Fall 2004 - Topic 7 ATOMS: Dalton and Beyond

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Fall 2004 - Topic 7 ATOMSDalton and Beyond

• Dr. Donna Brestensky, Chemistry

Please pick up a handout as you come in.
Animation Reference www.tvgreen.com
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Start of Modern Era of AtomsDaltons Atomic
Theory

• John Dalton
• (1766-1844)
• British chemist,
• lecturer, and
• meteorologist

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Daltons Atomic Theory (1803) - 1

1. All matter is made up of indivisible and
   indestructible basic particles called atoms.
2. All atoms of a given element are identical, both
   in mass and in properties. Atoms of different
   elements have different masses and properties.
3. Compounds are formed when atoms of different
   elements combine in the ratio of small whole
   numbers.

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Daltons Atomic Theory (1803) - 2

• Elements and compounds are composed of definite
  arrangements of atoms.
• Chemical change occurs when the atomic arrays
  are rearranged.

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Significance of Daltons Atomic Theory

• Continued to break down earlier views of
  elements
• Bridged gap between lab data and hypothetical
  atom.
• - way of calculating relative atomic weights.
• Explained Law of Definite Proportions Proust
  1799
• - All samples of a compound contain same weight
  
• proportions of constituent
  elements.
• Explained Law of Conservation of Mass
• - Initial Mass Final Mass
• - Only reorganizing of unchangeable atoms
• occurs in chemical reaction.

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Dalton inconsistencies uncovered

• The basic state of an element one atom?
• Perhaps basic natural state of an
• element may be a molecule made of
• 2 or more atoms.
• 2) Dalton Thou knowsno man can split the
  atom. No radioactivity, atomic particles.
• 3) Atoms of given element have same mass and
  properties? Not exactly isotopes exist

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Thinking about Atoms
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Current Definitions Matter Classification

• Element
• - pure substance
• - made of unique, (nearly) identical atoms
• - cannot be broken down into simpler substances
  by a
• chemical reaction.
• Compound
• - pure substance
• - made of atoms of at least 2 different elements
  
• - can be broken down into simpler substances by
  a
• chemical reaction.

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Identification of Elements

• Physical properties
• Chemical properties
• Relative atomic weights (better values)
• Flame test for solids/solutions
• Interaction with light
• line-absorption spectrum
• line-emission spectrum

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Flame Test for Element Identification
(From left) Sodium, potassium, lithium
strontium, barium, potassium.
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Spectroscopes Seeing Atomic Light
Original 1859 Bunsen- Kirchhoff spectroscope
Typical setup for viewing a line-emission spectru
m
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Elements Ages of Discovery
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Classification of the ElementsDevelopment of
the Periodic Table

• Dobereiner 1817 Triads, group properties
• Newlands 1863 row Octaves, group properties
• Mendeleev 1869
• first-published Period definition (see next
  slides)
• Meyer 1870
• 2nd-published Period definition
  volume/properties

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• Dmitri
• Mendeleev
• (1834-1907)
• Creator of the
• Periodic Table
• (probably formulated
• periodic idea at same
• time as Meyer)

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• Mendeleevs
• early notes
• for the
• Periodic Table
• (1869)

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Mendeleevs table, as orig. published

• Formatted sideways compared to modern table
• ? instead of a name element was predicted to
  exist but not known yet

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(No Transcript)
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Characteristics of Mendeleevs Table

• Organized 60 known elements
• - by similar properties in each vertical
  family (group)
• - by valence combining number
• (split out elements with multiple valence)
• - by roughly increasing atomic weight within
• each horizontal row (moved 17 elements
• based on properties rather than
  weight)
• Used to predict existence of new elements
• (of 10, found 7 other 3 do not exist)

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Comparison of eka-silicons predicted
properties and known Group 4 properties
Eka one beyond
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1880s Revision of Mendeleevs Table
Contains rare gases and 3 elements unknown at
time of first version, though their properties
were predicted germanium (Ge), formerly
eka-silicon gallium (Ga), formerly
eka-aluminum scandium (Sc), formerly
eka-boron.
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Modern Periodic Table Organization

• Elements are NOW placed in order of
• increasing atomic number ( of protons).
• - Why? Gives absolute order...
• atomic weights not characteristic
• (different-mass atoms called
  isotopes exist!)
• A relationship between nuclear charge and
• arrangement of elements in the Table was
• finally discovered in 1914 (Henry Moseley).
• In 1860s, Mendeleev could NOT have predicted
• a relationship to subatomic particles!

