ATOMS

• Leukippos, Demokritos ( 500 BC):
  thought experiment 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

• 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.

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 = '', row = ''
• 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 predictions about as yet undiscovered elements

THERMAL ENERGY, 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

PHASES OF MATTER

• Main phases (``states'') of matter are: solid, liquid, gas, plasma, but 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

• 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
  • 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.

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.

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 temperature
  (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 characteristic 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

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''
• 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