One day, Magnesium and Calcium went over to the electron shop to buy some electrons.

               Mg.                  Ca.

When the metal atoms finished buying their electrons, Magnesium and Calcium took a look at their valence electron shells and said to one another:

   Mg.                             Ca.

I'm not happy with all these electrons in my valence shell.

  Same. I want to lose a couple.

Oxygen and Sulfur, two non-metals  overheard Magnesium and Calcium's complaints about their valence electrons.

O               S                           Mg           Ca

Hmm....

   O                   S

                                                         Mg             Ca

Sulfur and I heard you metals aren't happy with your valence electron shells.

Oxygen said to Magnesium and Calcium:

Yes, we are unhappy. You know us metals prefer to lose electrons than to gain them.

Sulfur said to everyone:

             S

Listen. I have a solution. Calcium if you transfer 2 of your valence electrons over to me and if Magnesium transfers 2 of her valence electrons over to Oxygen, we can come together to form Ionic Bonds. 

                                              O              Mg               Ca

Calcium replied:

                                                                           Ca

 S                   O                 Mg                   

Oh yeah! This makes sense because if me and Magnesium transfer each of our 2 valence electrons over to each of you, we will all be happy with 8 valence electrons in our shells!

Oxygen added:

O

                                 S                Mg          Ca

Exactly! Sulfur and I just need 2 more electrons to have an octet valence shell. You guys will have an octet as well, meaning 8 electrons in your valence shell in total.

Ionic Bonds are formed! When an atom gains electrons, they gain a negative charge. When an atom loses electrons, they gain a positive charge.

                S   2+                     Ca 2-

                  O 2+                  Mg 2-

                   Calcium Sulfide

                      Magnesium Oxide