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Discovery of Atomic StructureSub-atomic
Particles

• Thomson 1897 electron mass-to-charge ratio
• Millikan 1909 electron charge
• Rutherford 1910-11 mass charge of nucleus
• Chadwick 1932 neutron
• Bohr 1913 electron energy levels
• Gell-Mann/Zweig 1964 quark theory

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• Joseph John
• Thomson
• (1856-1940)
• British physicist
• and mathematician
• Nobel Prize in 1906
• (existence of electrons)

1897 calcd electrons mass-to-charge
ratio in cathode-ray experiment
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• Thomsons Cathode-Ray Experiment

• Known before
• atoms are normally neutral
• (neither positive nor
• negative charge)
• When cathode rays are
• made, remaining atoms are
• positively charged (ions)

Schematic of actual 1897 apparatus (vacuum
inside)
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Cathode-Ray ExperimentThomson (1897)

• Undeflected gt Point 1
• Rays can be attracted to plate (hit Point 3) or
  deflected by magnetic field (hit Point 2).
• Rays have negative charge, which cant be
  separated from rays!

Vacuum tube w/fluorescent end coating,
electrodes, and high-voltage passing through.
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Thomsons Cathode-ray Results

• Calculated mass-to-charge ratio (using math and
  known field strengths) and energy of ray
  particles
• Mass-to-charge ratio for cathode rays was over
  1000 times smaller than that of a charged
  hydrogen atom (a proton), suggesting
• either cathode rays carried huge charge,
• or they were amazingly light relative to their
  charge gt supported in future

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Thomsons conclusions/questions

• We have, in the cathode rays,
• matter in a new state...a state in
• which all matter...is of one and
• the same kind this matter being
• the substance from which all the
• chemical elements are built up."

• I can see no escape from the conclusion that
  cathode rays are charges of electricity carried
  by particles of matter.
• but...
• What are these particles? Are they atoms, or
  molecules, or matter in a still finer state of
  subdivision? - J. J. Thomson

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• Thomsons plum pudding atom model

• Cathode rays (electrons) are...
• tiny corpuscles of negative charge
• surrounded by a sort of cloud of positive charge

Never had plum pudding? Think of a blueberry
muffin.
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Robert Millikan (1868-1953)

• U.S. physicist
• Nobel Prize in 1923
• (charge of electron 1909 oildrop expt.)
• With Thomsons
• result, this allowed
• calculation of
• electron mass.

Millikans experimental apparatus.
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Millikans Oil-Drop Experiment (1909)

• Spray oil... droplets go thru plates hole
• Hit air molecules with X-rays... knock off
  electrons.
• Electrons on oil drops now, charged.
• Adjust voltage... a drop is held stationary.
• Use drops mass, voltage to calculate drops
  charge (always whole multiple of 1.60 x 10-19 C).

Diagram of apparatus - electrical field
between plates is adjustable.
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• Ernest Rutherford
• (1871-1937)
• nuclear physicist,
• Thomsons student,
• New Zealander teaching
• in Great Britain
• Nobel Prize in 1908
• (radioactive decay)

1910-11 Gold foil experiments
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Rutherfords Experiments (1910-11) (done by
undergrad Ernest Marsden/physicist Hans Geiger)

• Fired beam of alpha particles at very thin gold
  foil.
• Alpha particles positive-charged helium ions,
• mass 4 amu He2

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Rutherfords Experiment prediction

• By Thomsons model,
• mass and charge of gold atom are too dispersed
  to deflect the positively-charged alpha
  particles,
• so...
• particles should shoot straight through the gold
  atoms.

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Rutherfords Experiment prediction pass
through like this
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Rutherfords experiment what actually happened
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• Rutherfords results, response in amazement

Most alpha particles went straight through,
and some were deflected, BUT a few (1 in
20,000) reflected straight back to the source!
It was quite the most incredible event that has
ever happened to me. It was almost as
incredible as if you had fired a fifteen inch
shell at a piece of tissue paper and it came back
and hit you.
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• Rutherfords Model of the Atom

• Expt. Interpretation
• gold atom has small, dense, positively-charged
  nucleus surrounded by mostly empty space
• in which the electrons must exist.
• like tiny solar system

Also, calculated nuclear mass as mass of
positively-charged protons. Protons
only half of actual mass suggests
neutral particles of same mass as proton?
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How the Nucleus Repels Alpha Particles

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• How much of an atom is empty space?

