In chemistry, every element is represented by a one- or two-letter symbol1. The first letter is always capitalized, the second is in small case. You can find the symbols listed in a periodic table, a chart which lists the chemical elements in order of atomic number in such a way that related elements appear in the same columns (that statement is oversimplified, but it will have to do for now)2. There are many periodic tables available on the Web, but the one I suggest you take a look at first is Michael Dayah’s Dynamic Periodic Table, which you can view by clicking here. Note that you can amplify the information on any element by moving your cursor over it.
As you can see, most symbols are abbreviations of the element’s name: H for hydrogen, O for oxygen, Cl for Chlorine, and so forth. But quite a few symbols are not that way: Fe for iron, Ag for silver, Au for gold, for example. When I was younger, I wondered why that was. Why not use Ir for iron, Sv for silver, and Gl for gold? Then it dawned on me — not every chemist is an American who speaks English. Most of the elements that have funny symbols have been known to mankind for a very long time, and different languages have different words for these elements. “Iron” is fer in French, hierro in Spanish, Eisen in German, zhyelyezo in Russian, and barzel in Hebrew. Which language should we use for the symbol? It made sense to choose a classical language that is universally recognized but no longer spoken, and in Latin the word for “iron” is ferrum. That is why the chemical symbol for iron is Fe3.
Here are the more common symbols you are likely to encounter in chemical formulas, at least in this blog, ordered by atomic number.4:
|Element||Average Atomic Mass
(see note below this chart)
Note: Average atomic mass refers to the average mass of all atoms of that element found in nature, measured in daltons. It is numerically equal to the molar mass of the element as measured in grams per mole.
Using the element symbols, we can then represent chemicals with a shorthand notation called a chemical formula7. Each element that makes up the chemical is represented by its symbol in the periodic table. If two or more atoms of the same element make up the chemical, the number of atoms appears as a subscript following the symbol.
Let’s start by showing how molecules are represented by formulas. The formula shows how many atoms of each element make up the molecule. Hydrogen molecules consist of two hydrogen (H) atoms each, so the chemical formula for hydrogen gas is8:
Water molecules consist of two hydrogen atoms attached to an oxygen (O) atom, so its formula is the familiar9:
A common form of gasoline is octane, whose molecule consists of a chain of eight carbon (C) atoms surrounded by 16 hydrogen atoms. Its formula is10:
Chemical formulas can also represent ionic compounds. Here the formula doesn’t describe a molecule, but rather the proportion that each element appears in the compound. By convention, cations always appear before anions. In a previous post, we encountered a chemical called calcium chloride, which consists of two chlorine (Cl) anions for every calcium (Ca) cation. Its chemical formula is11:
In a previous post, we discussed how molecules can form parts of larger compounds, either as polyatomic ions or as functional groups of larger organic (carbon-based) molecules. When we write a formula for a compound having polyatomic ions or functional groups, we want the formula to show these groups. For example, we came across the compound ammonium nitrate, which consists of two nitrogen (N) atoms, three oxygen atoms, and four hydrogen atoms12. One might think that its formula should be written:
but that would obscure the fact that ammonium nitrate is made of two polyatomic ions:
- The ammonium cation, which is a molecule consisting of a nitrogen atom with four hydrogen atoms attached. Its chemical formula is NH4.
- The nitrate anion, which is a molecule consisting of a nitrogen atom with three oxygen atoms attached. Its chemical formula is NO3
That is why the accepted formula for ammonium nitrate is12:
As you can see, the formula for ammonium nitrate simply puts the formulas for the ions together. The ratio between ammonium and nitrate ions is 1:1.
Acetic acid is the main component of vinegar. Its molecule consists of two carbon atoms, four hydrogen atoms, and two oxygen atoms. As such, its formula is commonly given as13:
But this formula obscures an important part of the acetic acid molecule, the carboxyl group COOH. To show the carboxyl group, its formula is frequently written as14:
(You may ask why I did not represent the carboxyl group as CO2H, although I’ve seen that notation as well. The COOH formula emphasizes that the carbon atom [C] is bonded to an oxygen atom [O] and to a hydroxyl group [OH]15.)
