1. Chapter 5.6 – 5.7: How Elements Form Compounds & Determining Charges on Ions

2. Lewis Electron Dot Diagrams
Gilbert Lewis proposed more simplistic diagrams than Bohr diagrams for atoms
With Lewis diagrams, only the valence shell electrons are shown around the symbol of an atom
Examples:
● ● ● ●
Li Mg ● ● Cl :
● ●

3. Lewis Electron Dot Diagrams
To determine the number of valence electrons for a representative element, simply look at the group number!
The last digit of the group number will equal the number of valence electrons
Group 1 – 1 valence e’s Group 15 – 5 valence e’s
Group 2 – 2 valence e’s Group 16 – 6 valence e’s
Group 13 – 3 valence e’s Group 17 – 7 valence e’s
Group 14 – 4 valence e’s Group 18 – 8* valence e’s
* Except helium – has only 2 valence electrons because it is in the first shell (but they are paired together)

4. Lewis Electron Dot Diagrams
How to draw Lewis diagrams:
1. Establish the total number of valence electrons.
2. Write the element symbol.
3. Place ONE dot (representing an electron) at the top of the element symbol. Continue placing single dots clockwise around each of the four sides of the element symbol until all four sides have ONE dot or until all electrons have been placed.
4. If any electrons remain, begin a second rotation around the element symbol, pairing up electrons as you go along each side.
EXCEPTION: Helium’s second valence electron will be placed beside the first. This is because helium is a first row element.

5. Lewis Electron Dot Diagrams
Activity:
Draw Lewis electron dot diagrams for the FIRST twenty elements on the periodic table.
(a) What trends do you notice horizontally?
(b) What trends do you notice vertically?
(c) Why is helium better off in Group 18 than Group 2 ???

6. How Elements Form Ionic Compounds
Ionic compound – a substance formed when a metal atom donates an electron(s) to a non-metal atom; once the electron has been transferred, the result is a positively charge metal ion (called a cation) and a negatively charged non-metal ion (called an anion).
The overall charge of an ionic compound is ZERO. In other words, the sum of the charges on all of the metal cations must balance the sum of the charges on all of the non-metal anions.
e.g. Na+ Cl - = NaCl K+ O2- = K2O Al3+ S -2 = Al2S3

7. 5.7 - Ion Charges & the Periodic Table
Charges on ions depend on the number of valence electrons
Group 1 atoms have 1 valence electron and will lose it to become a cation with a +1 charge (e.g. H+, Li+, Na+, K+)
Group 2 atoms have 2 valence electrons and will lose both to become a cation with a +2 charge (e.g. Be2+, Mg2+, Ca2+)
Group 13 atoms have 3 valence electrons and will lose all three to become a cation with a +3 charge (e.g. Al3+)
Group 15 atoms have 5 valence electrons and will gain three more to become anion with a -3 charge (e.g. N3-, P3-)
Group 16 atoms have 6 valence electrons and will gain two more to become anion with a -2 charge (e.g. O2-, S2-)
Group 17 atoms have 7 valence electrons and will gain one more to become anion with a -1 charge (e.g. F-, Cl-)
Groups 3-12 atoms require the use of the periodic table to determine the charge as many are non-intuitive and/or have more than one possibility

8. How Elements Form Ionic Compounds
Steps for using Lewis Electron Dot Diagrams to derive the correct formula for an Ionic Compound:
1. Write the Lewis electrons dot diagrams for the two ATOMS involved in making the ionic compound.
2. Transfer a unpaired electrons from the metal to the non-metal (by drawing arrows) until no more can be transferred (keeping in accordance with the OCTET rule).
3. If necessary, add another metal or non-metal and continue transferring electrons until ALL atoms have obeyed the octet rule (i.e. have achieved a full outermost shell).
4. On the mirror side, include the charges on all of the resulting ions from the transferred electrons.
5. Use subscripts to represent the ionic formula.

9. How Elements Form Ionic Compounds
Use Lewis Electron Dot Diagrams to derive the correct formula for magnesium chloride:

10. How Elements Form Ionic Compounds
Use Lewis Electron Dot Diagrams to derive the correct formula for aluminum sulfide:

11. How Elements Form Ionic Compounds
Try Q.1-4 p.189*
ABM #2*
ABM #3 & 4
* replace the word “Bohr” with “Lewis” for all instructions