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Chapter 4: Atomic Structure
 4.1: Studying Atoms
 4.2: The Structure of an Atom
 4.3: Modern Atomic Theory
4.1: Studying Atoms
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Greeks
Dalton
Thomson
Rutherford
Atomic Model
Ancient Greek Models of Atoms
 Democritus believed that all matter
consisted of extremely small particles that
could not be divided.
Ancient Greek Models of Atoms
 Democritus called these particles
atoms from the Greek word atomos,
which means “uncut” or “indivisible.”
 He thought there were different types
of atoms with specific sets of
properties
Aristotle
 Thought that all substances were built up
from only four elements.
Dalton’s Atomic Theory
 Proposed the theory that all matter is
made up of individual particles called
atoms, which cannot be divided.
 Compounds contain atoms of more
than on element.
 In a particular compound, atoms of
different elements always combine in
the same way.
Dalton’s Experiment
 The ratio of the masses of the
elements in the compound is always
the same.
 A 100 gram sample of magnesium
combines with 65.8 grams of oxygen.
 A 10 gram sample of magnesium
combines with 6.58 grams of oxygen.
Dalton’s Model
 The elements are pictured as solid spheres.
Thomson’s Experiment
Thomson’s Experiment
 Thomson’s experiments provided the first
evidence that atoms are made of even
smaller particles.
Thomson’s Model (plum pudding)
 In an atom, the negative charges were
evenly scattered throughout an atom filled
with a positively charged mass of matter.
Rutherford’s Experiment
Rutherford’s Gold Foil Experiment
Rutherford’s Model
 All of an atom’s positive charge is
concentrated in its nucleus.
 The nucleus is a dense, positively charged
mass located in the center of the atom.
Pretending to be
Alpha Particles Mv.
4.2:Structure of an Atom
 Protons, electrons, and neutrons are subatomic
particles.
 Because these particles are even smaller than an
atom, they are called subatomic particles.
 These three types of particles are arranged in an
atom as shown below.
What makes one element different
from another if?
 All protons are the same.
 All electrons are the same.
 All neutrons are the same.
Protons
 The number of protons
distinguishes an atom of one element
from the atom of another element.
 All atoms of the same element will
have the same number of protons,
and atoms of different elements will
have different numbers of protons.
Properties of Subatomic Particles
 Protons, electrons, and neutrons can
be distinguished by mass, charge,
and location in an atom.
Atomic Number
 The number of protons in an atom
of that element.
 Because the number of protons in an
atom remains the same during
physical and chemical changes, we
can refer to each element by the
number of protons its atoms contain.
This unique number is called the
atomic number.
Atomic Number
 Because atoms have an overall
neutral charge, atoms have an equal
number of protons & electrons.
# protons = # electrons
Mass Number
 the sum of the protons and neutrons in the
nucleus of that atom.
Isotopes
 atoms of the same element that have
different numbers of neutrons and
different mass numbers.
Isotopes
4.3 Modern Atomic Theory
"D'oh!"
Bohr’s Atomic Model Focused on
the Electrons
1803: John Dalton
 Atoms are tiny,
indestructible
particles, with no
internal structure.
1897: JJ Thomson
 Discovers the
electron
 He pictures
electrons
embedded in a
sphere of positive
electric charge.
1911: Ernest Rutherford
 Atoms have a
dense, positively
charged nucleus.
 Electrons move
randomly around
nucleus.
 Gold foil
experiment
1913: Niels Bohr
 Electrons move in
spherical orbits at
fixed distances
from the nucleus.
1926: Electron Cloud Model
 A visual model of
the probable
locations of
electron in an
atom.
1932: James Chadwick
 Confirms the
existence of
neutrons in the
nucleus.
 Neutrons have no
charge.