Why do ions want to be isoelectronic with a
nearby noble gas?
How is the rule used to find charges of
main group elements?
Book Resource: page 159-161
Valence Electrons
The outermost electrons in an atom are valence electrons. For main group (s + p orbital)
elements, the number of valence electrons in an atom corresponds to group number.
Group 1 - 1 valence electron
Group 2 - 2 valence electrons
Group 13 - 3 valence electrons
Group 14 - 4 valence electrons
Group 15 - 5 valence electrons
Group 16 - 6 valence electrons
Group 17 - 7 valence electrons
Group 18 – 8 valance electrons (full octet) 
Atoms of different elements with the same number of valence electrons (the groups)
have similar chemical properties because valence electrons are involved in reactivity for
chemical interactions and bonding.
The Octet Rule
Noble gases are non-reactive because they all have a complete outer shell. A full
valance shell has 8 electrons. The octet rule tells us that in chemical reactions,
elements will gain, lose or share (covalent bond) the minimum number of electrons
necessary to achieve the electron configuration of the nearest noble gas. For anions,
electrons are added to achieve a full octet of 8 electrons, and for cations, electrons are
removed until there is a full octet.
Group
1
2
13
14
15
16
17
18
# Valence
Electrons
Ion
Charge
Apply:
Why do Neon and Argon commonly exist in nature as elements
(see neon pizza signs), yet the fluorine in toothpaste is actually an
ionic compound (sodium fluoride) because the element fluorine
does not exist freely in nature?
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
Starting with elements, draw a diagram to
show how an ionic compound is formed. Label
the energy changes at each step.
Book Resource: page 170-172
Ionic Bonding
Sodium reacts with chlorine gas in an exothermic reaction to produce NaCl.
Why does the process of FORMING ionic bonds RELEASE energy?
Chlorine has a high affinity for electrons (has 7e-, wants 8e- : needs 1 e- ).
Sodium has a low ionization energy (has 1e-, prefers a full valence of 8e- : 1 extra e-).
Recall: ionization is the energy needed to _________________________
Chlorine gains the extra electron from sodium (which loses the extra electron)
The e- is transferred from the sodium atom to the chlorine atom to form Na+ and Cl- ions
the Na atom loses 1e-, so there is now 1 extra positive charge (more p than e-)
the Cl atom gains 1e-, so there is now 1 extra negative charge (more e- than p)
Each ion now has an octet of electrons in its valence shell!
Energy of Ionic Bond Formation
The formation of ionic compounds is usually extremely exothermic.
Recall: exothermic: _________ energy
endothermic: _________ energy
Step 1: sublime the metal
 energy is added to change the solid element to the gaseous state
Step 2: “Ionize” element to form a cation
The loss of an electron from a metal
•
Always endothermic (requires energy to remove e- from the atom)
Step 3:Break diatomic bond
Diatomic (remember H2, O2, N2, Cl2, Br2, I2, F2 - HONClBrIF)
Cl-Cl  2 Cl atoms
 Energy is needed to break bonds
Step 4: “Ionize” element to form an anion
The gain of an electron by a nonmetal:
•
Generally exothermic (energy released)
Step 5: Form lattice
Ionic compounds are stable due to the attraction between unlike charges:
•
•
•
The ions are drawn together
Energy is released (exothermic)
Ions form solid crystal lattice
•
Lattice energy:
the energy required to separate a
solid ionic compound into gaseous
ions
Net Bonding Formation: exothermic
QuickTime™ and a
decompressor
are needed to see this picture.
Apply:
Why do large “mountain” deposits of sodium chloride (ionic
exist,
compound)
yet sodium (element) and chlorine (element) are not found in nature?
Recall: sodium is an extremely reactive Alkali metal and chlorine is a halide, which reacts with
many elements
Using the arrangement of ions in a salt,
explain why salts are hard and brittle.
Book Resource: page 172-173
Ionic compounds are very hard and very brittle. This is because of the way that
they're held together. It takes a lot of energy to break the positive and negative charges
apart from each other. This is the reason that ionic compounds are so hard – ions in the
structure simply don't want to move around much, so they don't bend at all.
This also explains the brittleness of ionic compounds. It takes a lot of energy to pull
ionic charges apart from each other. However, if we give a big crystal a strong enough
bang with a hammer, so much energy is used to break the crystal that the crystal
doesn't just break in just one spot, but in a whole bunch of places. Think of a window
shattering versus denting a metal car door.
QuickTime™ and a
decompressor
are needed to see this picture.
Apply:
If you had a rock or hammer, how could you tell apart Fool’s Gold (ionic compound)
versus real gold (metal)?
QuickTime™ and a
decompressor
are needed to see this picture.
How does the arrangement
of ions
in a salt
affect melting and boiling points?
Book Resource: page 173
Ionic compounds tend to have “very, very high” melting and boiling points.
Most of the time, the melting point is so high that you can't even melt them with a
Bunsen burner in class. So, why are these temperatures so high? Well, it has to do with
the way that ionic materials are held together. Remember that ionic compounds form
crystals? These crystals are organized so that the positive and negative charges are
attracted to one another together. To break the positive and negative charges apart, it
takes a huge amount of energy (heat is energy). So, it would take a very high melting or
boiling point (lots and lots of energy) to break the ions apart in the crystal.
QuickTime™ and a
decompressor
are needed to see this picture.
Apply:
Table salt (sodium chloride) is an ionic compound, while sugar is not ionic.
Your prankster cousin always switches your sugar and salt shakers when he visits.
Before you go pouring salt into your pumpkin spice coffee, how could you test which
substance is in the sugar shaker? To spare your taste buds the jolt of salt, you must
determine which is salt (ionic)/sugar (not ionic) without tasting the two substances.
Why do salts conduct electricity as liquids
or when dissolved,
yet they do not conduct electricity as
solids?
Book Resource: page 174-175
Ionic compounds generally dissolve in water. When ionic compounds dissolve in
water, they are able to conduct electricity. The properties of water pull the positive
and negative ions apart from each other. Since the ions are pulled apart from one
another, negative charges from a current of electricity are able to flow through the
positive and negative ions floating amidst the water molecules. Thus, ionic compounds
conduct electricity when dissolved in water.
Does electricity travel through solid salt crystals?
No. The ions are stuck in one place due to the structure of the crystal lattice, so the
electricity cannot move around very well.
QuickTime™ and a
decompressor
are needed to see this picture.
Apply:
Where would you be more likely be electrocuted if your hairdryer fell in the tub:
 Bathtub of “pure” water?
 Your regular bathtub ?
 Bath after a day at the beach?
Pure water (without ions) is not a good conductor.
However, the water in your tub is not entirely pure. Tap water itself has ions, in addition
to the salts that wash off your body into the water, resulting in IONS. Thus, when the
dryer hits the water, the ions in the water conduct electricity, and ZAP!
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.