Theories of the Atom
Theories of the Atom
Democritus: The Atom
(around 400 BCE)
• Democritus thought matter could be
divided into smaller and smaller
pieces until a single indivisible
particle is reached
• He named this particle the atom
• He proposed that atoms are of
different sizes, in constant motion,
and separated by empty spaces.
• Democritus thought that the atom
was indivisible.
• The word atom means “cannot be
cut”
Theories of the Atom
Aristotle: Earth,
Water, Air, and Fire
(around 450 BCE)
–Aristotle rejected the
idea of the atom. He
proposed four basic
elements: earth, water,
air, and fire.
Theories of the Atom
John Dalton: The Billiard Ball
Model (1807)
• all matter is made up of tiny,
indivisible particles called atoms
• all atoms of an element are identical
• atoms of different elements are
different
• atoms are rearranged to form new
substances in chemical reactions but
they are never created or destroyed
The atom is a
tiny, solid,
indestructible
sphere
Theories of the Atom
J. J. Thomson: The Plum
Pudding Model (1897)
• Thomson used a device
called a cathode ray
tube to conduct his
experiments
• The particles he
detected were attracted
to the positive end of
the circuit, so they had
to be negatively
charged.
Theories of the Atom
J. J. Thomson: The Plum
Pudding Model (1897)
– Thomson theorized that atoms
contain negatively charged
particles called electrons.
– He believed that since atoms are
neutral, the rest of the atom is a
positively charged sphere.
Theories of the Atom
Theories of the Atom
Ernest Rutherford: The Nucleus
and the Proton (1909)
• Rutherford expected the particles to pass undeflected
through the atoms
• However, a small fraction of
the -particles were deflected
• To deflect the energetic particles, the nucleus must be
dense, with positive charge
• Rutherford theorized that the
centre, or nucleus, of the atom
consists of positively charge
particles called protons.
• the positively charged nucleus
is surrounded by a cloud of
negatively charged electrons,
and the atom is mostly empty
space.
Theories of the Atom
James Chadwick:
The Neutron (1932)
–Chadwick revised
Rutherford’s theory, and
proposed that the
nucleus contains
positively charged
protons and neutral
particles called
neutrons.
In the planetary model of the atom,
electrons orbit the nucleus the way
planets orbit the Sun in our solar
system.
Theories of the Atom
Niels Bohr:
Electron Orbits
(1913)
– studied the
hydrogen atom
and the light it
produces when
excited (heated,
electrified)
Theories of the Atom
Niels Bohr:
Electron
Orbits (1913)
Theories of the Atom
Niels Bohr:
Electron
Orbits (1913)
Electrons are
located in
specific orbits
(energy levels,
shells).
Bohr revised the atomic
theory to include these
points:
Each orbit can
hold a certain
maximum number
of electrons: 2 in
the first orbit, 8 in
the second, and 8
in the third.
Electrons can
jump from orbit
to orbit. They
release energy as
light when they
jump from higher
to lower orbits.
Each electron in an
orbit has a specific
amount of energy.
The farther the
electron is from the
nucleus, the higher
its energy.
Homework:
• Worksheet: Development of the Atomic Theory
• pg. 233 #2, 3, 5