Main->Readings->5th Grade Readings->Chemistry->Part 4: Chemical Notation
Vocabulary |
Chemists speak in code. In this code, every element has its own symbol (one or two letters) which stands for that chemical's name. The symbols for the elements are called chemical symbols. Every chemist knows the symbols well, and they can use the symbols to name every molecule that can be made. A group of symbols showing what elements make up a compound is known as a chemical formula. Chemists can use this code to talk about matter and the ways atoms can make compounds.The code is known as chemical notation.
Hydrogen (HIGH-dro-jenn), for instance, is an element. It is a colorless gas, lighter than air, which explodes if a spark hits it. When chemists want to talk about hydrogen, sometimes they use the word "hydrogen," and other times they just write this:
|
H
|
|
Some of the elements have symbols that don't seem to fit. For instance, the precious element gold is:
What does Au have to do with gold? Gold is an element which has been known since ancient times. Its name was not always "gold." Its Latin name was "aurum," and that is how it got its unusual symbol. There are a few other elements with unusual symbols, and most of them, like gold, have been known to scientists long before the modern chemical names were adopted. |
Every chemist knows what is meant by the upper case letter H. It can only mean hydrogen, nothing else.
But what about the element helium? Helium starts with an H, too. Helium is a colorless gas which is lighter than air, but it doesn't explode easily. That is why we use it to fill balloons. You wouldn't want to make a mistake and fill balloons with hydrogen instead, especially if you were going to use the balloons at a birthday party with candles on the cake! However, there is a way around that. When chemists want to talk about helium, they write this:
|
He
|
An upper case letter H followed by the letter "e" is always helium.
Every element (and there are more than 110 of them) has its own symbol. Most of them are not hard to guess from the names of the elements. For instance, C is carbon, O is oxygen, and Al is aluminum. You can find all the symbols of the elements in Appendix A at the end of the packet.
Why bother to have symbols for the elements? They are the building blocks of all matter. Elements are put together to make compounds. If chemists want to describe compounds, they need a short way of doing it, and chemical symbols work well.
For instance, water is a very common compound. You could say, "a molecule of water has two atoms of hydrogen and one atom of oxygen," or you could use the chemist's code and say:
|
H2O
|
Which means exactly the same thing. Chemists use a small "subscript" (a small number written at the bottom and to the right of the symbol) to show the number of atoms of an element there are in a molecule of a substance. If there is no number, that means there is one atom. Here are a couple of examples:
Carbon dioxide is a compound. Its molecule has one atom of carbon and two atoms of oxygen. A chemist writes it like this:
|
CO2
|
Sodium bicarbonate (baking soda, which people use for making cookies) is:
|
NaHCO3
|
You can write all kinds of compounds with this code. For instance, how about acetylsalicylic acid, otherwise known as aspirin? It's C9H8O4. That is, a molecule with 9 atoms of carbon, 8 atoms of hydrogen, and four atoms of oxygen.
Chemists use this code all the time without even thinking about it. It is shorter than writing the formulas out in words, and much shorter than drawing diagrams of the molecules, though sometimes they do that too.For instance, nitric acid (a very powerful and destructive substance) has a formula of HNO3 and can be drawn as:
Return to Table of Contents
Go on to Part 5: The Periodic Table
This page last modified August 15, 2002
Questions? Send me a note at dturner@haverford.org