1. Review of Ions
Ion: an atom or group of atoms that has a positive or negative charge
cation: a positively charged ion
anion: a negatively charged ion

2. Formation of Ions
Ions are formed by gaining or losing electrons, as shown below:
Na Na+ + e- forms a cation (loses an electron)
Cl + e Cl- forms an anion (gains an electron)
More than one electron can be gained or lost:
Mg Mg2+ + 2e-
S + 2e S2-

3. Ionic or Covalent Bonding
Whether a bond is ionic or covalent depends on the difference in electronegativity of the two atoms involved.
For differences greater than ionic bond
For differences less than covalent bond

4. Electronegativity Review
Periodic Trend for electronegativity: Ionic bonds are typically formed when atoms from the s block bond with atoms from groups 16 or 17 of the p block

5. Key Concept
The number of valence electrons determines the type of ion formed
Electron Configuration of Sodium
1s2 2s2 2p6 3s1
Which is the valence electron for Na?
Answer: 1s2 2s2 2p6 3s1

6. Formation of Ions:
Elements on the left side of the periodic table usually form cations (lose one or more electrons to form positively charged ions)
Elements on the right side of the periodic table usually form anions (gain one or more electrons to form negatively charged ions)
Why?

7. Na Consider Na, which has 1 valence electron (and 11 electrons total):
If the valence electron is lost, the resulting Na+ ion will have the same electron configuration as Ne, with a full valence shell. This is especially stable (and preferred)
Na+ has 11 protons, but only 10 electrons (thus the +1 charge): Na+

8. Na, 11 e
Na+, 10e
Note that the cation has a lower energy valence shell

9. On the right side of the periodic table is Cl, with 7 valence electrons (and 17 electrons total):
If Cl gains an electron, the resulting Cl- ion will have the same electron configuration as Ar, with a full valence shell. This is especially stable (and preferred)
Cl- has 17 protons, and 18 electrons (thus the -1 charge):

10. Chlorine Atom  Chloride Ion
1s22s22p63s23p6
Chlorine atom
1s22s22p63s23p5
18 electrons total
8 valence electrons
Dot structure of Argon
17 electrons total
7 valence electrons

11. Result from Electron Transfer to form Ions
Ionic Bonds
Result from Electron Transfer to form Ions
Ionic bonds are formed by transfer of electrons from one element to the other.
Each element (now an ion) will have a complete octet after the transfer of electrons.

12. The Ionic Bond
The electrical force of attraction between oppositely charged ions Na F
Na+ F−
The oppositely charged ions have a strong attraction

13. Summary:
Ions form when doing so makes the valence shell more stable full
The ion formed depends on the number of valence electrons for the starting atom
Few valence electrons (left side of the periodic table) tends to form cations (positively charged)
Nearly full valence shells (right side of the periodic table) tends to form anions (negatively charged)

14. number of valence electrons number of electrons to transfer
Learning Check
element
number of valence electrons
forms cation or anion
number of electrons to transfer
ion formed K Ca Si B O F Li C

15. Practice Form ionic compounds using group 1A, group 2A, and group 7A.
Use at least 3 elements from each group (which will require a minimum of 6 compounds).