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Atoms
Chapter 4
Section 1: “The Development of
the Atomic Theory”

The atom is the smallest unit of all
matter.
Section 1: “The Development of
the Atomic Theory”
Who: Democritis
When: 4th century B.C.
What: Suggested that
the universe was made
of indivisible units.
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He called these units
atoms. “Atom” comes
from atomos, a Greek
word that means “unable
to be cut or divided.”
Section 1: “The Development of
the Atomic Theory”
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Who: John Dalton
When: 1803
What: Created the Atomic
Theory:
1.
2.
3.
Every element is made of
tiny, unique particles called
atoms that cannot be
subdivided.
Atoms of the same element
are exactly alike.
Atoms of different elements
can join to form molecules.
Section 1: “The Development of
the Atomic Theory”
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Who: J.J. Thomson
When: 1897
What: Used a cathode-ray tube experiment to
discover negatively charged subatomic particles called
electrons.
Section 1: “The Development of
the Atomic Theory”

What: Proposed a new model of an atom called the
plum-pudding model.
Section 1: “The Development of
the Atomic Theory”
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Who: Ernest Rutherford
When: 1911
What: Used the gold foil experiment to propose that
most of the mass of the atom was concentrated at the
atom’s center (the nucleus).
Section 1: “The Development of
the Atomic Theory”
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Rutherford’s model
disclaimed
Thomson’s plumpudding model.
Section 1: “The Development of
the Atomic Theory”
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Who: Niels Bohr
When: 1922
What: Developed
Bohr Model of the
atom.
Section 1: “The Development of
the Atomic Theory”
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Who: James
Chadwick
When: 1932
What: Discovered
the neutron.
Section 1: “The Development of
the Atomic Theory”

Quantum Mechanical Look at the Atom
Section 2: “The Structure of
Atoms”
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Subatomic Particles of an Atom
Subatomic
Particle
neutron(no)
Location
Charge
Mass
nucleus
0
1.67x10-27
kg (1 amu)
proton(p+)
nucleus
+1
electron(e-)
electron cloud
surrounding nucleus
-1
1.67x10-27
kg (1 amu)
9.11 x10-31
kg
Section 2: “The Structure of
Atoms”
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Electric force from the attraction between
the positive proton and negative electron
holds the atom together.
Section 2: “The Structure of
Atoms”
The overall charge electric charge of an atom is
neutral or 0. This is because the number of protons in
an atom is equal to the number of electrons.
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This hydrogen atom has 1 proton and 1 electron (+1 + -1 = 0)
Section 2: “The Structure of
Atoms”
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Atomic number is the number of
protons in the nucleus of an atom. Each
element has a unique number of protons.
Mass number of an atom is the sum of
the numbers of protons and neutrons in
the nucleus of an atom.
Atomic mass is the average weighted
mass of an element ’s naturally occurring
differences in the number of neutrons.
Section 2: “The Structure of
Atoms”
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Atomic Number (Z) =
6
Mass Number (A) =
12
Section 2: “The Structure of
Atoms”
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Atomic Number (Z) =
1
Atomic Mass (A) =
1.00794 amu
Mass Number (A) =
1
# of protons =
1 (same as Z)
# of electrons =
1 (same as Z)
# of neutrons =
0 (# of n0 = A – Z)
Section 2: “The Structure of
Atoms”
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An isotope is an atom that has the same number of protons as
other atoms of the same element do but that has a different
number of neutrons (and a different atomic mass)
Because isotopes have the same number of protons and
electrons, they generally have similar chemical properties.
Section 2: “The Structure of
Atoms”
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On the periodic table,
the atomic mass
displayed is an average
of the atomic masses of
all of that elements
isotopes.
It is a weighted average
so the mostly
commonly found
isotopes have the most
impact on the mass.
Section 2: “The Structure of
Atoms”
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A mole is the SI base unit that describes
the amount of a substance.
Avogadro’s constant is the number of
particles in 1 mole of a substance and is
equal to 602213670000000000000000
or 6.022 x 1023.
Section 2: “The Structure of
Atoms”
The molar mass is the mass in grams of 1 mol of a
substance.
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This is equal to the atomic mass of atoms measured in grams
rather than amu’s.
Example: The atomic mass of magnesium is equal to 24.30
amu. The molar mass of magnesium is equal to 24.30 g.
Section 3: “The Modern Atomic
Theory
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Electrons determine charge in an atom.
Section 3: “The Modern Atomic
Theory
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Energy levels are
areas where
electrons can exist
around the nucleus
of an atom.
Section 3: “The Modern Atomic
Theory
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The more energy an
electron has the
higher the energy
level they can exist
on.
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Electrons move from
one level to the next
by gaining or losing
energy.
Section 3: “The Modern Atomic
Theory
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Each energy level holds a specific
maximum number of electrons:
1st = 2
2nd = 8
3rd = 18
4th = 32
Section 3: “The Modern Atomic
Theory
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Valence electrons are electrons that are
found in the outermost shell of an atom and
that determine the atom’s chemical properties
and reactivity.
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The octet rule says that once an atom has 8
valence electrons, it will no longer be reactive (with
the exception of the first energy level which only
needs 2).
Section 3: “The Modern Atomic
Theory
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An orbital is a region in an atom where
there is a high probability of finding
electrons.
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Niels Bohr proposed that an atom’s
electrons move around an orbital in a fixed
path like planets around the sun.
Modern atomic theory says that rather than
moving in fixed paths, electrons move like
waves on a vibrating string where its exact
location cannot be determined.
Section 3: “The Modern Atomic
Theory
Section 3: “The Modern Atomic
Theory
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An “s” orbital is shaped like a sphere and can
hold a maximum of 2 electrons.
Each “p” orbital is shaped like a bar bell.
There are 3 different types that can each hold
2 electrons. The “p” orbital, therefore, can hold
up to 6 electrons.
“d” and “f” orbitals are more complex. There
are 5 types of “d” orbitals and 7 types of “f”
orbitals each that hold 2 electrons each.
Therefore, in total they hold 10 and 14
electrons respectively.
Section 3: “The Modern Atomic
Theory
Section 3: “The Modern Atomic
Theory