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Atomic Structure
1. Democritus:
Around 300 BC, a Greek philosopher,
Democritus stated that everything is made up
of tiny, invisible particles
• He said the particles were indivisible
• He called them “atomos” – which means
unable to divide
2. John Dalton :
Then in the early 1800’s, an English school
teacher, John Dalton developed the first atomic
theory
• His theory was successful because it was
supported with scientific evidence
• By using experimental methods, Dalton
transformed Democritus’ ideas on atoms into
a scientific theory.
• His theory has 4 main points
4 parts of Dalton’s theory:
1. All elements are composed of tiny indivisible
particles called atoms.
2. Atoms of the same element are identical.
The atoms of any one element are different
from those of any other element.
3. Atoms of different elements can physically
mix together or can chemically combine in
simple whole-number ratios to form
compounds.
4. Chemical reactions occur when atoms are
separated from each other, joined, or
rearranged in different combinations.
• Atoms of one element are never changed
into atoms of another element as a result of
a chemical reaction
Much of Dalton’s atomic theory is accepted
today.
• One important change, however, is that atoms
are now known to be divisible.
• Also, atoms of the same element are not
identical.
3. JJ Thomson
• In the late 1800’s, an English physicist,
JJ Thomson performed an experiment with
a cathode ray tube
• His experiment led to the discovery of the
electron
• An electron has one unit of negative charge,
and its mass is 1/1840 the mass of a
hydrogen atom.
Thomson’s experiment:
4. Eugen Goldstein
• In 1886, Eugen Goldstein discovered protons
• If matter has negative particles, it must also
have positive particles.
5. James Chadwick
• In 1932, James Chadwick confirmed the
existence of yet another subatomic particle:
the neutron
• Neutrons are subatomic particles with no charge
but with a mass nearly equal to that of a proton.
Chadwick’s experiment:
6. Ernest Rutherford
• In the early 1900’s, Ernest Rutherford
performed the “Gold Foil Experiment”
• He observed that the alpha particles went
through the gold foil
• The Rutherford atomic model is known as the
nuclear atom.
• His model inferred that the atom was almost
entirely empty space
• Also concluded that the nucleus was
extremely tiny and contained the protons
and the neutrons
• The electrons are distributed around the
nucleus and occupy almost all the volume of
the atom.
• According to this model, the nucleus is tiny
and densely packed compared with the atom
as a whole.
• If an atom were the size of a football stadium,
the nucleus would be about the size of a
marble.
ISOTOPES
Mass number = protons + neutrons
• Electrons are so much less massive than
protons and neutrons that their mass is not
included in atomic mass
• The mass listed in the periodic table is the
weighted average of the atomic masses
of the naturally occurring isotopes
• Isotopes are atoms of the same element with
different masses
• Neutrons were actually discovered due to
isotopes
• In an isotope the number of protons and
electrons are the same – only the neutrons
differ
• One way to specify an isotope is to use the
nuclear symbol.
• The nuclear symbol includes the chemical
symbol, atomic number, and atomic mass.
The value shown in the periodic table is the
average atomic mass. It is a weighted average
For example: Chlorine has two isotopes
Chlorine-35 and Chlorine-37
• Cl-35 has an amu of 34.9689 with an
abundance of 75.771%
• Cl-37 has an amu of 36.9659 with an
abundance of 24.229%
(34.9689 x 75.771%) + (36.9659 x 24.229%) =
The atomic mass of chlorine