Ionic Bonding

Ionic Bonding (also called ion-ion attraction) is the attraction between a positive ion and an negative ion. That's it. Nothing else. Seriously, that all there is to say.

Alternate version with WAY too much information

Many other resources will tell you a long story about ionic bonding that goes something like this: 

A chlorine atom and a sodium atom meet. The chlorine atom pulls on the outermost electron of the sodium and pulls so hard that it completely removes the electron from the sodium. As a result the chlorine becomes a chloride ion (with a negative 1 charge) and the sodium atom becomes a sodium ion (with a positive 1 charge). The two oppositely charged ions then attract each other.

This is a nice story and it may happen lots of times, but the story is just that, a story. Only the last sentence has anything to do with ionic bonding. The rest may explain how the chlorine and sodium became ions, but it also may NOT be how they became ions. In the end, it doesn't matter HOW the two became ions, only that they did and that they now attract each other.

Coulombs Law: 

Ionic bonds vary in strength based on Coulombs Law, which in simple terms says that the attraction between two charged objects increases with the size of the charge and decreases with the distance. There is (of course) a mathematical formula for this, but that's really physics. For us, we need only to understand the effect of these two things:

Charge - the more charge ions are, the stronger the attraction. That means that you would expect the attraction between a calcium (+2) ion and a chloride (-1) ion to be stronger than the attraction between a potassium (+1) ion and a chloride (-1) ion.

Size - When thinking about the attraction between two ions, we can imagine that the attraction is focused on the nucleus. (A physics teacher would say that you can imagine the ion to be a "point charge".) That means, however, that the large an ion is, the further away the point charges are, and the weaker the attraction. That means that the attraction between a Lithium (+1) ion and a chloride (-1) will be stronger than the attraction  between a cesium ion (+1) and a chloride (-1) because Cs⁺¹ is MUCH larger than Li⁺¹.

Why Coulomb's Law Matters in Chemistry

The relative attraction between ions, as described above based on Coulomb's Law, shows up in chemistry in a number of ways:

Lattice Energy - The lattice energy is "essentially" the bond energy of an ionic solid. Just as bond energy is the energy released when two separate atoms come together from "infinitely" far away to form a covalent bond (measured per mole), the lattice energy is the energy released when ions come together from "infinitely" far away to form an ionic solid.

Solubility - Since dissolving an ionic solid involves water molecules breaking apart the ions and pulling them away ion by ion, the stronger the attraction between the molecules the harder it will be to dissolve. That helps to explain why alkali metal compounds are ALWAYS soluble and MOST phosphates are INsoluble.