Formula Writing - Ionic Compounds

One thing that I find surprising is that usually a short amount of time is spent on going over formula writing - at least in the districts I've experienced. The amount of time that it takes for students to pick up on the patterns of formula writing varies greatly and the future topics that focus on formula writing are ones that branch out to a lot of other topics.

- Reaction Types

- Reaction Prediction
- Solutions

Needless to say, it makes things easier to learn it right the first time. 

Formula writing is the practice of writing the chemical formula from the name of a compound and ionic compounds follows a mathematical pattern. To discern this, you need to know the general charges of the main group elements:

Now this should've been covered in earlier material that covered valence, ions, and ionization energy, but for the purposes of this topic, we'll condense that. 

If you expand the periodic table above, you'll notice some column numbers are followed by an A while others are followed by a B.

Those with the A are your main group elements or representative elements. These are the most abundant elements

Those with the B are your transition metals. 

Your 1A group, 2A group, and Aluminum will have a positive charge equivalent to the A column it is in (Al is 3+ , Ca is 2+ etc) These elements have relatively low ionization energies (that is the energy it takes for an element to lose it's electrons). So, in essence, it's fairly easy to remove an electron (or 2 or 3) to get the element into a stable state defined as a valence of 8, 2 in the case of Helium.

Remember, charges are written in the upper right hand corner

The non metals in groups 5A - 8A have high ionization energies so it is hard to remove electrons and they tend to gain electrons, which makes them negative. If you subtract 8 from their group numbers, you get their charge ex. N = 5-8 = -3. 6A will have a -2 charge. 7A will have a -1 charge. Another way of looking at this is Nitrogen has 5 valence electrons and it needs to gain 3 to get to 8, hence, the -3 charge. The elements in group 6A have 6 valence electrons and they need to gain two to get to 8, hence the -2 charge. 

Keep in mind, this is just the preliminary background information for formula writing. I can go into larger detail for the charges in a later post. 

Formula writing is, generally, going from a name to a formula; however, in the case of reaction prediction, you're given formulas that change as a result of the reaction. Based off of what you are given, you have to write the correct formula if the reaction occurs. The charges of the formula have to equal zero so that you have a neutral compound, the positive values must equal the negative values. 

Name to Formula

You have two things to consider: 

a) main group binary and ternary ionic compounds

b) transition metal elements

In general, the cation (positive ion) will always come before the anion (negative ion).

a) Binary ionic compounds contain two elements. Ternary ionic compounds contain three elements, usually a polyatomic ion is involved. 

Consider the picture for Magnesium Chloride (MgCl2) and Sodium Chloride (NaCl)

Key things to observe:

1.) The negative and positive charges are balanced. Na needs only one Cl while Mg needs two to balance. 

2.) The charges are not written in the final compound 

Consider the picture for Ammonium Chloride (NH4Cl), Sodium Phosphate (Na3PO4), and Magnesium Nitrate (Mg(NO3)2). 

Key things to observe: 

1.) you have groups of elements that have an associated charge.

2.) when you have multiple polyatomic ions, you have to put parentheses around them indicate the whole group appears 2 or 3 times in the compound. 

The memorization required is having to recognize that Sodium Chloride is a Na+ ion combined with a Cl- ion in a NaCl compound. Given that there is a set pattern with how ionic compounds combine - there is only one combination for sodium chloride just like there is only one combination for magnesium nitrate, there are NO PREFIXES used. 

b.) Transition metal elements. Transition metals can have a variety of charges. As a result, the name usually will carry the charge of the cation, you would have to remember the name and charge of the anion.

ex. Lead (II) Sulfate vs Lead (IV) Sulfate

The charge on the cation will be given by the name of the compound. Other than that, all the same rules for binary and ternary compounds apply for transition metal ionic compounds. 

Practice: (highlight next to the compound name for the answers) 

Sodium Nitride  Na3N

Aluminum Phosphate   AlPO4

Ammonium Fluoride  NH4F

Lithium Chloride   LiCl

Tin (IV) Chloride   SnCl4

Iron (II) Oxide   FeO

Iron (III) Sulfate  Fe2(SO4)3

Other Resources:

Writing Ionic Compounds