Electronegativity - Miss Zukowski's...
Transcript of Electronegativity - Miss Zukowski's...
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Electronegativity is defined as the ability of an atom in a
molecule to attract electrons to itself.
Electronegativity is a function of two
properties of isolated atoms:
• The atom’s ionization energy (how strongly
an atom holds onto its own electrons)
• The atom’s electron affinity (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 attract electrons from other atoms and resist having
electrons attracted away
We say these atoms are “highly electronegative”
Electronegativity
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Electronegativity trends: across a period
Electronegativity increases across a period because:
1. The atomic radius decreases.
2. The charge on the nucleus increases without
significant extra shielding. New electrons do not
contribute much to shielding because they are added
to the same principal energy level across the period.
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Electronegativity trends: down a group
2. Although the charge on the nucleus increases,
shielding also increases significantly. This is
because electrons added down the group fill new
principal energy levels.
Electronegativity decreases down a group because:
1. The atomic radius increases.
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Electronegativity is dictated by:
The number of protons in the nucleus
• across a period you are increasing the number of protons, but filling electrons in the same Bohr quantized energy level. You are only filling sub-shells, so electronegativity increases from left to right
The distance from the nucleus
• down groups, you are placing electrons into new quantizedenergy levels, so moving further away from the attractivepower of the nucleus. Outer shell becomes further awayfrom the nucleus.
The amount of shielding by the inner electrons
• level of shielding upon bonding electrons increases down groups, and adds to the reduction in electronegativity. Shielding is caused by repulsion of electrons for each other.
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Ions & Atomic Radius
Negative Ions
As extra electrons are added to a neutral atom
(eg O to make O-2) the ion has the same positive
nuclear charge (due to protons), and an
increased number of electrons surrounding the
nucleus.
• The electrostatic repulsion increases
• The volume occupied by the electrons
increases
NEGATIVE IONS are LARGER
than the corresponding neutral atom
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Ions & Atomic Radius
Positive Ions
As electrons are removed from a neutral atom
(eg Mg to make Mg+2) the ion has the same
positive nuclear charge (due to protons), and a
decreased number of negative electrons
surrounding the nucleus.
• The electrostatic repulsion decreases
• The volume occupied by the electrons
decreases
POSITIVE IONS are SMALLER
than the corresponding neutral atom
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Electronegativity & Ionic Bonds
• Formed between two atoms with large
differences in their ionization energies
and electronegativity's
• An electronegativity difference of
greater than 1.6 can be classified as an
ionic bond
• In this case we can essentially say
electrons are transferred from one
atom to another
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Glossary
bond – A strong force that joins atoms or ions together in molecules and giant lattices.
compound ion – An ion made up of a group of atoms, rather than one single atom.
ionic bond – The electrostatic force of attraction between oppositely charged ions.
ionic compound – A compound made up of ions.
ionic lattice – A giant 3D structure of closely packed, oppositely-charged ions.
negative ion – An atom or group of atoms that has gained electrons and so has a negative charge.
noble gas – An element that has a full outer electron shell and so is very stable and unreactive.
positive ion – An atom or group of atoms that has lost electrons and so has a positive charge.
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Why do atoms form bonds?
Shared electron pair = covalent bond
Bond formation begins with atoms “colliding.”
For example as two hydrogen atoms approach each other, their
kinetic energy increases as each electron cloud is attracted
to the other’s approaching positive nucleus.
Atoms continue moving together until the repulsive forces of
the two negative electron clouds and the two positive nuclei
slow the atoms and convert their kinetic energy into potential
energy.
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Why do atoms form bonds?
As the atoms get close to each other, their electron clouds
overlap enough to cause attractive forces to exceed
repulsive ones.
The two valence electrons will move into the region of space
between the atoms nuclei.
Shared electron pair = covalent bond
This force of attraction of a pair of
valence electrons between
two adjacent nuclei constitutes a
single covalent bond.
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Why do atoms form bonds?
The atoms of noble gases have completely
full outer shells and so are stable.
The atoms of other elements have
incomplete outer electron shells
and so are unstable.
This makes the noble gases very unreactive
and so they do not usually form bonds.
By forming bonds, the atoms of these
elements are able to have filled outer
shells and become stable.
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What is a covalent bond?
Non-metal elements usually just need one or two electrons
to fill their outer shells. So how do they form a bond?
The shared electrons join the atoms together. This is called
a covalent bond.
The two non-metal atoms cannot form a bond by transferring
electrons from one to another. Instead, they share electrons.
Each atom now
has a full, stable
outer shell.
incomplete outer shells
Cl
Cl
Cl
Cl
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Two common ways to represent a covalent bond are:
Only outer shells of electrons are involved in bonding, so the
inner shells do not always have to be included in diagrams.
solid
line
Cl Cl–
–Cl Cl
A covalent bond consists of a shared pair of electrons.
covalent bond
Cl
simplified dot and cross
diagram
Cl
How is a covalent bond drawn?
Cl Cl
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How are covalent bonds formed?
How do non-metal atoms form covalent bonds?
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Covalent bonding in hydrogen
Many non-metal elements, such as hydrogen, exist as
simple diatomic molecules that contain covalent bonds.
How is a covalent bond formed in hydrogen?
Some molecules contain double or triple covalent bonds.
How are these are formed?
H H H
Each hydrogen atom needs one more electron in its outer
shell and so each atom shares its single unpaired electron.
H
This shared pair of electrons forms a covalent bond and so
creates a diatomic molecule of hydrogen.
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Can compounds contain covalent bonds?
Covalent bonding can also occur between atoms of different
non-metals to create molecules of covalent compounds.
These covalent bonds can be single, double or triple.
How is a covalent bond formed in hydrogen chloride (HCl,
also represented as H–Cl)?
H ClH Cl
Hydrogen and chlorine both need one more electron to fill
outer shells. By sharing one electron each, they both have
a stable outer shell and a covalent bond is formed.
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Covalent bonding in water
Compounds can contain more than one covalent bond.
The oxygen atom shares 1
electron with 1 hydrogen
atom, and a second
electron with another
hydrogen atom.
H H
O
Oxygen (2.6) needs 2 more electrons, but hydrogen [1] only
needs 1 more. How can these three elements be joined by
covalent bonding?
What is the name of the molecule that is formed?
H2O (or H–O–H) is water.
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How is the ratio of atoms calculated?
To calculate the ratio of atoms in a stable covalent compound:
For example, how
many nitrogen and
hydrogen atoms
bond together in an
ammonia molecule?
1. Work out how many electrons are needed by each
non-metal element to complete its outer electron shell.
2. Work out the ratio of atoms that will provide enough
shared electrons to fill all the outer shells.
N H
(2.5) (1)
3electrons needed
electron configuration
ratio of atoms
element
1
1 3
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H
Covalent bonding in ammonia
How do nitrogen and hydrogen atoms form covalent bonds
in a molecule of ammonia?
NH3 or H N H
H
N H
(2.5) (1)
3electrons needed
electron configuration
ratio of atoms
element
1
1 3
H HN
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H
Covalent bonding in methane
How do carbon and hydrogen atoms form covalent bonds
in a molecule of methane?
CH4 or H C H
H
H
C H
(2.4) (1)
4electrons needed
electron configuration
ratio of atoms
element
1
1 4
H
H HC
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Covalent bonding in carbon dioxide
How do carbon and oxygen atoms form covalent bonds in
a molecule of carbon dioxide?
C O
(2.4) (2.6)
4electrons needed
electron configuration
ratio of atoms
element
2
1 2
CO2 or O C O
O OC
double bonds
A double bond is when two pairs of electrons are shared.
In carbon dioxide there are two double bonds
– one between each oxygen atom and the carbon atom.
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What are simple covalent structures?
