How are chemical formulas constructed?

Answer 1

Terms Defined:

* ion: a charged atom (positive or negative) * charge: the electric charge (positive or negative) of an ion * subscript: the "lower" number in a formula. In H2O, the 2 is the subscript

* criss-cross: making a formula by putting the charge of one ion as the subscript of the other, and reducing all numbers to lowest terms

There are two major types of chemical formulas: ionic and covalent. Ionic compounds are formed from a metal and a nonmetal, while covalent compounds are formed from nonmetals. The use of the metalloids is more complex, and their behavior can vary.

We'll begin with ionic. As a first step, it's easiest to write out the charges of the ions. If you are requested to write the formula for Sodium Chloride, you would write Na+1 and Cl-1. You then criss-cross these numbers, giving you Na1Cl1. Since it's assumed that if no number is shown, it's a "1," the proper notation is NaCl. Now a harder one: Magnesium Oxide. The ions are Mg+2 and O-2. Criss-crossing these numbers would give Mg2O2. However, when dealing with ionic compounds you can reduce these numbers just like a fraction, giving you MgO. Harder yet is Calcium Chloride, whose ions are Ca+2 and Cl-1. Remember that criss-crossing means that the charge of one ion becomes the subscript of the other (but with no +/- sign). CaCl2 is the correct formula for Calcium Chloride. Yet more complex is where both ions take subscripts. An example is Aluminum Oxide, whose ions are Al+3 and O-2. Criss-crossing results in Al2O3. The most confusing are variable charge ions, such as Iron (III) Oxide. The (III) means that the iron as a +3 charge. All transition metals except for silver, cadmium, and zinc should all have a roman numeral in their name. So for our Iron (III) Oxide, the ions are Fe+3 and O-2. Criss-crossing these gives us Fe2O3. The same rules as listed above apply to any polyatomic ions you may encounter. However, remember to put any subscripts applied to a polyatomic ion outside parentheses. The formula for Copper (III) Sulfate is Cu2(SO4)3 Here are some more examples for ionic:

Strontium Bromide: SrBr2

Silver Nitrate: AgNO3

Iron (II) Oxide: FeO

Zinc Chloride: ZnCl2

Aluminum Sulfide: Al2S3

Congratulations, we now can move onto covalent bonding, which is much simpler. All covalent compounds' formulas are given with prefixes which tell you how many atoms of each element are in the molecule. The first six prefixes are

1-mono

2-di

3-tri

4-tetra

5-penta

6-hexa

The first element of the compound does not have the "mono" prefix. So, for instance, Sulfur Dioxide's formula is SO2. A few more examples:

Dihydrogen Monoxide: H2O

Xenon Hexafluoride: XeF6

Nitrogen Tribromide: NBr3

Diphosphorus Pentoxide: P2O5

Note that the -mono, -tetra, -penta, and -hexa prefixes drop their last vowel before an element beginning with a vowel. Monoxide is correct rather than monooxide; tetroxide is better than tetraoxide, etc.

This covers most of bonding and chemical formulas; I hope you enjoy reading this as much as I enjoyed writing it!

Answer 2

Chemical formulas are deducted from the chemical composition of a substance; and the chemical composition is established by chemical analysis.