1.3 Atomic Theory
• Early ideas about matter
• The Greek philosopher Democritus believed that matter
was made of atomos that were the smallest pieces of
matter.
• Atomos is a Greek word that means
UNCUTTABLE!
• Alchemists experimented with matter and
tried to turn common metals into gold.
Their activities marked the beginning of
our understanding of matter.
See pages 28 - 29
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Democritus
• Democritus claimed that everything is made up
of atoms.
• These atoms are physically indivisible;
• between atoms lies empty space;
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Democritus
• atoms are indestructible;
• have always been, and
always will be, in motion
• there are an infinite number
of atoms and kinds of
atoms, which differ in
shape, and size.
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Development of Atomic Theory I
• John Dalton (1766 - 1844)
• Credited with developing a theory that was a new way of
explaining matter.
• He studied gases that make up Earth’s atmosphere.
Based on his studies, he suggested that:
• matter is made of small, hard spheres
that are different for different
elements
• the smallest particle of an element
is called an atom
• This is the basis for Dalton’s Atomic Theory.
See page 29
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Dalton’s Atomic Theory
1. All matter is made of small particles called atoms.
2. Atoms cannot be created, destroyed, or divided into
smaller particles.
3. All atoms of the same element are identical in mass
and size, but they are different in mass and size
from the atoms of other elements.
4.
Compounds are created when atoms of different
elements link together in definite proportions.
See page 30
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Atomic Theory II
• J. J. Thomson (1856 - 1940)
• Thomson studied electric currents in gas discharge tubes (like
today’s fluorescent lights). From his studies, he determined that the
currents were streams of negatively charged particles. These were
later called electrons.
• He hypothesized that atoms are made
of smaller particles. He proposed
the “raisin bun” model of the atom.
• This model is best visualized as a
positively charged bun with negatively
charge particles spread out in it like raisins.
See page 30
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Atomic Theory III
• Ernest Rutherford (1871 - 1937)
• After experimenting with charged particles, he
found that some particles were deflected in
directions not originally predicted.
• He suggested that the deflection of the charged
particles was because the atom contained a tiny
dense centre called a
nucleus, and electrons
moved around the nucleus.
See page 31
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Atomic Theory IV
• Niels Bohr (1885 - 1962)
• He studied gaseous samples of atoms, which were
made to glow by passing an electric current
through them.
• Based on his observations, Bohr proposed
that electrons surround
the nucleus in specific
“energy levels” or “shells.”
See page 31 - 32
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The Planetary Model
• The Bohr Model is a planetary model in which
the negatively-charged electrons orbit a small,
positively-charged nucleus similar to the planets
orbiting the Sun (except that the orbits are not
planar).
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The Planetary Model
• The Bohr model shows that the electrons in
atoms are in orbits of differing energy around the
nucleus (think of planets orbiting around the
sun).
• Bohr used the term energy levels (or shells) to
describe these orbits of differing energy. He said
that the energy of an electron is quantized,
meaning electrons can have one energy level or
another but nothing in between.
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The Planetary Model
• Bohr suggested that electrons will occupy these
energy levels (shells) when surrounding the
nucleus.
• However, each shell that surrounds the nucleus
can ONLY hold a specific number of electrons.
• This turns out to be a very big deal for us
because it ultimately determines why and how
compounds are made.
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Specific # of Electron per shell
• Due to shell characteristics, Bohr determined
each shell around the nucleus can hold the
following:
• Shell 1 = 2 Electrons
• Shell 2 = 8 Electrons
• Shell 3 = 8 Electrons
• Shell 4 = 18 Electrons
• Shell 5 = 18 Electrons
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The Planetary Model
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The Planetary Model
• The energy level an electron normally occupies
is called its ground state. But it can move to a
higher-energy, less-stable level, or shell, by
absorbing energy. This higher-energy, lessstable state is called the electron’s excited state.
• After it’s done being excited, the electron can
return to its original ground state by releasing
the energy it has absorbed, as shown in the
diagram on the next slide!
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Energy Storage and Release
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Inside the Atom
• An atom is the smallest particle of an element that
retains the properties of the element.
• All atoms are made up of three kinds of particles called subatomic
particles. These particles are:
• electrons
• protons
• neutrons
Take the Section 1.3 Quiz
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See pages 32 - 33