Writing and interpreting chemical formulas
Since the field of chemistry began, we've worked out a system of rules for writing, interpreting and naming chemical substances according to the atoms they include, and often, their arrangements.
It's important that you know the basics before you go on with your study of chemistry. This section should help.
One of the most important things you can remember about our chemical notation is that it reflects the fact that atoms always combine in certain whole-number (integer) ratios. That's one of the central ideas of chemistry.
Here's the basic outline →
Always, in chemical formulas ("formulae if you prefer the Latin pluralization), we use the 1 to 2-letter atomic symbols to stand for the elements of the periodic table.
Click on the table on the right to go to the section on the periodic table.
Specifying isotopes
The identity and most properties of an element is completely determined by the number of protons (p+) in its nucleus. The number of neutrons (n0), however, may vary, and we need a way to show that in our notation. To do that we use super- and subscripts, like this:
Although they're not directly specified, it's easy to determine the number of neutrons in such an atom by simply subtracting the subscript from the superscript; there are 4 - 2 = 2 neutrons in the helium isotope shown.
There are several isotopes of carbon, the fundamental building block of life on Earth. The version with 6 neutrons is the most abundant. "carbon-14" is radioactive, and decays to half its mass every 5,700 years, making it handy for dating previously-living artifacts.
The table below shows all of the isotopes of uranium (92 protons) found (or made synthetically in a lab) on Earth.
We say "on Earth" because elements as large as uranium come from supernova explosions elswhere in the galaxy. The isotopic abundances are probably different on different celestial bodies throughout the universe.
This notation will become important later as you learn about nuclear reactions, in which the number of neutrons, and protons in a nucleus can change.
We use subscripts in chemical formulae to indicate the number of atoms of an element present in am molecule or formula unit. There are no exceptions to this. Here are some examples:
The familiar formula of water is a good place to start. It indicates that each water molecule contains two hydrogen atoms and one oxygen. In terms of atoms, water always has a 2:1 ratio of hydrogen to oxygen.
Note also that if no subscript is written, we assume it to be one.
While there are plenty of naming rules in the field of chemistry (they fill books), many chemical compounds like water and many of the ones to follow have common names. We almost never refer to water as "dihydrogen oxide."
Methane is one of the simplest hydrocarbons (a compound containing only carbon and hydrogen). These are important as fuels. Methane is a major component in natural gas.
Sulfuric acid is a strong acid you'll learn about in the section on acids and bases. It's one of the more commonly-used acids in research and industry.
Some formulas contain more than one instance of a single element. There are some good reasons for this that you'll learn in time. One example is acetic acid:
We follow exactly the same rules for counting atoms in these kinds of formulae, we just need to add up all instances of a given atom.
The reason why acetic acid is often written this way is to convey a little bit of structural information – information about how the atoms are arranged in a molecule of the compound. In this case, there is a CH3, or "methyl group" bonded to a central carbon. Also bonded to that carbon is an OH (or "hydroxyl") group and a double-bonded oxygen. Don't worry about these details for now; just focus on counting atoms.
Finally, a molecule of cholesterol contains 27 carbon atoms, 46 hydrogens and a single oxygen. This is an example of how a simple chemical formula like C27H46O doesn't really give us much of a picture of how cholesterol looks.
A better representation of cholesterol is the shorthand stick figure below. Carbon atoms are implied at every line end and vertex, and in such a figure, it is implied that each carbon is bonded to as many hydrogens as possible. You need a lot more chemical knowledge to interpret such a diagram, so don't worry about it for now. This is just to show that we don't always use such simple formulae.
Very often in chemical formulae, we use parentheses to form subgroups of atoms within a molecule. Usually this has some meaning about the structure of the molecule, but don't worry about that for now.
Parentheses are useless in a chemical formula if they don't have a subscript, so we'll assume one is always there. In such a formula, the subscript outside the parentheses means that to count atoms, you must multiply that subscript by the numbers of atoms inside.
