Atomic Structure—Time line
B.C.
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Ancient Greeks:
Democritus and
Leucippus
All matter is made up of
tiny indivisible particles
that could not be
divided into anything
smaller (atomos-atoms)
Discontinuous theory
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Continuous theory of
Matter
A solid body could be
divided into smaller and
smaller pieces with no
end.
Evidence in support of the atomic
theory did not come until almost
2000 years later
Experiments and research
1770--Lavoisier
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French chemist explained the nature of
burning
2Sn + O2→ 2SnO
Mass of reactants: 2(119) + (32)=270
Mass of Products: 2(119+16)=2(135)=270
Mass of reactants=Mass of Products
Proved the law of conservation of matter -(matter
can be neither created nor destroyed)
1799--Proust
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French chemist devised the Law of
Definite Proportions. (in a pure
compound the proportion by mass of
the elements is always the same)
For example water—H2O
2H: 2 x 1 =2
1O: 1 x 16 =16
H:O 2:16 or 1:8
1803—Dalton
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English scientist devised the law of multiple
proportions (certain elements often combine
to form more than one compound, if the
mass of one element is fixed the mass of the
other element varies in a ratio of small whole
numbers.
For example: H2O
and H2O2
In water H:O is 2:16, while in hydrogen
peroxide it is 2:32. The hydrogen is fixed
(2:2) while the oxygen varies (16:32 or 1:2)
Try SO2 and SO3
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SO2 S:O 32:32
SO3 S:O 32:48
Sulfur is fixed (32:32)
Oxygen varies (32:48, 2:3)
Do you see any connections between
the ratios and the formulas.
1803—Dalton’s Atomic Theory
(combined the above three laws into a theory)
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All matter is
composed of very
small particles called
ATOMS.
All atoms of the
same element are
exactly alike (same
mass).
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All atoms of
different elements
are different
(different masses).
The joining of 2 or
more elements in
definite whole
number ratios forms
compounds.
Modern Atomic Theory
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Dalton’s theory had existed for almost 100 years. As
new discoveries were made Dalton’s theory had to be
revised to include the structure of the atom. The
major differences from Dalton include:
1. Atoms have a detailed structure that is altered
during a chemical change. Atoms can be changed
from one another during a nuclear change.
2. Atoms of the same element are not exactly alike
they do have different masses. (isotopes)
1897—J.J. Thomson
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All atoms contain units
of negative charge
called electrons.
Used a modified
Crooke’s tube to show
that electrons are
deflected by electric
and magnetic fields.
Thomson Model--Plum
Pudding Model—
electrons embedded in
a positive sphere.
Atomic Structure
Discovered
1909—Robert A. Millkan
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American scientist
performed Oil-Drop
experiment
Determined the charge
on an electron and
calculated its mass.
Electrons had a
negative charge and a
mass of 0.000548593
amu or 0 amu.
1911--Rutherford
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Gold-foil experiment: directed alpha particles from a
polonium atom at a pieces of gold foil. Most passed
through the foil without a change in direction.
Concluded that most of the atom is empty space
with a small, dense, positive nucleus and
electrons floating around them, These positive
particles with a mass of 1 amu were later known as
protons.
1913—Bohr Model
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Bohr put
electrons in
energy levels like
planets around
the sun.
1920—Schroedinger Wave Model
(Modern Atomic or Orbital Model)
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Electrons are not in
specific energy levels,
but rather in sublevels
and orbitals.
An orbital is the average
region where an
electron is most likely to
be found.
Forms an electron
cloud.
1932—James Chadwick
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Discovered neutrons to have 0 charge
with 1 amu.
Neutrons are located in the nucleus.
Summary: