How and Why Do Atoms Bond

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How and Why Do Atoms Bond?

• Module 12

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A chemical bond is a mutual electrical attraction
between the nuclei and valence electrons of
different atoms that binds the atoms
together. Chemical Bonds involve valence
electrons. Valence electrons are illustrated by
electron dot structures or Lewis structures. What
are electron dot structures?
Another example
The symbol of the element.
Cl
Na
Remember that Na has one valence electron and Cl
has seven
And a dot for every valence electron
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What is the electron dot structure for the
element Phosphorous? Phosphorous is element
number 15. It is in group 15. Therefore a
phosphorous atom has 5 valence electrons.
Notice that as you place valence electrons around
the symbol, you treat the symbol as a rectangle
and place one electron on each side until all
four sides have an electron. Then you begin to
pair the electrons.
P
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Group 13
Group 1
Group 17
Group 15




















Be
O

Na

B
C

N

F
Ne












Group 16
Group 2
Group 14
Group 18
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Why do atoms bond?? Atoms bond by gaining,
losing, or sharing electrons. Why do atoms gain,
lose, or share electrons? To attain a noble gas
electron electron arrangement.
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What are the different types of bonds? Ionic
Bonding electrical attraction between large
numbers of cations (positive ions) and anions
(negative ions). Metals-nonmetals. Metals tend to
lose electrons to attain a noble gas electron
arrangement (8 valence electrons). They attain a
positive charge when they do this. Nonmetals tend
to gain electrons to attain a noble gas electron
arrangement (8 valence electrons). They attain a
negative charge when doing this. Ionic bonds
exist between metals which become cations and
nonmetals which become anions. How??????
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Sodium is a metal which has one valence electron.
Sodium needs to lose one electron to become like
a noble gas.
-