Covalent molecules that contain only a few atoms are
called simple covalent structures.
weak bonds
between
molecules
strong bonds
within
molecules
Most substances that contain simple covalent molecules
have low melting and boiling points and are therefore liquids
or gases at room temperature, e.g. water, oxygen, carbon
dioxide, chlorine and hydrogen. Why?
The covalent bonds within these molecules are strong but
the bonds between molecules are weak and easy to break.
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What is the structure of a molecular solid?
A few substances that contain simple covalent molecules
are solid at room temperature. These are molecular solids.
The solid is formed because
millions of iodine molecules
are held together by weak
forces of attraction to create
a 3D molecular lattice.
Two iodine atoms form a single covalent
bond to become an iodine molecule.
weak
forces of
attraction
What properties would you
expect molecular solids to have
with this type of structure?
Iodine is a molecular solid at room temperature.
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What are the properties of molecular solids?
low melting and boiling points;
usually soft and brittle – they
shatter when hit.
The properties of a molecular solid, such as iodine, are:
Why do molecular solids have these properties?
The weak forces of attraction between the molecules can
be broken by a small amount of energy. This means that
the molecular solids are soft and brittle and melt and boil at
low temperatures.
Molecular solids are also unable to conduct electricity
because there are no free electrons or ions to carry a charge.
cannot conduct electricity.
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What are giant covalent structures?
In some substances, such as sand, diamond and graphite,
millions of atoms are joined together by covalent bonds.
All the bonds are covalent, so giant covalent structures have
very high melting and boiling points, and are usually hard.
The covalent bonds in these substances do not form
molecules but vast networks of atoms called giant covalent
structures.
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What is the structure of sand?
Sand is mostly made of the mineral quartz, which is silicon
dioxide. It has a giant covalent structure made up of silicon
and oxygen atoms.
Each silicon atom (2.8.4) is bonded to four oxygen atoms,
and each oxygen atom (2.6) is bonded to two silicon atoms.
Si
O
O
O
O
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What are the allotropes of carbon?
Diamond and graphite appear to be very different substances
but what do they have in common?
Both diamond and graphite are made up of carbon atoms.
These allotropes of carbon have different properties because
the atoms are bonded in different arrangements which create
different giant structures.
Different forms of the same element are called allotropes.
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How does structure affect properties?
How do the different structures of diamond and graphite
influence their properties?
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What are the properties of diamond?
In diamond, all the electrons in the outer shell of each
carbon atom (2.4) are involved in forming covalent bonds.
This affects the properties of this allotrope of carbon:
Diamond is very hard – the
hardest natural substance.
Diamond has a very high
melting and boiling point
– a lot of energy is needed
to break the covalent bonds.
Diamond cannot conduct
electricity – there are no
free electrons or ions to
carry a charge.
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What are the properties of graphite?
In graphite, only three of the four electrons in the outer shell
of each carbon atom (2.4) are involved in covalent bonds.
Graphite is soft and slippery
– layers can easily slide over
each other because the weak
forces of attraction are easily
broken. This is why graphite is
used as a lubricant.
Graphite conducts electricity
– the only non-metal to do so.
The free electron from each
carbon means that each layer
has delocalized electrons,
which can carry charge.
This affects the properties of this allotrope of carbon:
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Are there other allotropes of carbon?
A third class of carbon compounds have been discovered
in recent years. These are called fullerenes.
Buckminsterfullerene is one type of fullerene. It contains
60 carbon atoms, each of which is bonded to three others
by two single bonds and one double bond.
The atoms in this allotrope of carbon form a sphere, like the
shape of a football. The molecules can be called ‘bucky balls’.
They are large but are not classified as giant structures.
C
C
C
C
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Electronegativity
values for some
common elements.
Values given here
are measured on
the Pauling scale.
In a covalent bond between two different elements, the
electron density is not shared equally.
This is because different elements have differing abilities to
attract the bonding electron pair.
This ability is called an element’s electronegativity.