Here's an example. Cisplatin is an important drug used in chemotherapy, and it's written like this:
Here's how we count the atoms in that formula.
The NH3 in parentheses, known as an amine group, occurs twice. The 2 outside the parentheses is multiplied by the implied subscript of 1 on the nitrogen, for a total of two nitrogens, and by the subscript of 3 on the hydrogen for a total of six hydrogens.
Here's a rough picture of the structure of cisplatin. You can see why we group those NH3's. Amine groups appear in a wide variety of compounds, so you'll see them again.
Here are a few more examples of the use of parentheses in chemical formulas.
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It turns out that iron can come in two different "flavors," iron (II) and iron (III). It can also form the compounds FeSO4 and Fe3(SO4)3. Here's how we calculate the numbers of atoms in this molecule:
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There are 13 total atoms in this compound of copper, they are:
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Here's a complicated molecule (it's a magnetic compound of titanium), the formula of which contains nested parentheses.
See if you can follow how we count the atoms. Begin with the inner parentheses, work outward and keep organized.
Very often in chemistry we attach numerical coefficients to the front of a chemical formula. We do this mainly when we balance chemical equations, which is how we make sure that the law of conservation of matter (matter can neither be created nor destroyed) is followed.
Here's a simple example
This notation means "two water molecules." The 2 simply doubles everything. Although we never write them, parentheses around the molecule, H2O in this case, are always implied.
Now it's easy to see that the coefficient is used like a multiplier outside of parentheses. The 2 multiplies all of the subscripts (even the 1's that aren't written) inside of the parentheses:
So in two water molecules, there are a total of four hydrogen atoms and two oxygens. Easy-peasy.
Here's another example, one in which the molecular formula contains parentheses.
The first thing to do is to ignore the coefficient and tackle the parentheses:
Now we can add up the H's and O's, add in the iron and multiply everything by the coefficient, 8.
We always work these problems from the inside outward. Begin with the innermost parentheses, working outward. The coefficient is last, and we treat it as though there are parentheses around the whole molecule.
As you work with chemical formulas more, you'll be able to take some shortcuts, but for now, be careful and methodical.
Practice problems
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Problem 1 solution
Problem 2 solution
Problem 3 solution
Problem 4 solution
This section is meant just to give you a glimpse of structural notation. There's a lot to it and you'll probably need to learn a lot more about chemistry before you really get it. Still, you'll see this kind of notation from time to time and this little section might help.
The chemical formula
Gives us the right ratios of the atoms in this compound (ethanol), but it doesn't tell us much about what the molecule looks like. This rearrangement,
is a little better because it puts the OH, called a hydroxyl group, where it belongs, on the end. This one is a little better still because it shows three hydrogens bound to the left-most carbon, and the hydroxyl plus two hydrogens bound to the right C.
There is a shorthand notation for this kind of molecule. It looks like this.
In this kind of notation, the ends and vertices of the stick figures all stand for carbon atoms, to which are bound all of the hydrogens that the laws of chemistry will allow (carbon can only form four bonds). It's simple and it conveys a pretty good picture of the structure, which is also bent, as shown.
The formula
is the formula for cyclohexane, shown here.
The solid wedges indicate a bond sticking out of the plane of the screen toward you, and the dashed ones behind that plane. The shorthand notation is just a hexagon:
The structural formula for benzene, C6H6, can be written like this:
Solid lines here are single bonds and double lines are double bonds, a stronger type of bond that are very often found in alternation in such compounds. The shorthand notation is:
The benzene structure is also commonly abbreviated as a circle inside a hexagon:
The formula for butylene is
Again, it gets the ratio right, but the structural formula tells us a little more about what the molecule looks like, and the shape of a molecule is often the key to its function.
Finally, we can draw a shorthand stick figure to represent it. The key is that you have to know how many bonds carbon can form. You'll learn that later.
So that's a very brief introduction to some of the kinds of structural formulae we can write.
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