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Development of the Atomic
Theory
Democritus 460 BC - Greek philosopher proposes the
existence of the atom ; calls it Atomos meaning indivisible.
His theory: all atoms are small hard particles made of a single material formed into different
shapes and sizes always moving, and that they form different
materials by joining together
Democritus
• Democritus model was a small sphere that was invisible
John Dalton 1803 - British chemist; elements
combined in specific proportions to form compounds.
His theory: all substances are made of atoms that
cannot be created, divided, or destroyed atoms join with other atoms to make new
substances atoms of the same element are exactly
alike, and atoms of different elements are different in mass and size (elements)
John Dalton
• Dalton’s model was a solid sphere that was invisible
Edward Frankland 1852 - English chemist developed the
valence theory
His theory: every atom has a fixed number of bonds
(chemical links) that it can form for the atom to be stable, all of these
bonds must be used.
J.J. Thomson 1897 - English chemist and physicist
discovered 1st subatomic particles
His theory: negatively charged particles called
electrons and positively charged matter created a model to describe the atom as
a sphere filled with positive matter with negative particles mixed in
Referred to model as the plum pudding model
J.J. Thomson
• Thomson’s model was a sphere filled with positive matter and negative particles.
Ernest Rutherford 1912 - New Zealand physicist
discovered the nucleus
His theory: small, dense, positively charged particle
present in nucleus called a proton electrons travel around the nucleus, but
their exact places cannot be described
Ernest Rutherford
• Rutherford’s model had a nucleus of protons surrounded by electrons.
Niels Bohr 1913 - Danish physicist discovered
energy levels
His theory: electrons travel around the nucleus in
definite paths and fixed distances; orbits electrons can jump from one level to a
path in another level
Niels Bohr
• Bohr’s model had a nucleus surrounded by electrons orbits.
Erwin Shrodinger 1924 - Austrian physicist developed
the electron cloud model
His theory: electrons exact path cannot be
predicted regions, referred to as the electron
cloud, are areas where electrons can likely be found.
Erwin Shrodinger
• Shrodinger's model had a nucleus surrounded by an electrons cloud.
James Chadwick
1932 - English physicist discovered neutrons
His theory: neutrons have no electrical charge neutrons have a mass nearly equal to
the mass of a proton unit of measurement for subatomic
particles is the atomic mass unit (amu)
James Chadwick
• Chadwick's model had a complete nucleus with protons and neutrons.
Modern Theory of the Atom Atoms are composed of three main
subatomic particles: the electron, the proton, and the neutron.
Most of the mass of the atom is concentrated in the nucleus of the atom.
The protons and neutrons are located within the nucleus while the electrons exist outside of the nucleus.
In stable atoms, the number of protons is equal to the number of electrons.
Modern Theory of the Atom The type of atom is determined by the
number of protons it has.
The number of protons in an atom is equal to the atomic number
The sum of the number of protons and neutrons in a particular atom is called the atomic mass
Valence electrons are the outermost electrons and are where bonding takes place
Any Questions?