Chemistry 11
Unit 9 Chemical Bonding #3
Name:____________________________
Block:_________
Electronegativity
Electronegativity is defined as the ability of an atom in a molecule to
.
Electronegativity is a function of two properties of isolated atoms:
•
The atom’s
(how strongly an atom holds onto its own
electrons)
•
The atom’s
(how strongly the atom attracts other electrons)
For example, an element which has a large (negative) electron affinity and a high ionization (always
endothermic, or positive for neutral atoms)…
…Will
from other atoms and
electrons attracted away
We say these atoms are “highly electronegative”
Electronegativity trends: across a period
Electronegativity
across a period because:
1.
2.
Electronegativity trends: down a group
Electronegativity
1.
2.
1
down a group because:
having
Electronegativity is dictated by:
across a period you are
, but filling electrons in
the same Bohr quantized
. You are only filling sub-shells, so electronegativity
increases from left to right
down groups, you are placing electrons into
moving further
, so
from the attractive power of the nucleus. Outer shell
becomes further away from the nucleus.
level of
upon bonding electrons increases down groups, and adds to the
reduction in electronegativity. Shielding is caused by
for each other.
Ions & Atomic Radius
to a neutral atom (eg O to make O-2) the ion has the
Negative Ions: As extra
(due to protons), and an
number of
surrounding the nucleus.
•
The electrostatic repulsion
•
The
occupied by the electrons
Positive Ions: As
from a neutral atom (eg Mg to make
Mg+2) the ion has the
(due to protons), and a
number of negative electrons surrounding the nucleus.
•
The electrostatic repulsion
•
The
occupied by the electrons
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Ionic Bonding Summary:
Electronegativity & Ionic Bonds
•
Formed between two atoms with
in their
ionization energies and
electronegativity's
•
An electronegativity difference of
can be
classified as an ionic bond
•
In this case we can essentially say
from one atom to another
Calculating Bonds with ∆𝑬𝑵 Values
ASSIGNMENT #3a
ASSIGNMENT #3a: complete the following questions
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ASSIGNMENT #3A: complete the following questions
Why do atoms form bonds?
Bond formation begins with atoms “
.”
For example as two hydrogen atoms approach each other, their
as each electron cloud is
to the other’s approaching
.
Atoms continue moving together until the
of the two
and the two positive nuclei slow the
atoms and convert their kinetic energy into
.
As the atoms get close to each other, their
cause
The two
enough to
to
ones.
electrons will move into the region of
space between the atoms nuclei.
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This
of a pair of valence electrons between two
adjacent nuclei constitutes a
.
What is a covalent bond?
elements usually just need one or two electrons to fill their outer shells. So
how do they form a bond?
The two non-metal atoms cannot form a bond by transferring electrons from one to another. Instead, they
.
The shared electrons join the atoms together. This is called a
How is a covalent bond drawn?
A covalent bond consists of a
of electrons.
Only
of
electrons are involved in bonding, so the inner shells do not always have to be included in diagrams.
Two common ways to represent a covalent bond are:
5
.
Comparing covalent and ionic bonding
Covalent Bonding
Ionic Bonding
Covalent bonding in hydrogen
Many non-metal elements, such as hydrogen, exist as simple
that
contain covalent bonds.
How is a covalent bond formed in hydrogen?
Each hydrogen atom needs
in its outer shell and so each atom
shares its
electron.
This shared pair of electrons forms a covalent bond and so creates a diatomic molecule of hydrogen.
What are the types of covalent bonds?
Molecule
Dot & Cross Diagram
Line Diagram
Can compounds contain covalent bonds?
Covalent bonding can also occur between atoms of
to create
molecules of
.
These covalent bonds can be
.
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How is a covalent bond formed in hydrogen chloride (HCl, also represented as H–Cl)?
Hydrogen and chlorine both need one more electron to fill outer shells. By sharing one electron each, they
both have a
and a covalent bond is formed.
Covalent bonding in water
Compounds can contain
.
Oxygen (2.6) needs
only needs
more electrons, but hydrogen [1]
more.
How can these three elements be joined by covalent bonding?
The oxygen atom
with 1 hydrogen atom, and a
with another hydrogen atom.
How is the ratio of atoms calculated?
To calculate the ratio of atoms in a stable covalent compound:
Covalent bonding in ammonia
How do nitrogen and hydrogen atoms form covalent bonds in a molecule of
Atomic Ratio
Line Diagram
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Dot & Cross (Bohr) Diagram
?
Covalent bonding in methane
Atomic Ratio
Line Diagram
Dot & Cross (Bohr) Diagram
Covalent bonding in carbon dioxide
Atomic Ratio
Line Diagram
Dot & Cross (Bohr) Diagram
A
is when
of electrons are shared. In carbon dioxide
there are two double bonds
– one between each oxygen atom and the carbon atom.
What are simple covalent structures?
Covalent molecules that contain only a few atoms are called
.
Most substances that contain simple covalent molecules have
and
and are therefore liquids or gases at room temperature, e.g. water,
oxygen, carbon dioxide, chlorine and hydrogen. Why?
The covalent bonds
molecules are
these molecules are
and easy to break.
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but the bonds
What is the structure of a molecular solid?
A few substances that contain simple covalent molecules are
at room temperature.
These are
.
is a molecular solid at room temperature.
Two iodine atoms form a
to
become an iodine molecule.
The solid is formed because millions of iodine molecules are
held together by
to create a 3D molecular lattice.
What properties would you expect molecular solids to have with this type of structure?
What are the properties of molecular solids?