Just like elements can appear more than once in a chemical formula, so can polyatomic ions and functional groups. When this happens, the group is enclosed in parentheses. Calcium hypochlorite is a compound used in sanitizing swimming pools. It consists of two ions: the calcium cation (an element, symbol Ca) and the hypochlorite anion (a molecule consisting of a chlorine atom attached to an oxygen atom, formula ClO). There are twice as many hypochlorite ions as calcium ions, therefore the formula is16:
Chemical formulas are used in describing chemical reactions, in which a chemical breaks up into simpler chemicals, or two or more chemicals recombine into new chemicals. We can represent chemical reactions with a notation called a chemical equation. On the left side are the formulas for the chemical or chemicals that start the reaction, on the right side are the symbols for the product or products of the reaction. Plus signs (+) connect the compounds on either side of the equation. Separating the two sides in the middle is a right arrow (→) showing the direction of the reaction17. There must be as many atoms of each element on the right side of the equation as there are on the left side.18
For example, carbon burns in the presence of oxygen to form the compound carbon dioxide. This is how we represent the reaction with a chemical equation19:
C + O2 → CO2
A chemical equation can be read in two ways. First, it can show how various atoms, ions, molecules, and functional groups combine and separate to form new molecules and ionic compounds. Second, it specifies the proportions that amounts of chemicals react with each other as measured in moles. By knowing the molar mass of each substance, you can then calculate the masses of the reacting substances and their products20. Bear in mind that the molar mass of a homogeneous chemical is equal to the sum of the molar masses of the elements in the proportions that make it up21.
There are two ways to read the equation above describing carbon combining with oxygen:
- One atom of carbon combines with the two atoms in an oxygen molecule to form one carbon dioxide molecule.
- One mole of carbon (12 grams) combines with one mole of oxygen gas (32 grams) to form one mole of carbon dioxide (44 grams).
Here’s what happens when hydrogen and oxygen gas combine to form water22:
2H2 + O2 → 2H2O
Again, you can read this equation in two ways:
- Two molecules of hydrogen gas combine with one molecule of oxygen gas to form two molecules of water. Note that the same numbers of atoms of both elements appear on both sides of the equation: four hydrogen atoms and two oxygen atoms.
- Two moles of hydrogen gas (4 grams) combine with one mole of oxygen gas (32 grams) to form two moles of water (36 grams).
Chemical equations can also describe ionic reactions. Sodium metal will combine with chlorine gas to form table salt23:
2Na + Cl2 → 2NaCl
Two atoms of sodium metal will react with one molecule of chlorine gas to form table salt. Note that the formula 2NaCl does not represent two molecules since table salt is an ionic compound. The "2" is necessary to keep the number of atoms on each side of equation balanced.
But the second way to read the equation is that two moles of sodium (46 grams) combine with one mole of chlorine gas (about 71 grams) to form two moles of table salt (117 grams). Here the need for the "2" is more obvious.
Finally, polyatomic ions and functional groups can be represented in chemical equations. Calcium hypochlorite is made by mixing calcium hydroxide (hydroxide is a polyatomic anion) with chlorine gas24:
2Ca(OH)2 + 2Cl2 → Ca(ClO)2 + CaCl2 + 2H2O
Two calcium cations and four hydroxide anions react with two chlorine gas molecules to form the ionic compounds calcium hypochlorite and calcium chloride, plus two molecules of water.
Alternatively, two moles of calcium hydroxide (148 grams) combine with two moles of chlorine gas (142 grams) to form one mole of calcium hypochlorite (143 grams), one mole of calcium chloride (111 grams), and two moles of water (36 grams).
In the next post, we will discuss the nature of energy.
- Strictly speaking, there are elements with three-letter abbreviations, but they are all newly discovered elements with half-lives too short to be of any practical value.
