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Physical
Science 9
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Atomic Structure
Chapter 3.1
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Chapter 3.1
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Physical
Science 9
Atomic Structure
Back in the 4th century, a Greek
Philosopher, Democritus, suggested that
the universe was made of invisible units
called ATOMS.
This theory was used more or less into
the 1700’s.
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Atomic Structure
“Atoms
“Every element
“Atoms
of
ofdifferent
the same
is made
elements
element
of tiny,
can
areunique
exactly
join to
form
particles
alike..”
molecules.”
called atoms that cannot be
subdivided.”
In 1908, John Dalton proposed his own
atomic theory. His theory was widely
accepted because there was much
evidence to support it.
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Less than 100 years later, scientists
were able to prove part of Dalton’s
theory incorrect. They proved that it is
possible to split atoms into “subatomic”
particles.
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Atomic Structure
It is now known that atoms have three
subatomic particles…
Protons, neutrons and electrons
Protons and Neutrons reside in
the Nucleus of the atom.
Electrons orbit around the
nucleus.
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Atomic Structure
Sub-atomic
Particle
Mass
Charge
Proton
1 amu
+1
Neutron
1 amu
0
Electron
Practically
Zero
-1
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Sub-atomic
Particle
Mass
Charge
Proton
1 amu
+1
Neutron
1 amu
0
Electron
Practically
Zero
-1
Protons and Neutrons both have
a mass of 1 amu (atomic mass
unit). The electrons have virtually
no mass, so the mass of the atom
is determined by the number of
protons and neutrons.
All atoms are electrically neutral (they have no charge). This means that
every atom MUST have the same number of protons and electrons. The
number of neutrons can vary because they do not affect the charge of
the atom.
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Atomic Structure
All lithium atoms have three
electrons and three protons.
The charge of the protons is 3+
(because there are 3 protons, each
with a charge of +1.
Lithium Atom
The charge of the electrons is 3-.
If you combine the charge of the
protons and electrons, the net
charge is zero.
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The mass of the lithium atom is 6
amu, since there are 3 protons
and 3 neutrons in the nucleus.
Lithium Atom
Remember, the electrons’ mass is
almost zero, so it does not
contribute to the mass of the
atom.
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The atom is held together by a
force known as the “Strong” or
nuclear force. This force is strong
enough to keep the protons from
repelling each other in the
nucleus.
Lithium Atom
The electrons are also held in
orbit around the nucleus by this
nuclear force, since the mass of
the nucleus does not have enough
mass to attract the electrons
using gravity.
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Atomic Structure
Chapter 3.1
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Physical
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Atomic Structure
Niels Bohr (not to be confused with I.M.
Boring) was the first to compare
electrons to planets.
His introduction of “energy levels”
helped to explain how atoms react to
one another.
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A Bohr diagram is a model of an atom
that shows the arrangement of the
elements in different energy levels.
This atom of aluminum has three
energy levels circling the nucleus.
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The first (or innermost) energy level of
an atom can only hold 2 electrons
before it’s full.
The 2nd energy level can hold up to
eight electrons before it’s full.
The 3rd energy level can hold up to 18
electrons.
The formula for calculating how many
electrons an energy level can hold is…
E = 2n2
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…where E is the number of
electrons and n is the energy
level number.
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Atomic Structure
Using the formula, how many electrons
can the 4th energy level hold?
E = 2n2
The answer is…
32
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You are now
officially smarter
than this guy!
Congratulations!
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Physical
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Atomic Structure
Although Bohr’s model of the atom was
a breakthrough, by 1925 it no longer
was able to explain some of the
observations made by scientists.
Electrons did reside in energy levels,
but not in circular orbits as in Bohr’s
model.
It is now believed that electrons behave
more like waves on a vibrating “string”
then like particles orbiting the nucleus.
X
This concept is known as
the wave-particle duality of
nature.
The smaller the particle, the more it acts like a wave instead of a particle.
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When a fan blade is rotating,
you cannot see the individual
blades as they turn. It is not
possible to tell where exactly
each blade of the fan is, but
you can say in what area the
blades are.
Electrons are the same. You can
never know where exactly any
electron is, but you can limit the
area in which it is most of the
time.
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The region in which an
electron is likely to be is called
an orbital.
There are four types of
orbitals…
s, p, d and f
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Electrons generally fill the
lowest energy levels first.
s, p, d and f
Within that energy level,
electrons occupy the lowest
orbital first. Orbital s has the
lowest energy level, followed by
p, d, and f.
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s orbital – holds two electrons
p orbital – holds six electrons
d orbital – holds 10 electrons
f orbital – holds 14 electrons
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Valence Electrons
Valence electrons are electrons on the OUTERMOST energy level of
an atom. For reasons you don’t yet understand, the most valence
electrons an atom can have is 8.
This Carbon atom has a total of 6
electrons…two in the 1st energy level,
and four in the 2nd energy level.
The 2nd energy level is the outermost
energy level, so ALL Carbon atoms
have four VALENCE electrons.
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Valence Electrons
How many valance electrons does this Sodium atom have?
The answer is… 1 (in the 3rd
energy level).
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We’re Done with 3.1!
Chapter 3.1
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