Assembling a salt

Common salt or ‘table salt’ is made up of sodium ions and chloride ions. Its chemical name is sodium chloride. With chemical symbols, it’s written as: NaCl. The chemical formula shows that there are the same number of positive ions and negative ions. The ratio between them is "one to one".

Here is another salt. It contains magnesium ions and chloride ions: magnesium chloride. It has exactly twice as many chloride ions as magnesium ions, that is, the ratio of "one to two". The chemical formula of magnesium chloride is MgCl2 Aluminum chloride is also a salt, with the chemical formula AlCl3 For each aluminum ion there are three chloride ions. What determines how many ions are needed of each kind, to form these compounds?

It has to do with the charges of the ions. To form an ionic compound, there must be equal numbers of positive and negative charges. Sodium ions and chloride ions have the charges plus one and minus one. It is enough with one of each for the charges to cancel each other out. Magnesium ions have a charge of two plus.

We’ll need two chloride ions with one negative charge each. And that is why the formula is MgCl2 In aluminum chloride, three chloride ions are needed to get a neutral compound. The chloride ions still carry one negative charge each, so the aluminum ions must have a charge of three plus. Imagine the ions as pieces of a jigsaw puzzle. A negative ion has a “tab” for each additional electron, while a positive ion has a "hole" for each missing electron.

It’s enough with one piece of each kind, to form the ionic compound NaCl. The magnesium ion is missing two electrons, so its puzzle piece has two holes. And the aluminum ion with a charge of three plus, has three holes. The pieces show how many chloride ions are needed for the charges to match. One for sodium, two for magnesium, and three for aluminum.

Here is a salt that contains the elements sodium and sulfur. The ions formed by sulfur are called sulfide ions. This is sodium sulfide. Here we need two sodium ions for every sulfide ion. The formula is Na2S.

Can we figure out the charge of the sulfide ion? Yes. Since it requires two sodium ions, each with a charge of one plus the sulfide ion must have a charge of two minus. We can show it as a jigsaw-piece with two tabs. Two “plus” ions together with one “two-minus” ion, form a neutral salt.

What if we want to assemble a salt using magnesium ions and sulfide ions? What will the formula be? A “two-plus” ion and a “two-minus” ion. One of each is enough for the charges to cancel each other out. Magnesium sulfide has the chemical formula MgS.

This way, we can assemble any ionic compound, by making sure that the number of plus charges and the number of minus charges are the same. That the pieces of the jigsaw puzzle fit together. Let’s try with aluminium sulfide, for instance. Aluminum with a charge of three plus, and sulfide with a charge of two minus. How many will we need of each?

You can pause the film if you need to. We need to get the same number of positive charges and negative charges. The same number of "holes" and "tabs". With two aluminum ions, we have a total of six "holes". And with three sulfide ions, we get six "tabs".

The chemical formula of aluminium sulfide is Al2S3 There are ions with an even higher charge. Tin, with the chemical symbol Sn, can form ions having a charge of plus four. That means a tin puzzle piece has four holes. Nitrogen, with the chemical symbol N, forms ions with a charge of minus three. These ions are called nitride ions and have jigsaw pieces with three tabs.

How many will we need of each ion to form the salt tin nitride? Let’s add more and more pieces until all tabs and holes are connected. With three tin ions, we get a total of twelve positive charges and with four nitride ions we get twelve negative charges. The chemical formula of tin nitride is Sn3N4