2. 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.
Total # of # of Valence # of Electrons Oxidation
Element Atomic Symbol
Electrons1 Electrons2 gained or lost3 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 3 lose 3 +3
Silicon Si 14 4 gain/lose 4 + 4 or - 4
Phosphorus P 15 5 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.
Mg O
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+ +O2- MgO
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.
Li N
14. Example 3.
Lithium + Nitrogen
A second Lithium atom provides another electron, still
leaving Nitrogen 1 short. So, add another.
Li N
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.
Li N
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
H P
H
23. Example 2.
3 Hydrogen + Phosphorus
Next, draw circles showing the electrons that are shared.
H
HP
H
24. Example 2.
3 Hydrogen + Phosphorus
Finally, draw lines showing the bond between the atoms
and then write the formula.
H
HP H3P
H
25. Example 3.
2 Hydrogen + Sulfur
First, write the Lewis Diagram for each element.
H
H S
26. Example 3.
2 Hydrogen + Sulfur
Next, draw circles showing the electrons that are shared.
H
HS
27. Example 3.
2 Hydrogen + Sulfur
Finally, draw lines showing the bond between the atoms
and then write the formula.
H
HS H2S