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The Atom: History Recap
Democritus: The indivisible atom
• Matter is composed of empty
space through which the atoms
move
• The differing properties of
matter are due to the different
sizes, shapes and movement of
the atoms.
Solid, homogeneous,
indestructible and
indivisible (400 BC)
• Changes in matter are due to changes in the arrangements
of the atoms and not from changes in the atoms
themselves.
(atomos = “uncuttable”)
Dalton’s atomic theory: particulate nature of matter
(1803)
• Indivisible atom
• All atoms of an element are identical (same size, mass
and properties) and unique (not like any other element’s
atoms)
• Atoms combine in simple whole number ratios to form
compounds.
• Chemical reactions cause atoms to be separated,
combined or rearranged.
+
2 H2
1 O2
2 H2O
J.J. Thomson: The ‘plum pudding model’
• 1897-Measures the mass to
charge ratio of the electron and
discovers that it is smaller than a
hydrogen atom. The ‘indivisible’
atom is no longer indivisible.
• 1904-Proposes that the atom
consisted of two parts…a large
positively charged mass with
electron ‘raisins’ mixed in with it.
Plum-pudding model
of the atom (1904)
• 1909-Robert Millikan determines an electron’s charge (-1)
and uses Thompson’s mass to charge ratio to calculate the
mass of an electron: 9.1 x 10-28 g
Ernest Rutherford: gold-foil experiment (1911)
Hypothesis: If Thomson’s
model of the atom is valid,
then a positive particle will
be able to pass through an
atom with little deflection.
Experiment: Bombard thin gold foil with alpha
particles (nuclei of helium atoms) and measure the
amount of deflection.
Ernest Rutherford: gold-foil experiment (1911)
• Most of the atom consisted of
empty space.
• All of the positive charge and
virtually all of the atom’s mass
is contained in a dense region in
the center called the nucleus.
• The electrons move rapidly
through empty space outside of
the nucleus.
The nuclear atom
• 1920-Rutherford refines model: the nucleus contains
positively charged particles called protons (+1 charge).
• 1932-Third subatomic particle, the neutron (0 charge)
discovered by James Chadwick.
Niels Bohr: planetary model of the atom (1913)
• Electrons travel along fixed
orbits around the nucleus.
• Each orbital has a specific
energy value.
• The energy of the orbits
increases with orbital
diameter.
Bohr planetary model
• Electrons only move to a
higher energy orbital after
absorbing a quanta (amount)
of energy equivalent to the difference in energy values
of the two orbitals.
• When electrons ‘fall’ back down into a lower energy
orbital, they release the quanta of energy.
Schrödinger: Electron cloud model (1926)
• In the quantum mechanical
model of atomic structure,
electrons do not travel along
fixed paths like planets
around the sun.
• Instead, electrons can be
found most frequently in
certain areas around the
nucleus.
z
y
x
Quantum mechanical model: 90% probability of finding electron within 3D
orbital with a maximum energy defined by the principal quantum number, n.
(n = the row number on the periodic table)
n=3
9 sublevels
18 electrons
3s
2 e-
3p
6 e-
3d orbitals
10 e-
Homework tonight: Writing Assignment #2
Write at least one paragraph to answer the following
question:
How does the development of the atomic model (Democritus
to Schrödinger) exemplify application of the scientific
method?
DemocritusDalton ThomsonRutherfordBohrSchrodinger
400 BC
1803
1904
1911
1913
1926