# Interaction radius of a molecule

What is the most commonly used radius of an atom or molecule to describe the area that must touch another area of another molecule for the two to react. Is there one radius that always works or does it depend on the reaction? Of course the actual distance at which bonds can form or break varies but I was wondering if at some distance the atoms can somehow bring themselves together for bond 'swapping'

The short answer: no, there is no such a general distance.

The long answer: atoms and molecules are not just hanging is the vacuum and waiting someone to grab them. They actually have rather high energy in translation, rotation and vibration modes and particles actually collide during a reaction some of these energies can be involved. The science that study these collisions is called reaction dynamics, and you can read e.g. Polanyi's Nobel lecture.You should note that:

• It is a quantum event, so technically speaking it is not deterministic.
• The orientation of the collision and the geometry of the potential energy surface along the reaction can be favourable, and rotation, vibration etc can help the reaction, or can prevent it.
• The details are also influenced by the medium (is it a reaction in gas or condensed phase, on surface?) and what kind of orbitals are involved in the reaction. About frontier orbitals and their roles you also can find lot of materials in textbooks, eg. any text that discusses the work of Fukui and Hoffmann.
• thanks, nice simple answer as well as explanation in the flaws in my reasoning. This is the type of answer I was looking for – J-S Apr 30 '15 at 12:24

According to Valence Bonding Theory, proposed by Heitler and London, bonding between two atoms A and B happens when the energy of the system is the lowest.

With experimentation was found that this minimum of energy, requires the distance between nucleuses to be $\mathbf{74~pm}$ and the energy $\mathrm{-4.72~ eV}$. Calculations say that the true distance should be $\mathbf{87~pm}$ and the energy $\mathrm{-3.14~eV}$, which means we should consider other factors which indicate the interaction between nucleuses and electrons.

• I am sorry, but you seem to talk about the H2 molecule and the H-H bond length. So it is neither general, nor on the actual topic. – Greg Apr 25 '15 at 15:04