Download 2.3 Periodic Table and Atomic Theory Bohr Diagrams

2.3 Periodic Table and Atomic
Theory
Bohr Diagrams
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Diagrams that shows how may electrons are in each
shell of an atom
Each shell can only hold a maximum number of
electron
Shell 1 = 2
– Shell 2 = 8 (total 10)
– Shell 3 = 8 (total 18)
– **Must fill each shell
completely before filling the next**
–
Bohr Diagram
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Stable Octet - has a full
outer shell
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Valence Shell - Outer
shell
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Valence electrons electrons in outer shell
Drawing a Bohr Diagram:
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1. Find out the # of protons, neutrons and
electrons
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–
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Remember…
Atomic # = # of protons & electrons (in an atom)
Mass # - Atomic # = # of neutrons
2. Start filling electrons in shells starting from the
inner most working outward
Bohr Diagram
# of protons = atomic #
# of protons = 15
15P
16N
# of electrons = # of protons
# of electrons = 15
# of neutrons = mass # - atomic #
# of neutrons = 31-15
# of neutrons = 16
Practice Together…
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Bohr model for:
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Sodium
Chlorine
You try…
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Lithium
Fluorine
Noble Gas Stability
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Noble gases are the most unreactive elements
Their atoms have full valence shells
–
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(they contain the maximum possible number of electrons
without going to the next shell)
When elements combine, they are trying to gain a
full outer shell of electrons by either giving away their
electrons to another element, or taking electrons
from another element.
Noble gases are happy the way they are – they do
not want to gain or lose electrons. This is why they
do not react easily with other elements!
Noble Gas Stability
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Other elements either gain or lose electrons
to try to become like the noble gas nearest to
them on the periodic table.
Non-metal atoms will gain electrons.
Metal atoms will lose electrons.
How Atoms become Ions
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
Atoms become ions by gaining or losing
electrons.
When this happens, the number of protons
and electrons becomes unequal.
–
Remember that a neutral atom (one with no
charge) has an equal number of protons and
electrons.
Example…

Lithium tends to form a +1 ion.
–
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It has 1 valence electron that it loses to become
like the closest noble gas on the periodic table,
Helium
A neutral lithium atom has 3 protons and 3
electrons
An Li+ ion has 3 protons and 2 electrons
–
Less electrons = less negatives = +1 charge
Example…

Oxygen tends to form a -2 ion.
–
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It is missing 2 electrons in it’s valence shell, by
gaining two electrons, oxygen becomes like the
closest noble gas on the periodic table, Neon
A neutral oxygen atom has 8 protons and 8
electrons.
An O-2 atom has 8 protons and 10 electrons.
–
It gained 2 electrons = more negatives = -2 charge