Download models of the atom

MODELS OF THE ATOM
"In science, a wrong theory can be valuable and better than no theory at all."
- Sir William L. Bragg
Greek model
Dalton’s
model
(400
(1803)
B.C.)
Thomson’s plum-pudding
model (1897)
Bohr’s model
(1913)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
Rutherford’s model
(1909)
Charge-cloud model
(present)
GREEK MODEL
Democritus
Greek philosopher
 Idea of ‘democracy’
 Idea of ‘atomos’

= ‘indivisible’
 ‘Atom’ is derived
 Atomos

No experiments to
support idea
“To understand the very large,
we must understand the very
small.”
Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE
DALTON’S ATOMIC THEORY OF MATTER
PAGE 102 IN TEXT BOOK!
His atomic theory of matter contains four
hypotheses:




Elements are made of tiny particles called atoms.
All atoms of a given element are identical. The atoms
of a given element are different from those of any
other element
Atoms of one element can combine chemically with
atoms of another element to form compounds. A
given compound always has the same relative
number and types of atoms.
Atoms are indivisible in chemical processes. Atoms
are not created or destroyed in chemical reactions. A
chemical reaction simply changes the way the atoms
are grouped together.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
CATHODE RAY EXPERIMENT
Thompson’s 1897 Experiment
 Using a cathode ray tube,
Thomson was able to deflect
cathode rays with an electrical
field.
 The rays bent towards the
positive pole, indicating that
they are negatively charged.
J.J. THOMPSON’S
A CATHODE RAY TUBE
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
THOMSON MODEL OF THE ATOM



J.J. Thomson discovered the
electron and knew that electrons
could be emitted from matter
(1897).
Thomson proposed that atoms consist
of small, negative electrons
embedded in a massive, positive
sphere.
The electrons were like raisins in a
plum pudding, called the ‘plum
pudding’ model of the atom.
RUTHERFORD’S APPARATUS
beam of alpha particles
radioactive
substance
circular ZnS - coated
fluorescent screen
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
Rutherford Model
 In
1911, Ernest Rutherford conducted
his gold foil experiment which helped
improve our understanding of atomic
structure.
 He
directed a narrow beam of alpha
particles at a very thin sheet of gold
foil.
particles (a) are He atoms
that have been stripped of their
electrons
 Alpha
RESULTS OF GOLD-LEAF EXPERIMENT
n+
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
What he expected…
Because he thought the mass was
evenly distributed in the atom.
-
-
-
-
What he got…
richocheting
alpha particles
DENSITY AND THE ATOM
Since most of the particles went through,
the atom was mostly empty.
 Because the alpha rays were deflected so
much, the positive pieces it was striking
were heavy.
 Small volume and big mass = big density
 This small dense positive area is the
nucleus

California WEB
THE PLANETARY MODEL
Nucleus
Electron
Orbit
+
The Bohr
model of the
atom, was
built upon the
incorrect idea
that electrons
orbit the
nucleus like
planets
around the
sun.
WAVE/CLOUD MODEL OF THE ATOM
In Schrödinger’s wave model of the atom, the
electron behaves as an energy wave as well as a
matter particle.
Einstein had predicted that energy and matter were
related in his equation E = mc2.
If we could see an electron it
might look like this.
THE COMPLETE ATOM
An atom is mostly empty
space with a nucleus
containing protons and
neutrons which contain most
of the mass of an atom.
Electrons are in energy
levels around the nucleus.
Electrons jump between
levels, emitting and
absorbing energy as they
jump.
EXTRA STUFF…SUBATOMIC PARTICLES
 Quarks

component of
protons &
neutrons

6 types (flavors)

3 quarks =
1 proton or
1 neutron
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
He
electron
e-
proton
p+
neutron
n0
-1
Outside of
1/1840 nucleus
+1
0
Structure of the Atom
1
Inside
nucleus
1
Inside
nucleus
STRUCTURE OF THE ATOM
There are two regions
1.
Nucleus



2.
Contains protons and
neutrons
Has a positive charge
Almost all the mass
Single Atom
Electron Cloud



Where electrons are found
Has a negative charge
Almost all the volume
Water Molecule
SIZE OF AN ATOM


Atoms are incredibly tiny.
Measured in picometers (10-12 meters)


Nucleus tiny compared to atom



California WEB
Hydrogen atom, 32 pm radius
Radius of the nucleus near 10-15 m.
Density near 1014 g/cm3
IF the atom was the size of a stadium,
the nucleus would be the size of a
marble.