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Atomic Theory
Greek “Thinkers”
Atoms – “indivisible” – smallest part of
an element that retains the property of
that element
1. All atoms have a definite size
(“incompressible”)
2. All atoms are neutral
3. All matter is composed of atoms too
small to be seen which cannot be divided.
Greek “Thinkers”
4. There is empty space between atoms
5. Atoms are completely solid
Later proved incorrect
6. Atoms are different sizes, shapes, and
weights
Alchemists
Middle Ages
Spent most of their time trying to
change metals into silver and gold
Transmutation – changing one
element into another
Added knowledge about the properties
of elements
Antoine Lavoisier 1789
Able to list 33 known
elements
Law of Conservation of
Mass
“Recycling” Law
Matter can be changed;
BUT it cannot be created
or destroyed.
John Dalton
1803
“The Father of
Modern Atomic
Theory”
Used all former ideas
to come up with his
atomic theory
John Dalton
1. Each element consists of individual
atoms
2. All atoms of the same element are
identical in mass and properties
3. All atoms of different elements are
different in mass and properties
John Dalton
4. Atoms combine with each other in
specific ratios to make compounds
5. Chemical reactions occur when
atoms are separated, joined, or
rearranged, but the atoms of one
element are not changed into the
atoms of another by chemical
reaction
Dmitri Mendeleev 1869
Russian Scientist
Devised the Periodic Table
Arranged the known
elements by atomic weight
and noticed that properties
re-occurred periodically
Left blanks for undiscovered
elements
400-1890’s
Atom was viewed as solid and indivisible
Marie & Pierre Curie
Henri Becquerel
Discovered Radioactivity
Polonium (Po), Radium (Ra), Uranium (U)
Smaller particles are emitted from the atom
Gamma Rays – neutral
Beta rays – electrons
Alpha rays - protons
J.J. Thomson
Discovered the electron using the cathode ray tube
Electrons are NEGATIVE particles
e- are very small in weight
Plum pudding model
Later found positive particles
(protons)
Robert Millikan
Determined the mass of the e-
(9.11 x 10-31 kg)
This mass is MUCH smaller than the
mass of the proton
Ernest Rutherford
Gold foil experiment
Found an atom has a very small positive nucleus
surrounded by tiny moving e and lots of empty
space
Protons must be in center
Modern Gold Foil
Experiment
James Chadwick
Discovered the neutron
No charge
Same weight as proton
Atomic Structure
Subatomic Particles
protons (p+) = positive
electrons (e-) = negative
neutrons (n0) = neutral
Atomic Structure
Atomic number = # p+
Bromine
A.N. = 35
35 protons
Protons determine the element
Atomic Structure
Atoms are neutral
# protons (p+) = # electrons (e-)
Bromine
a.n. = 35
35 protons (+)
Therefore, 35 electrons (-)
Atomic Structure
Atomic Mass
The weight of the atom is in the nucleus
Protons and Neutrons in the nucleus
Atomic mass = #p+ + #n0
Bromine a.m. = 80
80 = 35 + #n0
#n0 = 45
Atomic Structure
Atoms are neutral
# protons (p+) = # electrons (e-)
Atomic number = # p+
Atomic mass = #p+ + #n0
= neutrons = protons
C-14
Nucleus
C-12
Nucleus
= neutrons = protons
C-14
Nucleus
C-12
Nucleus
-Both are Carbon (both have 6 protons)
-Have different number of neutrons!
Isotopes
Atoms of the same element (same
atomic number) with different atomic
masses (different # of neutrons).
C-14 and C-12