- Freudenrich, Craig, How the Periodic Table Works. HowStuffWorks website. To view, click here. See also Western Oregon University website. A Brief History of the Development of the Periodic Table. To view, click here. See also the American Institute of Physics website, Marie Curie and the Science of Radioactivity: The Periodic Table of the Elements. To view, click here. I discovered a very entertaining periodic table on the web. Gray, Theodore and Whitby, Max. Photographic Periodic Table of the Elements. To view, click here.
- Our modern system of chemical symbols was invented by the Swedish chemist Jöns Jacob Berzelius (1779 – 1848). van der Krogt, Peter, Development of the chemical symbols and the Periodic Table, on Mr. van der Krogt’s personal website. To view, click here.
- Information taken from Michael Dayah’s Dynamic Periodic Table, which you can view by clicking here.
- Lead is the only element in this list that I could not find an average atomic mass accurate to two decimal places. All the sources I looked up listed the average atomic mass of lead as 207.2 daltons. I wonder why.
- This is not the average atomic mass of plutonium but the mass of its longest-lived isotope. This is because plutonium is not found in nature but only manufactured in nuclear reactors, so it can’t have an average mass of all its atoms found in nature.
- NDT Resource Center website (operated by the Collaboration for Nondestructive Testing Education). Chemical Formula. To view, click here. For a more in-depth explanation of chemical formulas, see the Washington University at St. Louis website, Chemical Formulas. To view, click here.
- The Chemical formula website. Hydrogen. To view, click here.
- The Chemical formula website. Water, H2O. To view, click here.
- Helmenstine, Anne Marie, About.com Chemistry website. Octane Chemical Structure. To view, click here.
- This is the formula for anhydrous calcium chloride, calcium chloride that has not absorbed water into its crystal structure. National Institute of Health’s PubChem website. Calcium chloride anhydrous. To view, click here.
- Helmenstine, Anne Marie, About.com Chemistry website. Ammonium nitrate. To view, click here.
- National Institute of Health’s PubChem website. Acetic Acid. To view, click here.
- Helmenstine, Anne Marie, About.com Chemistry website. Vinegar Chemical Formula. To view, click here.
- Pearson Prentice Hall website.Concept 6 Review: The Carboxyl Group. To view, click here. Also, see the U.S. Department of Energy’s NEWTON: Ask a Scientist! website. Acetic Acid CH3C00H not C2H4O2. To view, click here.
- Dictionary.com website. Calcium Hypochlorite. To view, click here. Note that their formula slightly varies from ours in that it shows the hypochlorite anion as OCl rather than ClO: Ca(OCL)2.
- Helmenstine, Anne Marie, About.com website. What is a Chemical Equation?. To view, click here.
- Helmenstine, Anne Marie, About.com website. Balancing Chemical Equations. To view, click here. The art of balancing chemical equations is called stoichiometry. See also the Purdue University Bodner Research Web website, Balancing Chemical Equations. To view, click here.
- University College Cork website. Combustion of Carbon. To view, click here.
- This follows from the definition of molar mass. The molar mass is the amount of grams in every mole. If you know how many moles of the substance you have, you just multiply by the molar mass to get the mass in grams. Similarly, if you have a known mass in grams of a certain substance and you know its molar mass, divide the mass by the molar mass to get the number of moles.
- This also follows from the definition of molar mass. One mole of a substance has as much mass in grams as a molecule of the substance has in daltons (for ionic compounds, treat the proportion of ions as reflected in its chemical formula as if it were a molecule. For example, calcium chloride is CaCl2). But the mass of a molecule is equal to the sum of the masses of atoms that make it up. Therefore, the molar mass of the substance must equal the sum of the molar masses of the elements in the proportions that they appear in the molecule. In our example, the molar mass of calcium chloride is equal to the molar mass of calcium plus twice the molar mass of chlorine.
- Purdue University Bodner Research Web website. Chemical Equations. To view, click here.
- Angelo State University website. Sodium + Chlorine: Pass the Salt, Please. To view, click here.
- Wikipedia. Calcium Hypochlorite To view, click here.