A language every chemist speaks
In English it’s oxygen. In Spanish it’s oxígeno, in German Sauerstoff, and in Chinese 氧 (yǎng). If every chemist used only their own word, sharing results would be a mess. So chemists agreed on an international element symbol元素符号 for every element. Oxygen is O, and it’s O in every chemistry book in the world.
Writing element symbols
Every symbol is one or two letters, and the capitals follow a strict rule:
- A one-letter symbol is always a capital: H, O, C.
- In a two-letter symbol, the first letter is a capital and the second is lowercase: Ca, Cl, Na.
Most symbols come from the element’s name. Carbon is C, so other elements starting with C need a second letter: calcium Ca, chlorine Cl, cobalt Co.
Capitals change the meaning. Co is cobalt, a single element. CO, with two capitals, means two elements, carbon and oxygen: it’s the formula of carbon monoxide (lesson 1.5).
Type a symbol and see whether it’s written correctly. Be sure to compare Co, CO, and co:
Type an element symbol.
Symbols from Latin
A few symbols don’t match the English names at all, because they come from the elements’ Latin names:
| Element | Symbol | Latin name |
|---|---|---|
| Sodium | Na | natrium |
| Potassium | K | kalium |
| Iron | Fe | ferrum |
| Copper | Cu | cuprum |
| Silver | Ag | argentum |
| Tin | Sn | stannum |
| Lead | Pb | plumbum |
| Mercury | Hg | hydrargyrum (“liquid silver”) |
| Gold | Au | aurum |
Some of these live on in English words: a plumber got the name from working with lead (plumbum) pipes, and “ferrous” metals contain iron.
What a symbol means
A symbol carries two meanings. Take O:
- On the large scale, it stands for the element oxygen.
- On the particle scale, it stands for one atom of oxygen.
For substances made of single atoms (metals, noble gases, and solid nonmetals such as carbon and silicon), the symbol also stands for the substance itself. Fe means the element iron, one iron atom, and the metal iron.
A number in front of a symbol counts atoms, so it only has the particle meaning: 2H means two hydrogen atoms. You can’t say “two hydrogen elements,” because elements are kinds of atoms, not numbers of atoms (lesson 1.2.6).
Symbols to know
These elements come up again and again. Learn to go from name to symbol and from symbol to name:
Example 1: What does each symbol mean?
Give the meaning of each: (1) N; (2) 2N; (3) Cu.
- (1) N. On the large scale, the element nitrogen; on the particle scale, one nitrogen atom. Nitrogen gas is made of molecules, so N does not stand for nitrogen gas.
- (2) 2N. A number in front means atoms only: two nitrogen atoms.
- (3) Cu. Copper is made of single copper atoms, so Cu has all three meanings: the element copper, one copper atom, and the metal copper.
Example 2: Fix the symbols
A student writes the symbols for potassium, sodium, and lead as P, So, and L. What’s wrong, and what are the correct symbols?
- P is already taken. P is phosphorus. Potassium’s symbol comes from its Latin name, kalium: K.
- So isn’t a symbol. Sodium’s symbol comes from natrium: Na.
- L isn’t a symbol either. Lead’s symbol comes from plumbum: Pb.
- Lesson. When a symbol doesn’t match the English name, check the Latin-based list. These nine are worth memorizing.
- Mg for magnesium
Capital M, lowercase g: correct.
- CL for chlorine
The second letter must be lowercase: Cl.
- Hg for mercury
Correct. It comes from the Latin hydrargyrum.
- co for cobalt
Every symbol starts with a capital: Co. (CO would mean carbon and oxygen.)
- So for sodium
Sodium is Na, from the Latin natrium.
- Zn for zinc
Capital Z, lowercase n: correct.
- Ir for iron
Iron is Fe, from ferrum. Ir is a different element, iridium.
- K for potassium
Correct. It comes from the Latin kalium.
- Ag for silver
Correct. It comes from the Latin argentum.
Common mistakes
Mistake 1: “Two-letter symbols can be all capitals, like CL or MG.” The second letter must be lowercase. CO and Co mean completely different things: carbon plus oxygen, or cobalt.
Mistake 2: “2O means two oxygen elements.” Elements are kinds of atoms, so you can’t count them. 2O means two oxygen atoms.
Mistake 3: “Every symbol comes from the English name.” Nine common ones come from Latin, including K for potassium, Fe for iron, and Pb for lead. Guessing from English gives the wrong element: Ir is iridium, not iron.
Quick check
Which symbol is written correctly?
A two-letter symbol is a capital followed by a lowercase letter: Mn (manganese), Mg (magnesium), Cu (copper), Al (aluminum).
Which of these can the symbol Fe NOT mean?
Fe means the element iron, one iron atom, and the metal iron. Two iron atoms are written 2Fe.
Summary
- Element symbols are international. One capital letter, plus at most one lowercase letter; the wrong capitals change the meaning.
- A symbol stands for an element and one atom of it; for substances made of single atoms, it also stands for the substance. A number in front counts atoms.
- Na, K, Fe, Cu, Ag, Sn, Pb, Hg, and Au come from Latin names, so they don’t match the English names.
On to AP (for later)
The periodic table you’re given on the AP exam shows symbols, atomic numbers, and atomic masses, but no names. You need to know the common symbols cold, including the Latin-based ones. AP questions often use symbols alone, as in “Compare the atomic radii of Na and Cl” (topic 1.7).