Atomic Structure An introduction to Atomic Theory
Atomic
StructureAn introduction to Atomic Theory
Instructional
Goals• Define Dalton‟s atomic theory
• Differentiate between historical atomic models
• Describe the tentative nature of science.
Overview• Leucippus, Democritus and Aristotle
• John Dalton
• J.J. Thompson
• Ernst Rutherford
• Neils Bohr
• Modern Quantum Mechanical Model of
the atom
Leucippus 480-420 BC
• He was a natural philosopher.
• He thought matter to be finite.
• His student was Democritus.
• Only the quotes others have of Leucippus and Democritus been found.
He believed matter is
finite.
Finite means having
boundaries or limits,
like “No refills.”
Democritus 460-371 BC
Democritus’ Hypothesized:
If matter is cut again
and again, eventually a
single indivisible part
will remain. This part
called atomos is Greek
for indivisible. His
idea of matter gave us
the word ATOM.
According to Aristotle
• Matter was continuous
or infinitely smaller .
• Waterfalls produce
smaller and smaller
particles we call mist.
• Four basic elements”
Earth, Fire, Water and
Air.
Aristotle 384-322 BC
• His works were
rediscovered in
Western Europe
around 1200 AD.
• Aristotle‟s theory
about the atom was
widely accepted and
promoted by the
Catholic Church for
generations.
• Was an English school teacher who
independently studied the nature of matter.
• Recorded observations of his experiments.
• Proposed the first atomic theory based on
empirical evidence.
http://maple.lemoyne.edu/~giunta/dalton.html
http://www.tannerm.com/index.htm
John Dalton 1766-1844
Dalton‟s Theory• All elements are composed of submicroscopic
indivisible particles called atoms.
• Atoms of the same element are identical.
• Atoms of different elements can mix together, or chemically combine with one another to form compounds.
• Chemical reactions occur when atoms are separated, combined, or rearranged, but atoms of one element do not change into
another element.
Dalton‟s Model of the atom
History Summary
Performed
experiments to
support his theory.
Scientific theory- Matter
finite: is composed of
indivisible particles
called “atoms.”
Scientist:
Dalton
His idea or matter
was not supported by
later experiments.
Matter is infinite:
composed of basic
elements: earth, air,
water, and fire.
Aristotle
His ideas agreed
with later theories,
but lacked
experimental
support.
Matter is finite:
composed of tiny
invisible, indestructible
particles.
Democritus
Highlights PhilosophyPhilosophers
The discovery of the Electron
• J.J. Thomson (1856-1940) used results from
cathode ray tube (commonly abbreviated CRT)
experiments to discover the electron.
http://dbhs.wvusd.k12.ca.us/Disc-of-Electron-Images.html
http://dbhs.wvusd.k12.ca.us/Disc-of-Electron-Intro.html
J.J. Thompson’s Experiments
Thomson devised a cathode ray tube with a paddle
wheel built inside. When the high voltage
electricity was turned on the paddle wheel began
to rotate and move away from the cathode and
towards the anode.
J.J. Thompson’s Experiments
Thomson devised another cathode ray tube with positive
and negatively charged plates. Thomson observed that
the particles were being repelled by the negatively
charged plate and attracted to the positive plate.
http://lausd.k12.ca.us/~kmcmahon/Atoms%20%26%20Period%20Table/page_id_4745.html
Magnetic vs Electric field
deflection ratio
J. J. Thomson's experiment resulted
in a value for e/m (the ratio of the
charge of the electron to the mass of
the electron).
Millikan’s Oil Drop Experiment
http://webphysics.davidson.edu/applets/pqp_preview/cont
ents/pqp_errata/cd_errata_fixes/section4_5.html
Thompson’s Ions MineIn the dusty lab‟ratory,
„Mid the coils and wax and twine,
There the atoms in their glory.
Ionize and recombine.
Chorus
O my darlings! O my darlings!
O my darling ions mine!
You are lost and gone forever
When just once you recombine.
In a tube quite electrodeless,
They discharge around a line,
And the glow they leave behind them
Is quite corking for a time.
Chorus
…In the weird magnetic circuit
See how lovingly they twine,
As each ion describes a spiral
Round its own magnetic line.
J.J. Thompson‟s ModelAtom is like
raisin bread.
The atom‟s
“dough” is
positive so
the electrons
stick like
raisins.
This is also referred to as the plum pudding model.
Rutherford‟s Hypothesis
Rutherford thought if J.J.‟s model was correct, alpha particles
would not be deflected through a thin piece of gold foil.
http://micro.magnet.fsu.edu/electromag/java/rutherford/
ZnS
screen
What he expected
Why? Because, he thought the mass
was evenly distributed in the atom.
He thought if the mass
was evenly distributed
in the atom…
alpha particles would pass straight through.
Rutherford‟s ExperimentMost of the alpha
particles went straight
through.
But some were
deflected; the atomic
model had to change.
+
How he explained it
• A small dense,
positive piece is at the center of the atom.