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• How much of an atom is empty space?

Most of it!

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• How much of an atom is empty space?

Most of it!

• In fact, if the nucleus of an atom were the size
  of a large room, the outermost electrons (far
  edge of the electron cloud) would be in
• The room next door
• The far side of campus
• Downtown Olean
• New York City

(click for the right answer)
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• James Chadwick
• (1891-1974)
• Rutherford student
• English nuclear physicist
• Nobel prize in 1935
• (existence of neutron)

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Chadwicks subatomic particle neutron

• Made rays of different atomic particle
• Not deflected by electric fields, so no charge
  (neutral) gt neutron
• Collide neutron with different-weight
  gases...measure their deflections
• gt calculate neutron mass
• similar to protons
• Neutrons penetrate and split various heavy atoms,
  b/c not repelled by nucleus (unlike alpha)
• gt atomic bomb

Actual 1932 apparatus Alpha particles from
polonium source (right) hit beryllium target
(left), making new rays
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Known Properties of Subatomic Particles
Property Particle Mass (amu), Mass (g) Relative Charge
Electron 0.00055 9.1093897 x 10-28 - 1
Proton 1.00728 1.6726231 x 10-24 1
Neutron 1.00866 1.6749286 x 10-24 0
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• Niels Bohr
• (1885-1962)
• Danish physicist
• Revised Rutherfords
• model of atom (1913)

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Bohr Looks at Emission Spectrum Hydrogens
Fingerprint
Observation when hit with electricity hydrogen
gives off light of specific wavelengths, NOT
continuous range!
The line-emission spectrum of hydrogen gas (the
bands visible to humans)
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Bohrs Model of Atom (1913)

• Hypotheses
• Circling electron maintains orbit ONLY at
  specific distances from nucleus (containing
  protons and neutrons).
• Only way electron could exist for long time w/o
  giving off radiation.
• Electron is more stable as distance r from
  nucleus decreases.

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Ongoing Study of Subatomic Structure

• Other ways to study atoms and atomic pieces
• in cloud chamber (Wilson 1911) or bubble chamber

One of first photographs of alpha particle
trails, in water mist
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Ongoing Study of Subatomic Structure

• typical coiled motion of electron in cloud
  chamber, under influence of varying magnetic field

Electron generated on left. Note tighter spiral
after electron gives off light
Ref The Particle Odyssey, p. 37
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Ongoing Study of Subatomic Structure

• So...
• theres evidence that protons (), neutrons
  (neutral), and electrons (-) exist in the atom.

End of the story?
NO! Still more to see and learn!
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More new particles antimatter!

• Rare simultaneous generation of an electron and a
  positron when certain high-energy light passes
  through chamber
• energy converts to mass gt Einsteins
  equation Emc2

(Note positrons i.e, antielectrons- are not
found in atoms.)
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Fermi National Accelerator Lab 6-km Tevatron
ring and 3-km Main Injector

• Chicago site for study of sub-subatomic particles
• proton and antiproton beams used

contrast to worlds-largest machine CERN 27-km
LEP collider (1989-2000)
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Proton and neutron are not fundamental!

• 1960s - Gell-Mann and Zweig - proposed protons
  and neutrons are made of smaller particles they
  named quarks (refers to term in James Joyces
  Finnegans Wake)
• Need to use 2 different quarks (UP and DOWN) held
  together by gluon particles
• UP quark has 2/3 charge, DOWN quark has 1/3

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Quark Evidence from Particle Destruction?

• (CERN) after collision of electron and
  positron... evidence of quarks?
• (DESY-PETRA) quark and anti-quark evidence?

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Computer modelling of other new particles?

• Beginning about 2006, CERNs new LHC (Large
  Hadron Collider) particle accelerator will search
  for clues to the Big Bang and the origin of mass.
• Does proposed Higgs particle really exist?
• Simulated tracks from proton-proton collision
  decay of Higgs particle

Ref The Particle Odyssey, p. 15