ionic bonding chemistry i. ionic bonding ionic bonding and the octet rule octet rule: atoms will...
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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 not stable
Mg2+: [Ne] 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
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
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 sodium ions Subscript
of “2” means the formula unit contains “2” sodium ions
This compound contains oxygen ions
Since there isn’t a subscript written next to the O, that means there is only 1 oxygen in 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
• 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)
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