1. Bonding Basics
It’s really very easy!

2. # of Electrons gained or lost3
Before you can figure out how atoms form molecules and compounds, you need some information. You need to be able to figure out all this from the periodic table.
Element
Atomic Symbol
Total # of Electrons1
# of Valence Electrons2
# of Electrons gained or lost3
Oxidation Number4
Iodine I 53 17
gain 1 -1
Lithium Li 3 1
lose 1 +1
Calcium Ca 20 2
lose 2 +2
Sulfur S 16 6
gain 2 -2
Boron B 5
lose 3 +3
Silicon Si 14 4
gain/lose 4
+ 4 or - 4
Phosphorus P 15
gain 3 -3
1. Total number of electrons equals the atomic number.
2. The number of valence electrons is the same as the column number on the periodic table
3. Less that 4 valence electrons, electrons are lost. More than 4 valence electrons, electrons are gained. Atoms with 4 (carbon, silicon) can gain or lose.
4. Atoms lose electrons to become positive ions. Atoms gain electrons to become negative ions.

3. Ionic Bonds
Form when one atom loses electrons and one atom gains electrons.
Always form between a metal and a nonmetal. (never between two nonmetals)
The oxidation numbers must add up to zero.

4. Example 1. Potassium + Iodine
First, write the Lewis Diagram for each element. K I

5. Example 1. Potassium + Iodine
Next, draw an arrow showing the transfer of electrons from one atom to the other. K I

6. Example 1. Potassium + Iodine
Now, show the charges on the newly formed ions. Potassium lost an electron to become +1, and Iodine gained an electron to become -1. K+ I-

7. Example 1. Potassium + Iodine
Finally, combine the 2 ions to form the final compound.
K+ + I- KI

8. Example 2. Magnesium + Oxygen
First, write the Lewis Diagram for each element. Mg O

9. Example 2. Magnesium + Oxygen
Next, draw an arrow showing the transfer of electrons
from one atom to the other. O Mg

10. Example 2. Magnesium + Oxygen
Now, show the charges on the newly formed ions. Magnesium lost 2 electrons to become +2, and Oxygen gained 2 electrons to become -2.
Mg2+
O2-

11. Example 2. Magnesium + Oxygen
Finally, combine the 2 ions to form the final compound.
Mg2+ +
MgO
O2-

12. Example 3. Lithium + Nitrogen
First, write the Lewis Diagram for each element. Li N

13. Example 3. Lithium + Nitrogen
Next, draw an arrow showing the transfer of electrons from one atom to the other. Notice that 1 Lithium does not provide enough electrons, so add more. N Li

14. Example 3. Lithium + Nitrogen
A second Lithium atom provides another electron, still leaving Nitrogen 1 short. So, add another. N Li

15. Example 3. Lithium + Nitrogen
Nitrogen now has eight valence electrons. It took 3 Lithium atoms to provide enough electrons to fill Nitrogen’s valence energy level. N Li

16. Example 3. Lithium + Nitrogen
Now, show the charges on the newly formed ions. Lithium lost 1 electron to become +1, and Nitrogen gained 3 electrons to become -3.
Li+
N3-

17. Example 3. Lithium + Nitrogen
Finally, combine the newly formed ions to make the compound.
Li+ + Li+ + Li+ + N3- Li3N

18. Covalent Bonds Form when one atom shares electrons with another atom.
Always form between between two nonmetals.
The oxidation numbers must add up to zero.

19. Example 1. Fluorine + Fluorine
First, write the Lewis Diagram for each element. F F

20. Example 1. Fluorine + Fluorine
Next, draw circles showing the electrons that are shared. F F

21. Example 1. Fluorine + Fluorine
Finally, draw a line showing the bond between the atoms and then write the formula. F F F2

22. Example 2. 3 Hydrogen + Phosphorus
First, write the Lewis Diagram for each element. H P H H

23. Example 2. 3 Hydrogen + Phosphorus
Next, draw circles showing the electrons that are shared. H H P H

24. Example 2. 3 Hydrogen + Phosphorus
Finally, draw lines showing the bond between the atoms and then write the formula. H H P
H3P H

25. Example 3. 2 Hydrogen + Sulfur
First, write the Lewis Diagram for each element. H S H

26. Example 3. 2 Hydrogen + Sulfur
Next, draw circles showing the electrons that are shared. H H S

27. Example 3. 2 Hydrogen + Sulfur
Finally, draw lines showing the bond between the atoms and then write the formula. H H S
H2S