4.2 How Elements
Form Compounds
Objectives
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Model two types of compound formation:
ionic and covalent at the atomic level.
Demonstrate how and why atoms achieve
chemical stability by bonding.
Compare, using examples the effect of
covalent and ionic bonding on the physical
properties of compounds.
How Elements Form Compounds
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In the 19th Century chemists tried to picture
how atoms combine.
Early models included atoms with “hooks” that
allowed them to attached to one another.
When Atoms Collide
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Atoms collide when they react
These collisions determine what kind of
compounds are formed.
When atoms collide it is the electron clouds that
interact with one another.
To be more precise it is the valence electrons
that interact.
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Remember: valence electrons are responsible for the
chemical properties of elements
Valence Electrons and Bonds
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To understand how valence electrons affect the
bonds of compounds let look at elements with
unusual properties….the noble gases
Noble Chemical Stability
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Noble Gases are located in Group 18
They are inert or unreactive
This lack of reactivity makes them useful as
incandescent light bulbs (argon or krypton) and
neon lights (orange-neon, blue-argon, yellowhelium)
Occur naturally in the environment
Noble Gases
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Not a single compound of these elements has
ever been found naturally in the environment.
In 1960 chemists were able to react fluorine with
krypton and xenon under high temperature and
pressure.
Since then a few additional compounds of
xenon and krypton have been synthesized, but
none with helium, neon or argon.
The Octet Rule
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Lack of reactivity of the noble gases must mean
that they are stable.
Elements in the same group have similar valence
arrangements.
Noble gases have 8 valence electrons, except for
helium that has 2.
Because the electron arrangement determines
chemical properties  unreactive nature of
noble gases
Octet Rule
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Atoms combine because they become more
stable by doing so.
Modern model of bonding is based on the fact
that the stability of noble gases is a result of
their valence electron arrangements.
Octet Rule- atoms can become stable by having
eight electrons in their outer energy level (or two
electrons in the case of some of the smaller
atoms).
Noble Gas Configuration
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Atoms obtain stability by achieving the same
configuration of valence electron as found in
noble gases; most stable configuration.
Valence Electron Review
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How many valence electrons do you find in
Group 1, 2, 13, 15, 16 and17?
Review
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What is the octet rule?
4.2 Continued
Ways to Achieve a Stable Outer
Energy Level
Forming Compounds
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When atoms collide with enough force their
outer electrons may rearrange to achieve a stable
octet of valence electrons.
When it occurs they achieve a noble gas
configuration and the atoms form compounds
How do the electrons rearrange?
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Electrons are transferred from one atom to
another
Electrons are shared between atoms
Electrons Can Be Transferred
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Example Sodium (Na) and Chlorine (Cl)
Na has one valence electron
 Cl has seven valence electrons
 Na transfers its 1 to Cl 7  Na now has 8 electrons
in its outer shell and Cl now has 8 in its outer shell
 Na now has 1 less electron  has +1 charge
 Cl now has 1 more electron has a -1 charge
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Ions
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Atom or group of atoms that has a charge b/c
they have either lost or gained electrons
Ions form when valence electrons rearrange by
transfer
Compounds composed on ions are ionic
compounds
Table 4.2 p. 134
Ions Attract Each Other
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Opposites attract
Positive sodium is attracted to the negative
chloride ion.
Positive ions are called cations
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Names stays the same
Negative ions are called anions
Add –ide to the end
 Sulfur  Sulfide ion
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Ionic bond
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Strong attractive force between ions of opposite
charge
Hold ions together in an ionic compound
Crystal is a regular, repeating arrangement of
atoms, ions or molecules
The Results of Ionic Attraction
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b/c of the strong attractive forces and the
degree of organization  solid at room temp
Raising temp causes particles to move faster 
particles overcome attractive forces and the
crystal organization breaks down
Representing Compounds with
Formulas
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Sodium Chloride
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1.
2.
Formula: NaCl
Write cation first followed by the anion
Always in the smallest whole number ratio
Review
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What is a negatively charged ion called?
