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Chemistry 11
Chemical Bonding
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Electrostatic Forces
An electrostatic force is a
forces existing as a result of
the attraction or repulsion
between 2 charged particles
All bonding is based on the experimentally-derived
relationships of electrostatics:
• Opposite charges attract (+ -)
• Like charges repel (+ + or - -)
• The greater the distance between two charged particles, the smaller
the attractive force
• The greater the charge on two particles, the greater the force of
attraction (or repulsion) between them
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Electron Shells REVISITED!
The periodic table shows that patterns in the properties of
elements are linked to atomic number.
What links atomic number and the properties of elements?
Electrons!
atomic number = number of protons
number of protons = number of electrons
atomic number = number of electrons
As atomic number increases by one, the number of electrons
also increases by one.
This means that the elements in the periodic table are
also arranged in order of the number of electrons.
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What is periodicity?
The term periodicity describes a repeating pattern in
properties of elements across periods of the periodic table.
The Russian chemist Dmitry
Mendeleev is credited with being the
creator of the first version of the
periodic table. He observed that when
the elements are arranged in order of
atomic mass, there are recurring
patterns in certain properties.
The modern periodic table can be used
to analyse trends in properties such as
Atomic Radius across periods and
down groups.
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Atomic Radius Trends
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What is atomic radius?
The atomic radius of an element is difficult to precisely define
because of the uncertainty over the size of the electron cloud.
Several definitions are used.
One definition is half the shortest internuclear distance found
in the structure of the element.
For non-metallic elements, the
covalent radius is often used
as the atomic radius. This is half
the internuclear distance
between two identical atoms in
a single covalent bond.
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covalent
radius
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More on atomic radius
For non-bonded adjacent atoms (e.g. in a covalent crystal of
a non-metallic element), the van der Waals radius is used as
a value for atomic radius. This is half the shortest internuclear
distance between two similar non-bonded atoms.
van der
Waals radius
For metallic elements, the metallic radius is often used as
the atomic radius. This is half the shortest internuclear
distance between two adjacent atoms in a metallic bond.
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Trends in atomic radius in period 3
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Trends in atomic radius in period 3
Atomic
Element
radius (nm)
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Na
0.190
Mg
0.145
Al
0.118
Si
0.111
P
0.098
S
0.088
Cl
0.079
Ar
0.071
The atomic radius of the elements
across period 3 decreases.
This might seem counterintuitive, because as the
numbers of sub-atomic particles
increase, the radius might be
expected to also increase.
However, more than 99% of the
atom is empty space – the
nucleus and electrons themselves
occupy a tiny volume of the atom.
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WHY? Increase in proton number
The number of protons in the nucleus of the atoms
increases across period 3.
proton
number
Element
11Na 12Mg 13Al
14Si
15P
16S
17Cl
18Ar
This increase in the number of protons increases the
positive nuclear charge of the atoms. The nucleus has
stronger attraction for the electrons, pulling them in
closer and so the atomic radius decreases across the
period.
increased
nuclear charge
pulls electrons
closer
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What is shielding?
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Explaining atomic radius in period 3
Atomic
Proton
Element number radius
(nm)
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Na
11
0.190
Mg
12
0.145
Al
13
0.118
Si
14
0.111
P
15
0.098
S
16
0.088
Cl
17
0.079
Ar
18
0.071
Proton number increases
across period 3, but
shielding remains
approximately constant.
This causes an increase
in effective nuclear
charge, leading to a
greater attraction
between the nucleus and
the outermost electrons.
This pulls these electrons
closer to the nucleus and
results in a smaller radius.
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How are electrons arranged?
Electrons are arranged in shells around an atom’s nucleus.
(also called energy levels-principle quantum number n1).
Each shell has a maximum number of electrons that it can
hold. Electrons will fill the shells nearest the nucleus first.
1st shell holds
a maximum of
2 electrons
2nd shell holds
a maximum of
8 electrons
3rd shell holds
a maximum of
18 electrons
This electron arrangement is written as 2,8,18.
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Electrons in period 1
Elements in period 1 only have electrons in the first shell.
Why are there only two elements in period 1?
1
1
2
3
4
5
6
7
0
H
He
1
2
The first shell can only hold a maximum of two electrons,
so period 1 only includes the elements hydrogen and helium.
What is special about the outer shell of helium?
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Electrons in period 2
Elements in period 2 all have a complete first shell.
What happens to electrons in the second shell in period 2?
2
1
2
3
4
5
6
7
0
Li
Be
B
C
N
O
F
Ne
2,1
2,2
2,3
2,4
2,5
2,6
2,7
2,8
The second shell is completed one electron at a time going
across the period from left to right.
What is special about the outer shell of neon?
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Electrons in period 3
Elements in period 3 have complete first and second shells.
What happens to electrons in the third shell in period 3?
1
2
3
4
5
6
7
0
3
Na Mg
Al
2,8,1 2,8,2 2,8,3
Si
P
S
Cl
Ar
2,8,4
2,8,5
2,8,6
2,8,7
2,8,8
The third shell is completed one electron at a time going
across the period from left to right.
What is special about the outer shell of argon?
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What are the patterns of electron arrangement?
Consider the electron arrangements of the first 20 elements
in the periodic table.
1
2
3
4
5
6
7
0
2
1
1
2
2,1
2,2
2,3
2,4
2,5
2,6
2,7
3
2,8,1
2,8,2
2,8,3
2,8,4
2,8,5
2,8,6
2,8,7 2,8,8
4
2,8,8,1 2,8,8,2
2,8
*Remember…will fill 4s2 before we complete the 3d subshell
What is the pattern of outer shell electrons in a group?
What is the pattern of outer shell electrons across a period?
What is the pattern of full electron shells in a group?
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Electron trends in the periodic table
Trends down a group:
 the number of outer shell electrons is the same;
 the number of complete electron shells increases by one.
The number of a group is the same as the number of
electrons in the outer shell of elements in that group,
except for group 0.
Trends across a period:
 the number of outer shell electrons increases by one;
 the number of complete electron shells stays the same.
The point at which a new period starts is the point at
which electrons begin to fill a new shell.
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Valence Electrons Revisited
Valence electrons are:
 All electrons in an atom except those in the [core] or in
filled d- or f- subshells / orbitals
 All electrons in ‘open shells’
An open shell is a shell containing less than it’s
maximum number of electrons
Valence electrons are considered reactable electrons
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Electrons and groups
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Glossary
 atomic number – The number of protons in an atom.
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Sometimes called the proton number.
electron arrangement – A shorthand way of writing the
number of electrons in an atom’s electron shells.
element – A substance made up of only one type of atom.
group – A column in the periodic table containing elements
with the same number of outer shell electrons and similar
chemical properties.
period – A row in the periodic table containing elements
with the same number of full electron shells.
periodic table – The table that lists all the elements in
order of increasing atomic number, arranged into groups and
periods.
property – Any characteristic of an element.
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Complete Assignment #1
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