What is electronegativity?
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The electronegativity of an element depends on a
combination of two factors:
1. Atomic radius
As radius of an atom increases, the bonding pair of
electrons become further from the nucleus. They are
therefore less attracted to the positive charge of the
nucleus, resulting in a lower electronegativity.
higher
electronegativity
lower
electronegativity
Electronegativity and atomic radius
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Electronegativity, protons and shielding
2. The number of unshielded protons
The greater the number of protons in a nucleus, the
greater the attraction to the electrons in the covalent
bond, resulting in higher electronegativity.
However, full energy levels of electrons shield the
electrons in the bond from the increased attraction of the
greater nuclear charge, thus reducing electronegativity.
greater nuclear
charge increases
electronegativity…
…but extra shell of
electrons increases
shielding.
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Types of Covalent Bonds:
We have discussed the two extreme cases of
bonding: complete electron transfer- ionic
bonding
And covalent bonds with completely equal
electron sharing.
Between these extremes are covalent bonds
which involve unequal electron sharing.
When atoms with different electronegativities form covalent bonds,
those ΔEN values may be minimal or significant.
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Non-polar bonds
If the electronegativity of both atoms in a covalent bond is
identical, the electrons in the bond will be equally attracted
to both of them, and form a non-polar covalent bond.
This results in a symmetrical
distribution of electron
density around the two
atoms.
Bonding in elements (for
example O2 or Cl2) is always
non-polar because the
electronegativity of the atoms
in each molecule is the same.
both atoms are
equally good at
attracting the
electron density
cloud of electron density
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Mostly Covalent bonds
both atoms are
equally good at
attracting the
electron density
Such bonds are designated
as being “mostly
covalent” because ΔEN
appears to be insignificant.
Another way to
characterize this is to say
that these bonds have very
little “ionic character.”
If ΔEN is less than 0.4, the bonding electrons
between the two atoms spend no more of their time
nearer one nucleus than the other.
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Electronegativity and Covalent Bonds
• As ΔEN increases beyond 0.4, the pair of
bonding electrons will be drawn closer
and closer to the nucleus of the atom
with the higher electronegativity.
• This unequal distribution of electron
density will give that end of the bond a
partially negative “pole” and the other a
partially positive “pole.”
• A bond “dipole” is said to exist and the
bond itself is known as a polar covalent
bond.
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Effect of electronegativity on polarization
The greater the electronegativity difference between the two
atoms in a bond the greater the polarization of the bond.
decreasing polarization
This can be illustrated by looking at the hydrogen halides:
H F Cl Br I
Molecule
Electronegativity
difference
between atoms
H–F H–Cl H–Br H–I
1.8 1.0 0.8 0.5
Pauling
elecronegativities
Element
2.2 4.0 3.2 3.0 2.7
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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 extremes of a scale.
Less polar bonds have
more covalent
character.
increasing polarization
More polar bonds have more
ionic character. The more
electronegative atom attracts 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.
If the polar bonds are
arranged symmetrically,
the partial charges cancel
out and the molecule is
non-polar.
Non-polar molecules
If the polar bonds are
arranged asymmetrically,
the partial charges do not
cancel out and the
molecule is polar.
Polar molecules
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Glossary (1/2)
allotrope – A structurally different form of an element with
different physical properties.
covalent bond – A strong bond between two atoms in
which each atom shares one or more electrons with the other.
covalent compound – A compound containing atoms
joined by covalent bonds.
double bond – A covalent bond in which each atom
shares two of its electrons.
giant structure – A structure containing millions of atoms
or ions bonded together. The structure extends in three dimensions until all available atoms are used up.
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Glossary (2/2)
molecule – A small group of atoms which are held together
by covalent bonds.
molecular solid – A solid substance made up of
molecules held together by weak forces of attraction, forming a lattice.
single bond – A covalent bond in which each atom shares
one of its electrons.
triple bond – A covalent bond in which each atom shares
three of its electrons.