Cl
Na
Chlorine is a nonmetal which has seven valence
electrons. Chlorine needs to gain one more
electron to attain a noble gas electron
arrangement.
When sodium loses its one valence electron, its
no longer neutral. Now the protons outnumber the
electrons by one and sodium has a 1 charge
What a great relationship. Sodium needs to get
rid of an electron and chlorine needs to attain
an electron!
When chlorine gains another electron, its no
longer neutral. The electrons now outnumber the
protons by one and chlorine attains a -1 charge
Now we all know that opposites attract. Sodium is
a positive ion (cation) and chlorine is a
negative ion (anion). They are now attracted to
each other since they have opposite charges.
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1 valence electron
7 valence electrons
Sodium Loses its one valence electron
Chlorine gains one valence electron
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Ionic bonds always involves an exchange of
electrons. Metals lose electrons and nonmetals
gain electrons. What if the exchange does not
involve an equal number of electrons???
Calcium has two valence electrons since it is in
group 2.
2
-
F
F
-
Ca
Fluorine, like chlorine, has seven valence
electrons.
Calcium needs to lose two electrons to attain a
noble gas electron arrangement, but fluorine only
needs to gain 1. What do we do??
Use TWO fluorines, where each fluorine can take
one of the two electrons from calcium
Now they are attracted to each other and form an
ionic bond
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Potassium and Iodine each become ions when
exchanging electrons. Each ion becomes very
stable when they attain a noble gas arrangement.
Since each ion has an equal but opposite charge,
the ions form a bond an ionic bond
Magnesium has two valence electrons and needs to
lose them both to become like a noble gas.
Chlorine, as we have seen needs only to gain one
electron to become stable. Therefore, two
chlorines are needed to take the two magnesium
electrons.
Iodine (I) has seven valence electrons like
chlorine. Iodine needs to gain one more electron
to attain a noble gas electron arrangement
Potassium (K) has one valence electron like
sodium. Potassium needs to lose its one valence
electron in order to attain a noble gas electron
arrangement.
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It is important to take note of two things Most
neutral atoms are unstable, because they do not
have a noble gas electron arrangement (8 valence
electrons). Ions (cations or anions) are formed
when atoms gain or lose electrons to attain a
noble gas electron arrangement. Ions are
therefore very stable (like noble gases), whereas
neutral atoms are generally unstable.
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How would aluminum and sulfur form a
bond? Aluminum is a metal in group 13. Sulfur is
a nonmetal in group 16. Aluminum has 3 valence
electrons and needs to lose all 3 to become
stable. Sulfur has 6 valence electrons and needs
to gain 2 to attain noble gas stability.
We can now begin to understand something about
chemical formulas. In this compound, two
aluminums are needed to transfer six electrons
and three sulfurs are needed to receive six
electrons.
The common multiple between the number of
electrons needed to be gained and lost is six (Al
loses 3 and S gains 2).
The formula for a compound between aluminum and
sulfur is
S
3
Al
Al2S3
S
3
Al
You will need 2 aluminums and 3 sulfurs to
account for all the electrons transferred.
S
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Compounds with primarily ionic bonds are usually
crystalline made of crystals. Crystals are
compounds with very strong bonds at fixed
geometric bond angles, so that the atomic
structure is reflected in the structure of the
compound.
At the atomic level sodium ions and chlorine ions
alternate three dimensionally and form a cubic
arrangement. Crystals often form this kind of
network of repeating ions.
Salt NaCl, for example has a cubic structure. The
atomic arrangement of the ions of salt is cubic
as well.
Nearly all ionic
bonds form crystalline arrangements. These bonds
are very strong, but brittle. They have very high
melting and boiling points.
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Compounds with ionic bonds usually dissolve in
water and strongly conduct electricity when
dissolved in water. These are called electrolytes.
A conductivity apparatus like the one below will
show whether a solution will conduct electricity
or not. Ionic compounds dissolved in water do
conduct electricity and the light bulb will glow.
If this apparatus were powered and salt was added
to the water, the salt water solution would
conduct the current and the light bulb would glow.
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What other types of bonds exist? A second type of
bond is called a Covalent Bond. What is a
covalent bond? A covalent bond is a bond between
two atoms (usually nonmetals) in which electrons
are shared to attain a noble gas electron
arrangement. How do atoms share electrons?
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How does a covalent bond form? Hydrogen is a good
example
?
?
H
H
Each hydrogen atom has one valence electron. It
needs one more electron to attain the noble gas
stability of helium.
Each hydrogen atom shares its electron with the
other so that each atom has available 2
electrons, giving it a noble gas electron
configuration (like helium)
Now both elements are happy ?
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Chlorine can form a bond with another
chlorine. Chlorine has seven valence electrons
and needs only one more electron to be stable.
one more electron needed
Cl
Cl
This is a single covalent bond a bond formed
when a pair of electrons is shared between two
atoms.
Each chlorine shares its single unpaired
electron. Each chlorine now has available its
seven valence electrons PLUS an electron shared
by the other chlorine.
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Molecule a neutral group of atoms held together
by covalent bonds Molecular compound - a
chemical compound whose simplest units are
molecules. Chemical formula the use of symbols
and subscripts to represent the relative number
and kinds of atoms making up a compound. Molecular
formula the number and kind of atoms in a
molecular compound. Diatomic molecules a
molecule containing two of the same atoms H2,
N2, O2, F2, Cl2, Br2, I2. We have seen the
diatomic molecules H2 and Cl2 on the previous
slides. Each forms a single covalent bond since a
single pair of electrons is shared between each
atom.
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What are double covalent bonds? Bonds formed
when two pairs of electrons (4 electrons) are
shared between two atoms. The diatomic molecule
oxygen, O2, is an example of a molecule with a
double covalent bond.
Each oxygen has six valence electrons and needs
two more electrons to become stable.
O
O
Each oxygen has two unpaired electrons.
Each oxygen will share its two unpaired electrons
with the other oxygen, so that each oxygen has
eight electrons available and becomes stable.
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What are triple covalent bonds? These bonds form
by sharing three pairs of electrons (6
electrons) between two atoms. This is not
terribly common. The diatomic molecule, Nitrogen
N2, is an example of a molecule with this type of
bond. Nitrogen is in group 15 and therefore has 5
valence electrons.
Each atom has 3 unpaired electrons and needs 3
more electrons to attain noble gas stability.
N
N
Now each atom has eight valence electrons
available and have attained noble gas stability
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Compounds containing covalent bonds are also
called molecular compounds. These compounds are
noncrystalline, typically have low boiling and
melting points, are nonbrittle, and are
nonelectrolytes (do not conduct electricity when
dissolved in water). These are opposite of ionic
compounds which are typically crystalline, high
melting and boiling points, and
brittle. Molecular compounds are composed of
molecules. Water is made of water molecules. What
does water look like at the atomic level? You
know the formula H2O!
Oxygen is happy like a noble gas!!!
When each hydrogen shares it one valence electron
with oxygen and oxygen shares its two unpaired
electrons, each atom has a noble gas arrangement
Oxygen (group 16) has six valence electrons
Each hydrogen has one valence electron
O
H
And the hydrogens are happy too!!!
H
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Covalent compounds or molecular compounds do not
conduct electricity when dissolved in water.
Covalent compounds are composed of only
nonmetals.
Table sugar is an example of a covalent compound.
Its formula is C12H22O11. Table sugar dissolved
in water will not conduct an electric current and
is therefore a nonelectrolyte.
The lightbulb stays unlit.
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Covalent bonds are sharing bonds and can share in
two ways equally or unequally. Some atoms have
a stronger desire (hunger) for electrons than
others. When this difference in electron hunger
exists, then these electrons are shared
unequally. Polar Covalent Bond bond in which
the electrons are not shared equally. This causes
one side of the bond to be slightly negative in
charge and the other side to be slightly
positive. Water is an example The term polar
means containing opposite sides.
In the water molecule above, oxygen has a greater
attraction for the shared electrons that the
hydrogens. Hence, the oxygen side of the bonds
are slightly negative and the hydrogen side of
the bonds are slightly positive.
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These are the symbols for slightly positive and
slightly negative
Here is an example of another polar bond, where
chlorine has a stronger attraction for the shared
electrons than does hydrogen. So Chlorine is
slightly negative and hydrogen is slightly
positive.
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What are nonpolar bonds? Nonpolar bonds are bonds
between atoms which share electrons equally.
Since the electrons are equally shared, there are
no charges and therefore no polarity. All of the
diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2)
contain nonpolar bonds since the bond exists
between identical atoms which have the same level
of attraction for electrons. Molecules are groups
of atoms bonded together by covalent bonds.
Molecules may also be polar or nonpolar. If one
side of the molecule is slightly positive and one
side slightly negative, then the molecule is
polar. We saw on a previous slide that a water
molecule is polar.
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Ammonia is also a polar molecule.
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We will learn later that polar substances mix
with polar substances. Nonpolar substances mix
with nonpolar substances. But polar substances
and nonpolar substances dont mix. Oil is
nonpolar and water is polar. They do not
mix. This ends Module 12.