Once upon a time there was a land full of atoms. Some atoms were happy and others were sad. For example, Neon was happy, as was Argon and Krypton. They were happy because they had a full sets of electrons in their energy levels (8 in the outermost level to be exact.) Everyone thought they were so cool and so happy, that they even had a cool name for them - the noble gases.
Not all of the atoms were so happy, however. "Oh, my life is awful!" said Lithium. "As an Alkali Metal, I only have one electron in my outermost energy level."
Fluorine had another problem. "You guys think you have it bad?" she asked. "Try being a Halogen like us!" chimed in Chlorine. "Yeah," added Bromine, "with seven electrons in our valence level, we're exactly one short of where we need to be, that magic number of 8!"
Then Lithium had an idea. "Hey, Chlorine!" he said. "What about if I give you my electron. Then you'll have the eight you need, and I'll have the full energy level underneath for myself!" Chlorine quickly agreed and took Lithium's electron, and they became the compound Lithium Chloride. In fact, everyone thought it was such a great idea that Sodium gave his electron to Bromine, making the compound Sodium Bromide and Potassium gave his electron to Fluorine, making the compound Potassium Fluoride.
And then everyone had complete sets of electrons in their energy levels and lived happily ever after as compounds. The End.