“Diatomic elements are molecules composed of only two atoms, every time, always. HowStuffWorks
Diatomic elements hate to be alone — so much so that they just aren’t found as single atoms.
Instead they’re always two atoms of the same pure element bonded together. It’s right in the name: Di– means "two," and atomic means "of the atoms." And elements are the basic building blocks of the universe.
But out of the entire periodic table, there are only seven diatomic elements:
- Hydrogen H2
- Nitrogen N2
- Fluorine F2
- Oxygen O2
- Iodine I2
- Chlorine Cl2
- Bromine Br2
Why Only Seven?
There are really only seven diatomic elements. Five of them — hydrogen, nitrogen, fluorine, oxygen and chlorine — are gases at room temperature and normal pressure. They’re sometimes called elemental gases. Bromine is always a liquid, while iodine can be a liquid or solid when at room temperature, depending on a number of factors. All seven are nonmetallic.
Other elements of course can bond together; those are called diatomic molecules. That’s how we get table salt (sodium + chlorine = NaCl, sodium chloride). Diatomic molecules like this are found everywhere. Some other elements can form diatomic molecules, but the bonds are very weak and unstable. They don’t stay diatomic for long. Only these seven diatomic elements form strong bonds and are found in this form almost always.
That’s not to say that diatomic elements are rare — on the contrary! Nitrogen and oxygen, in their diatomic forms N2 and O2, make up 99 percent of Earth’s atmosphere. That’s the opposite of rare.
Need an easy way to remember these seven? Try this mnemonic: Have No Fear Of Ice Cold Beer. The first letter of each word will remind you of each diatomic element.
Now That’s Elemental
Elements can also be monatomic, which means there’s only one atom. (Mon- means "one.") Helium is a monatomic element. And oxygen can be triatomic, with three atoms bonded together. That’s what we commonly call "ozone."