1. Electron Configuration and Ion Formation
Vocabulary:
valence shell
valence electrons
anion
cation
isoelectric
Lewis structure (Lewis dot diagram)
Banksy, British, 2002

2. Draw the orbital diagram for
chlorine 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 17 protons
potassium 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 19 protons

3. Identify the valence electrons
chlorine 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 17 protons
potassium 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 19 protons

4. Consequences
chlorine 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 17 protons
But a whole new energy level
potassium 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 19 protons
Only 2 more protons 

5. Consequences
That means strong attraction for these valence electrons (large Zeff).
chlorine 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 17 protons
potassium 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 19 protons
And weak attraction for this valence electron (low Zeff).

6. Consequences
That means easy to gain 1 more electron here (high electronegativity)
chlorine 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 17 protons
potassium 4s __ 4p ___ ___ ___ 3s ___3p ___ ___ ___ 2s ___2p ___ ___ ___ 1s ___ 19 protons
And easy to lose an electron here (low ionization energy)

7. Summary:
chlorine
4s __ 4p ___ ___ ___
3s___3p ___ ___ ___
2s___2p ___ ___ ___
1s ___
17 protons and 17 electrons = 0 charge
Elements that have a nearly full valence shell, like chlorine, have high electronegativities and can gain electrons easily.

8. Summary:
chlorine
4s __ 4p ___ ___ ___
3s___3p ___ ___ ___
2s___2p ___ ___ ___
1s ___
17 protons and 18 electrons = 1- charge
Elements that have a nearly full valence shell, like chlorine, have high electronegativities and can gain electrons easily.

9. Summary:
chlorine
4s __ 4p ___ ___ ___
3s___3p ___ ___ ___
2s___2p ___ ___ ___
1s ___
17 protons and 18 electrons = 1- charge
Atoms that have gained one or more electrons are now negative ions and are called anions (ˈan-ī-ən).

10. Summary:
potassium
4s __ 4p ___ ___ ___
3s___3p ___ ___ ___
2s___2p ___ ___ ___
1s ___
19 protons + 19 electrons = 0 charge
Elements that have a nearly empty valence shell, like potassium, have low electronegativities and can lose electrons easily.

11. Summary:
potassium
4s __ 4p ___ ___ ___
3s___3p ___ ___ ___
2s___2p ___ ___ ___
1s ___
19 protons + 18 electrons = 1+ charge
Atoms that have lost one or more electrons are now positive ions and are called cations (kat-ī-ən).

12. Notice
chlorine anion (1- charge) 4s __ 4p ___ ___ ___ 3s___3p ___ ___ ___ 2s___2p ___ ___ ___ 1s ___
potassium cation (1+ charge) 4s __ 4p ___ ___ ___ 3s___3p ___ ___ ___ 2s___2p ___ ___ ___ 1s ___
When each atom gained or lost electrons, the new outer shell (energy level) is full, like the noble gases.

13. Notice
chlorine anion (1- charge) 4s __ 4p ___ ___ ___ 3s___3p ___ ___ ___ 2s___2p ___ ___ ___ 1s ___
potassium cation (1+ charge) 4s __ 4p ___ ___ ___ 3s___3p ___ ___ ___ 2s___2p ___ ___ ___ 1s ___
Different atoms or ions that the same electron configuration are called isoelectric.

14. The Octet Rule
Atoms lose or gain electrons in order to have a full valence shell (a noble gas configuration). This will typically result in 8 valence electrons.

15. 7 valence electrons because in group 17
An easier way!
Rather than draw orbital diagrams, we can use Lewis Structures (Lewis Dot Diagrams) to show numbers of valence electrons.
Write symbol of element.
Determine number of valence electrons by position on periodic table
Use dots to represent electrons around symbol (one at a time on each side and then pair them up.)
7 valence electrons because in group 17 Cl

16. It needs one more to have an octet.
An easier way!
So when chlorine forms an ion, will it gain or lose electrons? How many?
It needs one more to have an octet. 1- Cl
What will its charge be now?

17. Lewis Structure Practice: potassium
Write symbol of element.
Determine number of valence electrons by position on periodic table
Use dots to represent electrons around symbol (one at a time on each side and then pair them up.)
1 valence electrons because in group 1 K

18. Lewis Structure Practice: potassium
So when potassium forms an ion, will it gain or lose electrons? How many?
It will lose one electron to have a full valence 1+ K
What will its charge be now?

19. Lewis Structure Practice: nitrogen
Write symbol of element.
Determine number of valence electrons by position on periodic table
Use dots to represent electrons around symbol (one at a time on each side and then pair them up.)
5 valence electrons because in group 15 N

20. Lewis Structure Practice: nitrogen
So when nitrogen forms an ion, will it gain or lose electrons? How many?
It will gain three electrons to have a full valence 3- N
What will its charge be now?

21. Short Cut
Using periodic trends, we can predict the ion charge for the main group elements. 1+ 2+ 3+
4+/- 3- 2- 1-