The formula of a salt

The name of salt is formed from the positive and negative ion's names, but the name doesn't tell you the proportion between the positive and negative ions. This is implied, which causes confusion among many students. In this article we will explain how to use the periodic table to determine the proportion between the ions and determine the chemical formula.

The formula shows the proportions

A salt crystal is not a gathering of separate molecules, but is rather a clump, or giant network of positive and negative ions attached in a regular pattern. For chemical formula for the salt shows the proportions between the positive and negative ion. This is what we call an empiric formula. Magnesium chloride has the empirical formula MgCl2, which shows there are twice as many chloride ions as there are magnesium ions in a crystal of magnesium chloride.

One could say that the empirical formula shows the smallest repetitive block of a salt. This block is called formula unit. Don't mix this up with molecules. Molecules are separate building blocks which are attached with strong bonds within, and weak bonds between, while the formula unit for a salt crystal has the same ionic bonds within as between the blocks.

Determining the charge of the ions

The first step toward determining the empirical formula for a salt is to find which charge the positive and negative ions have. For simple atomic ions, such as the sodium ion, this is done through the periodic table. The group number tells us how many valence electrons the atom has, which means we can determine the charge of the ion through the octet-rule. Sodium, for example, is in group 1, has one valence electron, and forms ions with the charge 1+. Oxygen is in group 16, has 6 valence electrons, and forms ions with the charge 2−.

Net charge of zero

Salts are always uncharged as a total, which means that the charge of the positive and negative ions "cancel each other out". If the positive ion is monovalent, and the negative ion is monovalent, we need one of each to reach a net charge of zero. Na+ and Cl gives the salt NaCl. A salt is never written with charges, since it's charge neutral.

When dealing with ions that are not monovalent, we need to combine them differently. Ba2+ and Cl gives the salt BaCl2. In this case we have two chloride ions per barium ion to balance out the charges.

It becomes even more complex with divalent and trivalent ions. Fe3+ and SO42− creates the salt Fe2(SO4)3.

Parenthesis are used if you have several units of a molecular ion. Fe2(SO4)3 means we have 2 Fe3+, and 3 SO42−. If we would not have used parenthesis, it would have been impossible to understand how the ions look. Writing the same salt as Fe2S3O12 (incorrect way to write it) makes it incredibly hard to understand that it contains three sulfate ions in the salt.

Several possible charges

Something which is good to know, is that some ions may vary in charge. This is true especially true for ions from the transition metals. The iron ion can be either Fe2+ or Fe3+. When you see the formula unit of the salt you can determine the charge of the iron. FeSO4 means we have a divalent positive iron ion, since the sulfate ion is divalent negative. Other ions that often vary in charge are the copper ion and the lead ion. In the cases where an element can create ions with differing charges, you can specify the charge by adding the oxidation state in the same, for example iron(II) sulfate.

The next article in the series is about naming salts.

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