The
between the molecules can be
.
This means that the molecular solids are
and
Molecular solids are also
are
by a
and melt and boil at
because there
or ions to carry a charge.
What are giant covalent structures?
In some substances, such as
millions of atoms are joined together by
,
and
,
bonds.
The covalent bonds in these substances do not form molecules but vast
called
All the bonds are covalent, so giant covalent structures have
boiling points, and are usually
.
What is the structure of sand?
Sand is mostly made of the mineral
is
covalent structure made up of
atoms.
and
, which
. It has a giant
and
Each silicon atom (2.8.4) is bonded to four oxygen atoms,
atom (2.6) is bonded to two silicon atoms.
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and each oxygen
What are the allotropes of carbon?
Both diamond and graphite are made up of
.
Different forms of the same element are called
.
These allotropes of carbon have
because the atoms are
bonded in
which create different giant structures.
What are the properties of
diamond?
What are the
properties of
graphite?
In diamond, all the electrons in the outer shell of
each carbon atom (2.4) are involved in forming
covalent bonds.
In graphite, only three of the four electrons in the
outer shell of each carbon atom (2.4) are involved
in covalent bonds.
This affects the properties of this allotrope of
carbon:

This affects the properties of this allotrope of
carbon:




What is electronegativity?
In a
between two different elements, the
is
This is because different elements have
This ability is called an element’s
Electronegativity values for some common elements.
Values given here are measured on the Pauling scale.
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.
.
Electronegativity and atomic radius
The electronegativity of an element depends on a combination of two factors:
1. Atomic radius
As radius of an atom
2. The number of unshielded protons
, the
The
bonding pair of electrons become further from
the number of
in a nucleus, the
the nucleus.
They are therefore
to the electrons in the
to covalent bond, resulting in
the positive charge of the nucleus, resulting in a
.
.
However,
energy levels of electrons
the electrons in the bond from the
increased attraction of the greater nuclear charge,
thus
electronegativity.
Types of Covalent Bonds:
We have discussed the two extreme cases of bonding:
- ionic bonding
And covalent bonds with completely
Between these extremes are covalent bonds which involve
electron sharing.
form covalent bonds, those ΔEN
When atoms with
values may be minimal or significant.
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Non-polar bonds
If the electronegativity of both atoms in a covalent bond is
, the electrons in the bond will
be
to
both of them, and form a
covalent bond.
This results in a
around the two
atoms.
Bonding in
(for example O2 or Cl2) is
because the electronegativity of the atoms in each molecule is
.
Mostly Covalent bonds
If ΔEN is
, the bonding electrons between the two atoms spend no more of their
time nearer one nucleus than the other.
Such bonds are designated as being “
” because ΔEN appears to be
Another way to characterize this is to say that these bonds have
Electronegativity and Covalent
Bonds
As ΔEN increases
,
the pair of bonding electrons will be
and closer to the nucleus of the atom with
the
.
This
distribution of electron density will give that end of the bond a
partially
A
and the other a partially
“
” is said to exist and the bond itself is known as a
ASSIGNMENT 3B: complete the following questions on a separate page
Hebden Questions pg 178 # 69-71
pg 179 # 72
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“pole.”
Effect of electronegativity on polarization
The
between the two atoms
in a bond the
of the bond.
This can be illustrated by looking at the hydrogen halides:
Ionic or covalent?
Rather than saying that ionic and covalent are two distinct types of bonding, it is more accurate to say that
they are at the two
.
Less polar bonds have more
More polar bonds have more
.
.
The
atom
the electrons in
the bond enough to ionize the other atom.
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Polar molecules
Molecules containing polar
bonds are not always polar.
Practice Problems:
Covalent Bonding
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ASSGINMENT #3C: Complete the following Review Questions
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SLIDE 165
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