What is a positively charged ion called?
What is the noble gas configuration?
Electrons Can Be Shared
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What happens when hydrogen and oxygen
collide?
Oxygen has 6 valence electrons
 Hydrogen has 1 valence electron
 Can hydrogen lose its one valence electrons?
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Colliding atoms
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Transfer electrons only when one atom has a
stronger attraction for valence electrons
In the case of sodium and chlorine, chlorine
attracts sodium’s valence electrons strongly and
sodium is holding its electrons weakly
In the case of hydrogen and oxygen neither
atom attracts strongly enough to take electrons
from the other.
Hydrogen and Oxygen
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They will share their electrons
When atoms collide with enough energy to
react, but neither atom attracts electrons
strongly enough to take electrons from the
other, the atoms combine by sharing valence
electrons.
Let’s Look at Water
Formation of Compounds
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Like the formation of sodium chloride all the
components present before the reaction are still
present after the reactions
The valence electrons no longer reside in the
same position.
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They have rearranged
Electrons Sharing Produces
Molecules
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Covalent bond- the attraction of two atoms for a
shared pair of electrons
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Electrons are shared
Neither atom has an ionic charge
Covalent compound- compound whose atoms are
held together by a covalent bond, also called molecular
compounds
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Water is an example
Molecules-uncharged group of two or more atoms held
together by covalent bonds (water molecules)
More Than Two Electrons Can Be
Shared
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Let’s look at Carbon Dioxide
Carbon has 4 valence electrons
 Oxygen has 6 valence electrons
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Covalent and Ionic bonds
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Two nonmetallic elements usually form
molecular or covalent compounds by sharing
electrons
Nonmetallic and a metallic element usually form
ionic compounds
Review
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What is a covalent bond?
What is the definition for a molecular
compound?
Give me an example of a molecular compound?
Review
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What is a covalent bond?
What is the definition of a molecular
compounds?
What are two examples of a molecular
compound?
How do ionic and
covalent compounds
compare?
Bonding
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When elements combine they either form ions
or molecules…there are no other possibilities.
When particles change dramatically, when they
change from sodium to sodium ions or from
hydrogen and oxygen to water molecules.
These changes explain why compound have
different properties from the elements they are
composed of.
Explaining the Properties of Ionic
Compounds
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Physical properties are a result of the wellorganized, tightly bound ions.
Ions forms strong, three-dimensional crystals
Properties of Ionic Compounds
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Generally ionic compounds are crystalline solids
at room temperature.
Generally hard, rough and brittle
High melting points and boiling points
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This is because of the strong attraction between ions
Usually dissolve in water
Properties of Ionic Compounds
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Solutions will conduct electricity
Liquid state (melted state) will conduct
electricity
They are electrolytes- any compound that
conducts electricity when melted or dissolved in
water
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In order to conduct electricity the ions must be free to move.
Must take on or give up electrons
Solid state does not conduct electricity. Why?
Explaining the Properties of
Covalent Compounds
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Composed of molecules
Held together by covalent bonds-make the
molecule a stable unit
Molecules have no ionic charge
Attractive forces between molecules are usually
weak
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Interparticle forces- forces between particles that
make up a substance
Properties of Covalent Compounds
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Many are liquids or gases at room temperature
Low melting points and boiling points
Do not conduct electricity
Some do not dissolve in water
Vegetable oil and gasoline
 In general they are less soluble in water than ionic
compounds
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Ionic Vs. Covalent
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It is the strength between interparticle forces in
covalent compounds compared to the strong attractive
forces of ions in ionic compounds is what explains
many of the differences in their physical properties
Because covalent compounds do not have ions you
would expect that they don’t conduct electricity
Ionic compounds are more soluble in water because
ions are attracted by the water molecules, while
molecular compounds are not
Ionic Vs. Covalent Compounds
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High melting point
High boiling point
Solid at room
temperature
Soluble in water
Excellent conductor of
electricity
Many crystalline
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Low melting point
Low boiling point
Liquid or gas at room
temperature
Less soluble in water
Poor conductor of
electricity