Halogens can generally be found as diatomic molecules, e.g., I2. What type of bonding occurs in these molecules?
The term halogens refers to elements in group 17 of the periodic table. Group 17 can be found to the far right of the periodic table. It contains the elements fluorine, chlorine, bromine, iodine, astatine, and tennessine. Astatine is the rarest element naturally found on Earth. It is also radioactive, with its most stable isotope having a half-life of 193 days. Meanwhile, tennessine is an entirely synthetic element. Samples of astatine and tennessine are not large enough for us to have assessed their molecular behavior. So we can ignore these for the purposes of this question.
A diatomic molecule is a molecule composed of two atoms. Out of the remaining group 17 elements, all of them conventionally form diatomic molecules, F2, Cl2, Br2, and I2. Since all these elements are in group 17, they all have seven electrons in their outer shell. Take fluorine, for instance. Fluorine has an atomic number of nine, meaning an atom of fluorine has nine protons and therefore nine electrons. That means two electrons in the inner shell and seven electrons in the outer shell. Another name for the outer shell is the valence shell. The valency refers to the bonding ability of an atom.
When two fluorine atoms are mixed together, they’re attracted to one another and form a single bond, sharing one electron each. This means that the valence shell at each atom is full and the atom is more stable. The fluorine atoms are sharing valence electrons. This type of bonding is called covalent. Co- refers to shared, and valent refers to the valence electrons in the outer shell. We know the bonding is not ionic because there’s no transfer of electrons. And we know it’s not metallic because the electrons in the valence shell are not donated to a sea of delocalized electrons. The principles that apply to fluorine also apply to chlorine, bromine, and iodine.
So the type of bonding that occurs in all the diatomic halogens is covalent.