The elements on the right side of the periodic table that only need one or two electrons to fill their outer shells (like F) have a strong tendency to remove electrons from those elements on the left side of the periodic table (like Li), which need to add 6 or 7 more electrons to have full, stable outer shells. When an electron gains or loses electrons, the number of electrons versus protons it has becomes unbalanced electrically.

An atom with an imbalance of + to - charge is called an ion. Positive ions have more protons than electrons and thus a + charge. Negative ions have more electrons than protons and thus a - charge. Because of their charges, positive and negative charges act like the opposite ends of electromagnets and thus they attract each other. This attraction is known as an ionic bond (See the lower diagram above.).

Ionic bonds are very weak (in relation to the strength of covalent bonds) and are very easy to break. Water which is able to ionize too, is able to separate positive ions from their negative ions. An illustration of this would be placing salt (NaCl) in water. The Na and Cl are rapidly separated by the water molecules, and the salt thus dissolves!