You’ll find four major categories of chemical names out there. Three of them are scientific, and have rules that are easy to follow (until you get to large, complex molecules anyway). The fourth are simply made-up names – either a name assigned by the discoverer of the chemical or a trade name chosen by a corporation that they felt would make their newest wonder-drug more marketable to the public. There are no real rules pertaining to made-up names, save that they should not have the same name as another chemical. Below are some basic rules for how to name simple compounds scientifically.
Class 1 – Inorganic, Ionic compounds (instructions for binary only)
Ionic compounds are what the name implies, a compound made from ions. The positively and negatively charged ions (cations and anions) attract one another and form a regular structure that we perceive as a crystal.
To name an ionic compound, you simply have to name the ions that it is made of, cation first, followed by the anion. (Simply assumes that you already know your element and ion names.)
If you don’t already know how to identify the ions in a compound, here’s a quick guide:
Any element that appears on the left end of the periodic table is a metal and has a positive charge as an ion. (That makes it the cation, so it gets named first.) To name a metal ion, you simply say the name of the metal with the word “ion” following it. In a chemical name, you drop the word “ion” again, so you just say the name of the metal.
Any element that appears on the right end of the periodic table will be your non-metal (or, less often, a metalloid). These have a negative charge in ionic compounds, so they are the anions and get named second. To name the anion, take the original name of the element, drop the ending and replace it with -ide. (carbon becomes carbide, bromine becomes bromide, phosphorus becomes phosphide…)
The most common example in this category is table salt, which is NaCl, or sodium chloride.
Class 2 – Inorganic, Covalent compounds (instructions for binary only)
Covalent compounds are formed from atoms that are covalently bonded. These rules apply when the two elements are both found on the right side of the periodic table (so no metals are involved). The naming is similar to ionic compounds, with two major differences. First, there are no cations/anions, so you have to use a different method to determine the order, and second you have to specify how many of each atom are found in the compound. Here’s how:
To choose which element comes first, you find the one with the lower electronegativity. If you have a table of electronegativities handy, great! If you don’t you can make a good guess by looking at the periodic table. Fluorine (top right) has the highest electronegativity. Whichever element is farther from fluorine will likely have the lower electronegativity and should be named first.
The element that is named first should keep its regular name, and the second element should be changed to an -ide, as in ionic compounds.
Because the atoms covalent compounds can occur in multiple ratios, you have to say how many atoms of each element are in the compound. We do this with numerical prefixes on the names.
1 = mono-
2 = di-
3 = tri-
4 = tetra-
5 = penta-
6 = hexa-
7 = hepta-
8 = octa-
9 = nona-
10 = deca-
11 = undeca-
12 = dodeca-
The only exception is that if there is only one of the element being named first, we don’t include the mono- prefix.
Applying these rules, you can get compounds like these. (Please imagine that the numbers are subscripted.)
carbon monoxide – CO – one carbon, one oxygen – the poisonous gas in car exhaust
dihydrogen monoxide – H2O – two hydrogens, one oxygen – also known as water. Don’t fall for the DHMO scam – visit DHMO.org if you’re curious
nitrogen tri-iodide – NI3 – not very common, but it explodes with a puff of purple smoke
Class 3 – Organic, Covalent compounds (single-bonded carbon chains [alkanes] only)
Compounds that are made up primarily of carbon and hydrogen (but can also include oxygen, nitrogen, sulfur, chlorine, and pretty much anything else you like) are referred to as orgainc compounds. Because they can get complex very quickly, they have a special set of naming rules. To keep this from becoming a textbook, I’ll only go over the basics of naming a chain made of single-bonded carbons and hydrogens. These compounds are referred to as “alkanes”.
To make matters easy, hydrogens are ignored. The name only depends on how the carbons are connected. (This is because hydrogens only make one bond, so they are only on the outside fringes, and don’t determine structure at all.)
Every single-bond carbon chain will have the suffix -ane. The -ane tells everyone who reads it that the compound has only single bonds in it.
If all the carbons are attached in a row (a straight chain compound), the naming is simple. A prefix is chosen based on how many carbons are in the chain. (Sadly, these aren’t the same prefixes used in inorganic compounds, so you have to learn another set.)
1 Carbon = methyl-
2 = ethyl-
3 = propyl-
4 = butyl-
5 = pentyl-
6 = hexyl-
7 = heptyl-
8 = octyl-
9 = nonyl-
10 = decyl-
(you can see that they match up with the inorganics once you get to 5)
In naming the straight chain compound, the “yl” portion of the prefix is dropped.
C4H10 has four carbons, and is named “butane” – found in lighters
CH4 has but one carbon, and is “methane” – a greenhouse gas produced by cows and landfills
C8H18 has eight carbons and is “octane” – a major ingredient in gasoline
To give you an idea of what is meant by a “straight chain”, here’s a poor depiction of octane.
(hopefully this shows up right)
H H H H H H H H
| | | | | | | |
| | | | | | | |
H H H H H H H H
The alternative is to have a “branched chain” which has one or more carbon chains coming off of the main chain. To name these requires a little more effort. First you have to find the longest carbon chain in the compound. This is the one we will name using the rules you already know. Any branches that come off of it get named based on how many carbons they have, using the same prefixes, but this time keeping the “yl” as a part of the name. Lastly, you have to specify which carbon in the main chain the branch is attached to, which we do by numbering the chain.
Here’s a sample branched compound, leaving the hydrogens off to make life easier:
(hopefully this shows up right)
The longest chain that can be found is to follow from the top carbon down to the right end, giving us eight carbons. That means this will be an octane.
Everything that wasn’t in that eight-carbon chain needs to be named, which leaves three side-chains branching off of our octane. On the left is one carbon making a branch all on its own. One carbon is a methyl- chain. On the bottom left is another methyl- chain. In the bottom middle is a two carbon chain, which is an ethyl- chain.
Now we know all the pieces (an ethyl, two methyls and an octane) so we just ahve to say how they’re put together. We need to number the carbons on the octane to do this. Start numbering at whichever end is closest to a side chain. In this case that will be at the top, so that the first methyl comes off at carbon #3, the second at #4, and the ethyl at #5. To put it all together, we list the side chains in alphabetical order, with the number to indicate where they are. Rather than writing “methyl” twice, we will say “dimethyl” instead.
That’s beyond any common chemical you’ll find, and so your lesson ends here.
See Also: Polyatomic ions, Hydrates of compounds, IUPAC rules, Transition metal ions, Substituent groups, Alkenes, Alkynes, Nomenclature