Halogen bond the electron density on H or X appears to be reduced. Electron density transfers results in a penetration of the van der Waals volumes. Halogens participating in halogen bonding include: iodine (I), bromine (Br), chlorine (Cl), and sometimes fluorine (F). All four halogens are capable of acting as XB donors (as proven through theoretical and experimental data) and follow the general trend: F < Cl < Br < I, with iodine normally forming the strongest interactions. Dihalogens (I, Br, etc.) tend to form strong halogen bonds. The strength and effectiveness of chlorine and fluorine in XB formation depend on the
Fluorine other elements from carbon to magnesium are twenty or more times as common. This is because stellar nucleosynthesis processes bypass fluorine, and any fluorine atoms otherwise created have high nuclear cross sections, allowing further fusion with hydrogen or helium to generate oxygen or neon respectively. Beyond this transient existence, three explanations have been proposed for the presence of fluorine: Fluorine is the thirteenth most common element in Earth's crust at 600–700 ppm (parts per million) by mass. Elemental fluorine in Earth's atmosphere would easily react with atmospheric water vapor, precluding its natural occurrence; it is found only in combined mineral