A diatomic molecule is a molecule made of two atoms joined together, such as Cl₂, Br₂ or I₂; the halogens exist naturally as diatomic molecules.
A diatomic molecule is a molecule made of two atoms joined together, such as Cl₂, Br₂ or I₂; the halogens exist naturally as diatomic molecules. Malay: molekul dwiatom · Chinese: 双原子分子.
The prefix “di-” means two, so a diatomic molecule is simply a two-atom molecule. In this chapter the idea matters most for the halogens: a single halogen atom has seven valence electrons and is not stable on its own, so two atoms share a pair of electrons and bond together, giving each a stable octet. That is why chlorine is written as Cl₂ and never as a lone Cl in its free, uncombined state. Several other common elemental gases behave the same way, including hydrogen (H₂), oxygen (O₂) and nitrogen (N₂).
Example. Chlorine gas is made of Cl₂ molecules, bromine liquid of Br₂ molecules and iodine solid of I₂ molecules. When you write an equation for a halogen reacting, you must use the diatomic formula, for instance 2Na + Cl₂ → 2NaCl, because forgetting the “2” is a very common way to lose a mark.
Confusion to avoid. Do not confuse a diatomic molecule (two atoms, like Cl₂) with a monatomic element (single atoms, like the noble gases). Diatomic is two, monatomic is one. Also keep the element apart from its ion: chlorine gas is the diatomic molecule Cl₂, while the chloride in a salt is the single ion Cl⁻.
In the Periodic Table of Elements chapter the diatomic molecule explains the physical form of the halogens and is the reason their formulae carry a subscript two, a detail that recurs whenever you balance a halogen equation. Because the halogens share this diatomic form, it is one more property that a whole group holds in common, alongside their seven valence electrons and their trend in reactivity.
Want a teacher to make this click?
We teach SPM Chemistry one to one, so your child understands it and scores it.
from RM50/hr · One-hour paid trial · Same-day reply