salt, sand, gemstones, metals, ceramics, most rocks and minerals; • have well-defined melting point = temperature at which intermolecular bonds break; • amorphous materials (glasses): • only short-range order, no long-range order • have no well-defined melting point -- gradual softening; • plastics: • composed of intertwined chains of polymers; • can be molded into any shape; • huge spread in properties to fit • almost any application.

8. EMISSION AND ABSORPTION SPECTRA • EMISSION SPECTRA: • continuous spectrum • solid, liquid, or dense gas emits continuous spectrum of electromagnetic radiation (“thermal radiation”); • total intensity and frequency dependence of intensity change with temperatur (Kirchhoff, Bunsen, Wien, Stefan, Boltzmann, Planck) • line spectrum • rarefied gas which is ``excited'' by heating, or by passing discharge through it, emits radiation consisting of discrete wavelengths (“line spectrum”) • wavelengths of spectral lines arecharacteristic of atoms • ABSORPTION SPECTRA: • light from continuous-spectrum source passes through colder rarefied gas before reaching observer; • see dark lines in continuous spectrum: • first seen by Fraunhofer in light from Sun; • spectra of light from stars are absorption spectra (light emitted by hotter parts of star further inside passes through colder “atmosphere” of star) • dark lines in absorption spectra match bright lines in discrete emission spectra • Helium discovered by studying Sun's spectrum

9. Historical notes: • Robert Boyle (1627-1691) (Ireland, London) • 1661: ``element'' = substance that cannot be decomposed into simpler substances • Boyle-Mariotte gas law • improved Guericke's air pump • Antoine Lavoisier(1743-1794) (Paris) • (executed during French Revolution) - • “the father of modern chemistry” • burning = oxidation • composition of water • realized importance of quantitative studies of proportions in chemical reactions - developed precise balance for these studies • John Dalton (1766-1844) (Manchester) • law of simple proportions” • law of multiple proportions” • introduced atomic theory into chemistry; • law of partial pressures; • color blindness • Amadeo Avogadro (1776-1856) (Torino) • Avogadro's law (1811) • Robert Brown (1773-1856) • English botanist, observed “Brownian motion”

10. Historical notes, cont’d • Dimitri Ivanovich Mendeleev (1834-1907) (Petersburg) • periodic table of elements (1869) • Lothar Meyer (1830-1895) (Tübingen) • changes in hemoglobin due to breathing; • periodic system of elements (1869) • Ludwig Boltzmann (1844-1905) (Wien (Vienna)) • strong proponent of atomic/molecular picture of matter; • kinetic theory of heat, • application of statistics to thermodynamics ---- “statistical physics” • relation between entropy and probability. • Albert Einstein (1879-1955)(Ulm, München, Bern, Zürich, Prague, 1914 to 1933 Prof. in Berlin; since 1933 in US, at Princeton • explanation of Brownian motion (1905); • explanation of photoelectric effect (1905); • special relativity (1905); • general relativity (1916); • Nobel prize in physics 1921