1. Ionic Bonding
Chemistry I

2. Ionic Bonding

3. 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)

4. 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

5. Cl-: [Ne]3s23p6 = [Ar] 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

6. 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

7. varies /

8. 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

9. Examples

10. 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.

11. 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

12. Sodium Chloride Formula unit: NaCl
Represents the ratio of Na+ to Cl- in the dimensional structure

13. Mono-: one Monatomic ions Poly-: many Polyatomic 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

14. Na2O Parts of a formula This compound contains sodium ions
Since there isn’t a subscript written next to the O, that means there is only 1 oxygen in the formula unit
Subscript of “2” means the formula unit contains “2” sodium ions
This compound contains oxygen ions

15. Ca3(PO4)2 This compound contains calcium ions
Subscript of “2” outside of parantheses means the formula unit contains 2 phosphate ions
This compound contains a polyatomic ion phosphate, PO4-3
Subscript of “3”, the formula unit contains 3 calcium ions

16. Na2SO4 This compound contains the polyatomic ion sulfate, SO4-2
Since there are no parantheses, the formula unit contains “1” sulfate ion

17. Rules for writing ionic formulas
1) The positively charged ion is written first

18. 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)

19. 3) never write “1” as a subscript

20. 4)Put parantheses around polyatomic ions if they need a subscript (other than “1”)

21. 5)Reduce subscripts to simplest form

22. 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

23. 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