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The Atom
What is an atom?
• An atom is the smallest part of an element
• The atom remained mostly a mystery because
it is unable to be seen with even a microscope
• Over time there have been many models of
what an atom looks like
Ancient Greek Model
• Greek philosophers asked “What would
happen if you kept cutting a piece of paper in
half until there is nothing left?”
• They believed that at some point a single,
smallest particle, that could not be divided
would remain
Ancient Greek Model
• The Greek model was not highly accepted at
the time
• Aristotle refuted these claims and most
people agreed with his argument
Dalton’s Atomic Theory
• In 1766, Dalton concluded that gas is made up
of billions of particles
• He gathered his evidence by measuring the
masses of elements in compound form
Dalton’s Atomic Theory
• He concluded that no matter how large or
small the sample of a compound is, the ratio
of the masses of the elements is always the
• Compounds have fixed compositions
Dalton’s Atomic Theory
• Dalton developed a theory –
“All matter is made up of individual particles
called atoms, which cannot be divided.”
Dalton’s theory cont.
• All elements are composed of atoms
• All atoms of the same element have the same
mass, and atoms of different elements have
• Compounds contain atoms of more than one
• In particular compounds, atoms of different
elements always combine in the same way
Dalton’s Atomic Model
Thomson’s Atomic Model
• In 1856, J.J Thomson used an electrical current
to learn more about the atom
• He used an electrical current, metal discs, and
a beam of light to study his theory
Thomson’s Atomic Model
• He found that particles in a beam of light have
a negative charge
• He found that these particles of light were
even smaller than an atom
Thomson’s Atomic Model
• Thomson’s experiments provided the first
evidence that atoms are made up of even
smaller particles
• We call these “Subatomic Particles”
Thomson’s Atomic Model
• Thomson concluded that an atom has a
neutral charge (Neither positive, nor negative)
• He designed an atom that has negative
charges scattered throughout a positively
charged mass
Thomson’s Atomic Model
Rutherford’s Atomic Theory
• In 1899 Rutherford discovered that uranium
emits fast moving particles that have a
positive charge
• He named theses particles “Alpha particles”
• He studied what would happen to alpha
particles as they pass through a thin sheet of
Rutherford’s Atomic Theory
• 1/20,000 alpha particles were deflected 90
degrees off of the gold
• The rest bounced straight off of the gold foil
Rutherford’s Atomic Theory
• Rutherford concluded that the deflected alpha
particles must have come close to a charged
• After studying this theory he concluded that
atoms contain a nucleus
The Nucleus
• A dense, positively charged mass located at
the center of an atom
• According to Rutherford, all of an atom’s
positive charge is concentrated in its nucleus