• Alpha particles are deflected by it if they get closeenough
•The atom is
mostly empty
space
Rutherford‟s
Model of the AtomClick here for
simulation
Summary
Have your neighbor
• explain the reason we
no longer accept the
plum pudding/raisin
bread model of the
atom
•Describe the Rutherford
model of the atom.
•What is the nucleus?
•What is in the nucleus?
•How big is the nucleus?
•Where are the electrons?
Because less than 1% of
the alpha particles were
deflected, so 99% must
have passed through
empty space.
Protons and neutrons
1/10,000 of an atom
Nucleus is a dense center of mass
surrounded by empty space
Neils Bohr 1885-1962
• Atoms have energy levels that contain electrons.
• Electrons must have a specific or quanta of energy to change levels.
• His model is the first quantum model of the atom.
http://www.colorado.edu/physics/2000/quantumzone/index.html
Neils Bohr’s Theory• The dense nucleus is composed of positive
protons
• Electrons travel in elliptical orbits of definite energies
• A force of attraction draws electrons to the nucleus
• Centrifugal force pulls them outward
• Electron energy increases with distance from the nucleus
Bohr Model
Problems with Bohr‟s Model
• The electrons must radiate energy when they move
• Loss of energy should propel them closer to the nucleus
• All atoms therefore must collapse
• The model does not predict reality- atoms don‟t collapse
• Hydrogen is the only atom the model seems to explain
The modern theory of the atom
Electrons are negatively charged
particles found around the nucleus.
Atoms are the building blocks of matter.
The nucleus
concentrates
protons and
neutrons. This
region in the
atom has the
most mass,
and the least
volume.
Protons have a
positive
charge.
The Quantum Model• Is a mathematical model of the atom as
opposed to a simple visual model
• Predicts electron positions in terms of probability clouds
• Describes electron positions and energies, as approximations
• Answers questions raised by previous theory’s
Current Animated Atomic Model
Atomic Model SummaryDalton‟s Thompson‟s Rutherford‟s
Bohr‟s Quantum
Mechanical
Model
For Review• How did Democritus idea of matter differ from
Aristotle’s?
• What is John Dalton’s Atomic Theory?
• What evidence demonstrated electrons had charge?
• Describe the evidence for a small dense nucleus.
• Describe the modern quantum model of the atom.
• Explain, using examples, the tentative nature of science.
Relative Mass
Atoms are compared according to how much mass they have. For example, it takes 12 hydrogen atoms to equal the mass of 1 carbon atom.
VocabularyRelative Atomic Mass
Mass Number
Atomic Mass UnitAverage Atomic Mass
Isotope
Electron
Neutron
Proton Video lessons
http://web.jjay.cuny.edu/~acarpi/
NSC/3-atoms.htm
So a relative mass unit was invented, the
AMU, or atomic mass unit.
Carbon has 12 times the mass of hydrogen,
or we say hydrogen weighs 1 amu and
carbon weighs 12 amu.
12 X 1 amu hydrogen = 1 X 12 amu carbon
So if carbon has a mass of 12 amu that also
means it has a counted number of 12
subatomic particles in its nucleus: 6
neutrons and 6 protons. So 12 amu is also
called the MASS NUMBER.
To find the number of protons, look up the
atomic number. To find the number of neutrons,
subtract the atomic number from the mass
number.
Vocabulary Terms
Mass number = protons plus neutrons = Z
(Z or Mass number = #p+ added to #no)
Atomic number = number of protons = A
Nuclear symbol Z 14
A 6
carbon-14
http://www.colorado.edu/physics/2000/isotopes/
Atoms of the same element can have different
numbers of neutrons; the different possible
versions of each element are called isotopes. For
example, the most common isotope of hydrogen
has no neutrons at all; there's also a hydrogen
isotope called deuterium, with one neutron, and
another, tritium, with two neutrons.
Isotopes
Hydrogen Deuterium Tritium
Atomic Theory and the
Periodic Table
• Atoms are the smallest particles of an element that display the properties of that element
• There are 92 naturally occurring elements
• Recently scientists found theoretically predicted elements 114 and 118 and____?
• Modern Atomic Theory is helping to complete the Periodic Table of Elements
How does this story illustrate
the definition of science?
Science is a tentative, truth seeking, self correcting relevant way of learning.
An Introduction to the
Elements
http://www.privatehand.com/flash/elements.html
References
• http://inst.augie.edu/~jkbjerga/hist.html
• http://www.tannerm.com/index.htm
• http://www.watertown.k12.wi.us/hs/teachers/buescher/atomtime.html
• http://dbhs.wvusd.k12.ca.us/Greeks.html
• http://www.tannerm.com/atoms/chem_elements/list.htm
• http://www.tannerm.com/atoms/periodic_table/Periodic.htm
• http://www.liv.ac.uk/Chemistry/Links/links.html
• http://windows.engin.umich.edu/cgi-bin/tour_def?link=/sun/Solar_interior/Sun_layers/Core/atom_model.html&sw=false&cd=false&fr=f&edu=mid