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, contd

• 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
  (11, 12, 23, 13,...).
• 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, contd

• 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)

10
Historical notes, contd

• 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