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 indestructible basic particles called and atoms .
2) All atoms of a given element in mass and in properties are identical , both . Atoms of different elements have different masses and properties.
3) Compounds are formed when atoms of different elements combine numbers .
in the ratio of small whole
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 Bunsen Kirchhoff 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 are
charges of electricity conclusion
carried by that [cathode rays]
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 X rays... 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 r 2 r 1 The first three allowed energy levels, at distances r 1 , r 2 , and r 3 from nucleus.
r 3 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 under influence of electron in cloud chamber, 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=mc 2 (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
together by
gluon
particles (
UP
and
DOWN
) held •
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