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 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

+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 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

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