Section 7.2
IONIC BONDS
AND IONIC
COMPOUNDS
Forming Ionic Compounds
• Since ions have charges,
ions with opposite
charges will be attracted
to each other. The force
that pulls them together
(and holds them
together) is an
electrostatic force
known as an ionic bond.
Ionic Bonding
• In an ionic bond, one
element has a much
stronger attraction for
electrons than the other
atom, so an electron is
essentially transferred to
the more electronegative atom.
• The result is 2 or more anions and cations
bonded together – an ionic compound.
Realplayer\7.2 Formation of ions and ionic bonding.flv
Comparing Electronegativities
• Whether or not a metal and a nonmetal will
form an ionic bond can be determined by
comparing their electronegativities.
• If the difference between the two
electronegativity values is 2.0 or more, then
they will ionize and form an ionic bond.
• Consider calcium (1.0) and fluorine (4.0):
END = 4.0-1.0 = 3.0  ionic bond
Forming Ionic Compounds
• When ionic bonding occurs and a new ionic
compound is formed, the new substance can
be represented with a chemical formula.
• A chemical formula shows the types of atoms
in the substance, as well as how many of each
type are in one unit of the substance.
• One unit of an ionic compound is referred to
as a formula unit (NOT a molecule).
Why a “Formula Unit?”
• An ion is actually attracted to
any oppositely charged ion
nearby; this creates strong
attractive forces (bonds) in
multiple directions and is why
ionic compounds have specific
crystalline structures (more on
this later).
• A formula unit is the lowest
whole-number ratio of ions in an
ionic compound.
Considering Table Salt
• As can be seen in the diagram to
the right, there is one chloride
ion for each sodium ion (a 1:1
ratio), which is why table salt has
the formula NaCl.
Using Electron Dot Structures
• It is also easily seen from the electron dot
structures of sodium and chlorine that they
would combine in a one-to-one ratio.
Balancing Charges
• Another possible way to figure out the
chemical formula of one formula unit of an
ionic compound is to consider the charges on
the ions. Figure out how many of each are
needed for the charges to be balanced (make
sure when you are finished that you have the
simplest whole-number ratio).
• Since sodium ion and chloride ion have+1
and -1 charges (respectively), they balance
with one of each.
Now you try it.
Use electron dot structures to figure out the
chemical formulas of the ionic compounds
formed when the following elements combine:
Magnesium and chlorine
Confirm with their charges:
Now you try it.
Aluminum and chlorine
Confirm with their charges:
Now you try it.
Aluminum and sulfur
Confirm with their charges:
Properties of Ionic Compounds
• Most ionic compounds are
crystalline solids at room
temperature. This results
from the strength of the
attraction between the ionic
particles.
• The crystalline structure is a
direct result of the repeating
3-dimensional patterns that
come from the charges as
well as the sizes of the ions
involved.
Coordination Numbers
• When ions form a crystal, a cation is attracted
to all nearby anions.
• The coordination number tells the number of
ions of opposite charge that
surround an ion in a crystal.
• What is the coordination
number of sodium ions in
sodium chloride?
Coordination Numbers
• What is the coordination number of cesium
ions in cesium chloride?
Coordination Numbers
• What is the coordination number of titanium
ions in titanium dioxide?
• What is the
coordination number
of oxide ions?
HINT: Look here!
Properties of Ionic Compounds
• Solid ionic compounds do not conduct
electricity, even though they are composed of
charged particles, because the ions are locked
rigidly into their crystal lattice – they are not
free to move, and neither are their electrons.
• Melting an ionic solid allows the ions to flow,
so charged particles can move and liquid ionic
compounds are excellent conductors of
electric current. (However, this would take
extreme temperatures!)
Properties of Ionic Compounds
• Ionic compounds will also dissolve in water.
When they dissolve, they become dissociated
into their ions in the solution.
Properties of Ionic Compounds
• Since the ions are
free to move around
in the water, saltwater solutions are
excellent conductors
of electric current as
well, and they do not
require extreme
temperatures (like
melting would).
References
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http://practicalmaintenance.net/wp-content/uploads/Ionic-Bond.jpg
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http://3.bp.blogspot.com/-0lmakBuwdIc/TVvctuCdf8I/AAAAAAAAADc/TKzKB8OzE5E/s1600/ionic.bond.jpg
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http://img.tfd.com/MosbyMD/thumb/ionization.jpg
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http://www.biology.arizona.edu/biochemistry/problem_sets/ph/graphics/nacl2.gif
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http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_51.gif
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http://www.websters-online-dictionary.org/images/wiki/wikipedia/commons/thumb/e/e9/Sodium-chloride-3Dionic.png/200px-Sodium-chloride-3D-ionic.png
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http://www.visualphotos.com/photo/1x7467111/cubic_crystals_of_rock_salt_known_as_halite_or_bg8086.jpg
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http://www.middleschoolchemistry.com/img/content/multimedia/chapter_4/lesson_6/ionic_bond_sodium_chloride_1.jpg
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http://www.middleschoolchemistry.com/img/content/multimedia/chapter_4/lesson_6/ionic_bond_sodium_chloride_3.jpg
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http://2.imimg.com/data2/AL/GL/MY-2779887/1-250x250-250x250.jpg
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http://upload.ecvv.com/upload/imageCache/Product/20119/China_Magnesium_chloride20119191430029.jpg
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http://cst-www.nrl.navy.mil/lattice/struk.picts/e2_1.png
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http://www.chem1.com/acad/webtext/states/state-images/hex_close_packing.png
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http://www.biology.arizona.edu/biochemistry/problem_sets/ph/graphics/nacl2.gif