Chemical Symbols and Writing Formulas
A chemical symbol is a one- or two-letter abbreviation for an element, and a chemical formula uses these symbols with subscript numbers to show which elements are in a compound and in what ratio. In brief: H is hydrogen, O is oxygen, and the formula H2O shows two hydrogen atoms bonded to one oxygen atom; the first letter of a symbol is always capital and the second, if present, is lower case.
Symbols and formulas make up the shorthand language of chemistry. They are used all over the world, regardless of language. This article explains how symbols are chosen, how to read and write formulas, how to find the formula of an ionic compound by balancing charges, lists the symbols of common elements in a table, works through examples and gives the history of chemical notation.
What are chemical symbols?
Each element has a symbol of one, two or (for some recently named elements) three letters. The first letter is always a capital letter and any following letter is lower case. Co is cobalt, while CO is a compound of carbon and oxygen. This rule is important, as a capital letter in the wrong place changes the meaning completely.
Many symbols come from the English name, such as C for carbon and Ca for calcium. Others come from the Latin or other older names, for example Na (natrium) for sodium, K (kalium) for potassium and Fe (ferrum) for iron.
Symbols of common elements
| Element | Symbol | Element | Symbol | Origin of symbol |
|---|---|---|---|---|
| Hydrogen | H | Sodium | Na | Latin: natrium |
| Carbon | C | Potassium | K | Latin: kalium |
| Nitrogen | N | Iron | Fe | Latin: ferrum |
| Oxygen | O | Copper | Cu | Latin: cuprum |
| Sulfur | S | Silver | Ag | Latin: argentum |
| Chlorine | Cl | Gold | Au | Latin: aurum |
| Calcium | Ca | Lead | Pb | Latin: plumbum |
| Magnesium | Mg | Tin | Sn | Latin: stannum |
How do you read a chemical formula?
A formula lists the symbols of the elements present. A subscript number after a symbol tells how many atoms of that element are in one unit. If there is no number, it means one. A coefficient in front of the formula, which is not part of the formula itself, shows how many units there are.
| Formula | Elements | Atoms in one unit | Name |
|---|---|---|---|
| H2O | H, O | 2 H + 1 O = 3 | Water |
| CO2 | C, O | 1 C + 2 O = 3 | Carbon dioxide |
| NH3 | N, H | 1 N + 3 H = 4 | Ammonia |
| C6H12O6 | C, H, O | 6 + 12 + 6 = 24 | Glucose |
| Ca(OH)2 | Ca, O, H | 1 Ca + 2 O + 2 H = 5 | Calcium hydroxide |
Parentheses group atoms. In Ca(OH)2, the subscript 2 applies to everything inside the parentheses, so there are two oxygen atoms and two hydrogen atoms. The coefficient in 3H2O means three molecules of water, which contain 6 hydrogen atoms and 3 oxygen atoms.
How do you write a formula?
- Identify the elements in the compound and write their symbols.
- Determine how many atoms of each element are present in one molecule or formula unit.
- Write each number as a subscript to the right of its symbol. Omit the number 1.
- For molecular compounds named with prefixes, such as carbon dioxide or dinitrogen tetroxide, the prefixes give the numbers (di = 2, tri = 3, tetra = 4).
How do you write the formula of an ionic compound?
An ionic compound is electrically neutral overall, so the total positive charge equals the total negative charge. Use the charges of the ions to find the ratio.
- Write the ions with charges, for example Mg2+ and Cl−.
- Find the lowest numbers of each ion that balance the charges: one Mg2+ (+2) needs two Cl− (−2).
- Write the formula with subscripts: MgCl2.
The crisscross shortcut is to use the numerical value of the charge of one ion as the subscript of the other, and then reduce to the lowest ratio.
Worked examples
- Sodium oxide. Na+ and O2−. Two Na+ (+2) balance one O2− (−2). The formula is Na2O.
- Aluminum oxide. Al3+ and O2−. The lowest common multiple of 3 and 2 is 6, which needs two Al3+ (+6) and three O2− (−6). The formula is Al2O3.
- Calcium chloride. Ca2+ and Cl−. The formula is CaCl2.
- Counting atoms. In 2Al2O3 there are 2 × 2 = 4 aluminum atoms and 2 × 3 = 6 oxygen atoms.
- Reading a formula. C2H5OH has 2 carbon, 6 hydrogen (5 + 1) and 1 oxygen atom, a total of 9 atoms. It is ethanol.
Where do the symbols come from?
Alchemists used pictures and signs for substances, often based on planets, such as the Sun for gold. John Dalton created his own circular symbols in the early 1800s. In 1813, Jöns Jacob Berzelius proposed using letters from the Latin names of elements, with a capital first letter and an optional lower-case letter, and this system has remained in use. Newly discovered elements are given temporary three-letter symbols until the official name is chosen by the International Union of Pure and Applied Chemistry (IUPAC).
Which mistakes are common?
Mistake 1: Wrong capitalization
CO is carbon monoxide, and Co is cobalt. NO is nitric oxide, and No is nobelium.
Mistake 2: Changing subscripts to balance an equation
Subscripts are part of the identity of a substance. Only coefficients may be changed when balancing.
Mistake 3: Confusing subscripts and coefficients
O2 is one molecule with two atoms, while 2O is two separate atoms. 2O2 is two molecules with four atoms in total.
Mistake 4: Writing ionic formulas with the charges
The formula of sodium chloride is NaCl, not Na+Cl−. Charges are used to work out the formula but do not appear in it.