Ionic Bonding
Chemistry I
Ionic Bonding
Ionic Bonding and the Octet Rule
• Octet Rule: Atoms will lose, gain, or share
electrons to achieve an octet (eight
valence electrons; a noble gas
configuration)
• In ionic bonding,
– Metal atoms lose electrons to become like the
noble gas immediately before them in the
periodic table
• Metals become positively charged cations
Example:
Mg: [Ne]3s2
Mg+: [Ne]3s1
Mg2+: [Ne]
not stable
stable
– Nonmetal atoms gain electrons to become
like the noble gas following them on the
periodic table
• Nonmetals become negatively charged anions
Example
Cl: [Ne]3s23p5
Cl-: [Ne]3s23p6 = [Ar]
stable
• We can predict how many electrons an
atom will lose or gain to achieve a stable,
noble gas configuration, therefore, we can
also predict the ionic charge
+1 +2
varies
+3 +/-4 -3 -2 -1 0
Representing the formation of an
ionic bond with Lewis structures
• Draw the Lewis structure for each atom in the compound
• Show the transfer of valence electrons from the metal to
the nonmetal with an arrow
• Continue adding atoms and showing transfers of
electrons until the total number of electrons lost equals
the total number of electrons gained.
• Rewrite the dot structures showing the ions formed
– All metal ions should have no dots and have a positive charge
– All nonmetal ions should have 8 dots and have a negative
charge
Examples
Writing formulas for ionic
compounds
• Chemical formulas
– Represent the composition of substances
– Show the kinds and numbers of atoms in the
smallest representative unit of a substance.
• Formula Unit
– The smallest representative unit of an ionic
compound
– The lowest whole number ratio of ions in the
compound
– Does not show the ionic charges
• Sodium Chloride
– Formula unit: NaCl
– Represents the ratio of
Na+ to Cl- in the
3-dimensional
structure
• Mono-: one
• Monatomic ions
– Composed of only one
atom which has an
overall + or – charge
– Examples: Na+1, O-2
• Poly-: many
• Polyatomic ions
– Composed of a group
of atoms (many) with
an overall charge
– May contain subscripts
(which you cannot
change!)
– Will usually contain at
least two different
elements
– Examples: OH-1, CO3-2
Parts of a formula
Na2O
This compound
contains
Subscript
sodium ions
of “2”
means the
formula
unit
contains
“2” sodium
ions
Since there isn’t a
subscript written
This
next to the O, that
compound means there is
contains
only 1 oxygen in
oxygen ions the formula unit
Ca3(PO4)2
This compound
contains calcium
ions
Subscript of “3”,
the formula unit
contains 3
calcium ions
This compound
contains a
polyatomic ion
phosphate, PO4-3
Subscript of “2”
outside of parantheses
means the formula unit
contains 2 phosphate
ions
Na2SO4
This compound
contains the
polyatomic ion sulfate,
SO4-2
Since there are no
parantheses, the
formula unit contains
“1” sulfate ion
Rules for writing ionic formulas
• 1) The positively charged ion is written first
• 2) compounds are neutral, so the
algebraic sum of the charges must add to
zero (the total positive charge must cancel
out the total negative charge)
• 3) never write “1” as a subscript
• 4)Put parantheses around polyatomic ions
if they need a subscript (other than “1”)
•
• 5)Reduce subscripts to simplest form
Other things to keep in mind…
• Any ionic compound will be made of only
one type of positive ion with one type of
negative ion
• You must know the charge of each ion
– Predict monatomic ion charges from periodic
table
– Look up charges of polyatomic ions in chart
Shortcut: the crisscross method
• Cross down the numerical value of each
charge to become the subscript of the
other ion
• Make sure you consider if you have the
simplest ratio; if not, reduce it