Transcript Document

Fall 2004 - Topic 7
ATOMS:
Dalton and Beyond
Dr. Donna Brestensky, Chemistry
*** Please pick up a handout as you come in. ***
Animation Reference: www.tvgreen.com
Start of Modern Era of Atoms:
Dalton’s Atomic Theory
John Dalton
(1766-1844)
British chemist,
lecturer, and
meteorologist
Dalton’s 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.
Dalton’s Atomic Theory (1803) - 2
4) Elements and
compounds are
composed of definite
arrangements of
atoms.
Chemical change
occurs when the
atomic arrays are
rearranged.
Significance of Dalton’s 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.
Dalton: inconsistencies uncovered…
1) 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 knows…no man can split the atom.”
No: radioactivity, atomic particles.
3) Atoms of given element have same mass and
properties? Not exactly: isotopes exist…
Thinking about Atoms…
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.
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
Flame Test for Element Identification
(From left) Sodium, potassium, lithium;
strontium, barium, potassium.
Spectroscopes: Seeing Atomic Light
Original 1859
BunsenKirchhoff
spectroscope
Typical setup
for viewing a
line-emission
spectrum
Elements: Ages of Discovery
Classification of the Elements:
Development 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
Dmitri
Mendeleev
(1834-1907)
“Creator of the
Periodic Table”
(probably formulated
periodic idea at same
time as Meyer)
Mendeleev’s
early notes
for the
Periodic Table
(1869)
Mendeleev’s
table, as orig.
published
• Formatted
sideways
compared to
modern table
• ? instead of a
name: element
was predicted to
exist but not
known yet
Characteristics of Mendeleev’s 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)
Comparison of
eka-silicon’s predicted properties
and known Group 4 properties
Eka:
“one
beyond”
1880s Revision of
Mendeleev’s 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.
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!
Discovery of Atomic Structure;
Sub-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
Joseph John
Thomson
(1856-1940)
British physicist
and mathematician
Nobel Prize in 1906
(existence of electrons)
1897: calc’d electron’s mass-to-charge
ratio in cathode-ray experiment
Thomson’s 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):
Cathode-Ray Experiment:
Thomson (1897)
• Undeflected => Point 1
• Rays can be attracted to
+ plate (hit Point 3) or
deflected by magnetic
field (hit Point 2).
• Rays have negative
charge, which can’t be
separated from rays!
Vacuum tube w/fluorescent
end coating, electrodes, and
high-voltage passing through.
Thomson’s 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 => supported in future
Thomson’s 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
Thomson’s “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.
Robert Millikan (1868-1953)
U.S. physicist
Nobel Prize in 1923
(charge of electron:
1909 oildrop expt.)
With Thomson’s
result, this allowed
calculation of
electron mass.
Millikan’s experimental apparatus.
Millikan’s Oil-Drop Experiment (1909)
• Spray oil... droplets go thru
plate’s hole
• Hit air molecules with Xrays... knock off electrons.
• Electrons on oil drops…
now, charged.
• Adjust voltage... a drop is
held stationary.
• Use drop’s mass, voltage to
calculate drop’s charge
(always whole multiple of
1.60 x 10-19 C).
Diagram of apparatus electrical field between
plates is adjustable.
Ernest Rutherford
(1871-1937)
nuclear physicist,
Thomson’s student,
New Zealander teaching
in Great Britain
Nobel Prize in 1908
(radioactive decay)
1910-11: Gold foil experiments
Rutherford’s 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 [He+2]
Rutherford’s Experiment: prediction
By Thomson’s 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.
Rutherford’s Experiment:
prediction
pass through
like this …
Rutherford’s experiment:
what actually happened
Rutherford’s 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.”
Rutherford’s 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?
How the Nucleus Repels Alpha Particles
+
How much of an atom is empty space?
+
How much of an atom is empty space?
Most of it!
+
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)
+
James Chadwick
(1891-1974)
Rutherford student
English nuclear physicist
Nobel prize in 1935
(existence of neutron)
Chadwick’s subatomic particle: neutron
• Made rays of different atomic particle
• Not deflected by electric fields, so no
charge (neutral) => neutron
• Collide neutron with different-weight
gases...measure their deflections
=> calculate neutron mass:
similar to + proton’s
Actual 1932 apparatus:
Alpha particles from
polonium source (right)
hit beryllium target (left),
making new rays
• Neutrons penetrate and split various
heavy atoms, b/c not repelled by
nucleus (unlike alpha)
=> atomic bomb
Known Properties of Subatomic Particles
Property
Particle
Electron
Proton
Neutron
Mass (amu),
Mass (g)
0.00055
9.1093897 x 10-28
1.00728
1.6726231 x 10-24
1.00866
1.6749286 x 10-24
Relative
Charge
-1
+1
0
Niels Bohr
(1885-1962)
Danish physicist
Revised Rutherford’s
model of atom (1913)
Bohr Looks at Emission Spectrum:
Hydrogen’s 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)
Bohr’s Model of Atom (1913)
H's electron
r1
r2
The first three allowed energy levels,
at distances r1, r2, and r3 from nucleus.
r3
H's nucleus containing 1 proton
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.
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
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
Ongoing Study of Subatomic Structure
• So...
there’s evidence that protons (+), neutrons (neutral),
and electrons (-) exist in the atom.
End of the story?
NO! Still more to see and learn!
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 => Einstein’s equation E=mc2
(Note: positrons – i.e, antielectrons- are not found in atoms.)
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 world’s-largest machine: CERN 27-km LEP collider (1989-2000)
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 Joyce’s 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
Quark Evidence from Particle Destruction?
• (CERN) after collision of
electron and positron...
evidence of quarks?
• (DESY-PETRA) quark
and anti-quark evidence?
Computer modelling of other new particles?
• Beginning about 2006,
CERN’s 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?
Ref: The Particle Odyssey, p. 15
• Simulated tracks from
proton-proton collision:
decay of Higgs particle