Atoms
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Leukippos, Demokritos (500 BC):
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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:
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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
Atoms, cont’d
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main findings during early (“chemical”) period:
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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
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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
THERMALENERGY,HEAT,
TEMPERATURE
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observation of “Brownian motion” (1827):
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small seeds (e.g. burlap) suspended in liquid show
erratic motion (random motion'')
kinetic theory of heat: (Boltzmann, Maxwell,...)
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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
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Main phases (“states”) of matter:
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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:
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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
Phases, cont’d
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liquid:
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gas:
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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
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:
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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:
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crystalline solids:
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amorphous materials (glasses):
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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;
only short-range order, no long-range order
have no well-defined melting point -- gradual
softening;
plastics:
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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
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EMISSION SPECTRA:
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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:
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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:
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Robert Boyle (1627-1691) (Ireland, London)
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Antoine Lavoisier(1743-1794) (Paris)
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law of simple proportions”
law of multiple proportions”
introduced atomic theory into chemistry;
law of partial pressures;
color blindness
Amadeo Avogadro (1776-1856) (Torino)
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(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)
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1661: ``element'' = substance that cannot be
decomposed into simpler substances
Boyle-Mariotte gas law
improved Guericke's air pump
Avogadro's law (1811)
Robert Brown (1773-1856)
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English botanist, observed “Brownian motion”
Historical notes, cont’d
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Dimitri Ivanovich Mendeleev (1834-1907)
(Petersburg)
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periodic table of elements (1869)
Lothar Meyer (1830-1895) (Tübingen)
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Ludwig Boltzmann (1844-1905) (Wien (Vienna))
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changes in hemoglobin due to breathing;
periodic system of elements (1869)
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
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explanation of Brownian motion (1905);
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explanation of photoelectric effect (1905);
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special relativity (1905);
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general relativity (1916);
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Nobel prize in physics 1921