Chemistry - What is the difference between physical and chemical bonds?

Solution 1:

In short: the definition of a chemical bond is not unique and a clearly-drawn line. The simplest and most common definition is the sharing of electrons between two or more nuclei. In contrast, other interactions are often said to be intermolecular (which is somewhat more specific than the term “physical”.

In a longer commentary, I see can have five different types of definition of the chemical bond (vs. intermolecular interactions).

  1. Let's start from the beginning, in this case using the words of Linus Pauling, winner of the 1954 Nobel Prize for “determining the nature of the chemical bond linking atoms into molecules”. In The Nature of the Chemical Bond (1960), he gives the following definition:

    The Chemical Bond Defined.—We shall say that there is a chemical bond between two atoms or groups of atoms in case that the forces acting between them are such as to lead to the formation of an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent molecular species.

    A bond is what links atoms into molecules, and molecules are defined at the discretion of the chemist. You can find the same definition still in use in some high-school textbooks, but it isn't very helpful…

  2. The complete opposite: consider all interactions as chemical bonds, whose strenght can vary. I actually hadn't heard that one before I researched textbooks to write this answer, but you can find it in some textbooks, like this one:

    A chemical bond is the physical phenomenon of chemical species being held together by attraction of atoms to each other through sharing, as well as exchanging, of electrons and is a phenomenon that is fully described by the laws of quantum electrodynamics. In general, strong chemical bonds are found in molecules, crystals or in solid metal and they organize the atoms in ordered structures. Weak chemical bonds are classically explained to be effects of polarity between molecules which contain strong polar bonds. Some very weak bond-like interactions also result from induced polarity London forces between the electron clouds of atoms, or molecules. Such forces allow the liquification and solidification of inert gases. At the very lowest strengths of such interactions, there is no good operational definition of what constitutes a proper definitional "bond".

    This view has some grounding, because all interatomic interactions stem from the behaviour of the system's electrons (in addition to nuclei–nuclei Coulombic forces). However, it does not allow to make a strong distinction between interactions whose energies differ by orders of magnitude. Chemists like molecules, and they like categorizing things between intra- and inter-molecular, as it's a nice model (making it easier for our mind to handle).

  3. You can classify interactions by energy: decide that chemical bonds are those interactions that have an energy higher than a certain threshold, let's say 50 kJ/mol. This makes things clean, and makes sure that you can easily classify interactions. However, the choice of a threshold is problematic.

  4. Finally, what I believe is the most common description is to look at the nature of the interaction, and classify it following a certain convention. The two other answers so far have focused on this part and listed the various “usual” types of bonds and intermolecular interactions, so I won't say more on that.

  5. I said five types, right? Well, the fifth is mine, of course. Not only mine, but that of the New Oxford American Dictionary as well, which I quite like:

    chemical bond
    a strong force of attraction holding atoms together in a molecule or crystal, resulting from the sharing or transfer of electrons.

    Short and powerful. What I like in that is that it gives a general prescription, allowing one to argue individual cases and not based too much on convention. What are the features of a chemical bond? Well, it has to be an attractive force between atoms, sure… but I think the most relevant criterion of all is sharing (or transfer) of electrons. That is, after all, what chemistry is about: description of electronic clouds around two or more atoms. And I think when this criterion is applied to the list of interaction types commonly classified, it works quite well (whithout being dogmatic).

    Also, what I like in it is that a given interaction type can be considered one way or another depending on its strength. The best example for that may be the hydrogen bond, which is the archetype of the almost-chemical-bond…

Solution 2:

The answer is that there is one (or perhaps two) types of bonds. A bond occurs when two atoms are attracted in a net-electrostatically favorable way. (Of course, the electrons and protons are subject to their quantum nature)

Why two? In the Quantum Theory of Atoms in Molecules, the "procedure" is:

  1. Ascertain whether a bond path exists between two atoms (atomic basins). This is a yes/no answer -- the is a bond or no bond -- because it is electrostatically favorable or unfavorable.
  2. If a bond path does exist, then the sign of the Laplacian of the electron density at the bond critical point tells you whether the interaction is "covalent" or "not covalent" though these terms are not strictly applicable. +/- yeilds only TWO possible types of bonds.

BTW: The Laplacian of any field is a measure of whether the electron density at a particular point is a "sink" or a "source." The reality is: there is an entire, continuous spectrum of values of the Laplacian. None of them present themselves as being called "dipole" or "van der Waals"

In summary, there is one language of bonding, and that is couched in Quantum Physics. You will never hear of van der Waals forces in a physics [quantum electrodynamics] class, because there is no such thing. There is no van der Waals term in the Schroedinger equation.

The difference between chemical and physical bonds as taught in class is simple: chemistry as a whole has not shaken the pre-quantum revolution description of a bond. It is disastrously complicated for students and professionals in the field, as there is a never ending squabble as to whether something is a "dipole-dipole" or "dipole-induced dipole" or "three center, two electron" But these are all arguments built on shaky scaffolding.

The fact is that the rules of physical bonding are almost "boring." The second fact is that most bond descriptions in chemistry have only certain elements of reality. To the credit of chemists, they need to quickly and efficiently describe certain oft-appearing motifs in bonding--without resorting to a quantum chemistry calculation--and they have done a pretty good job. It's just that these descriptions are somewhat ambigious and always open to interpretation, hence not purely quantum mechanical.

Solution 3:

All bonds are physical, of course. But not all bonds are chemical – as a somewhat stupid example consider a magnet attached to your fridge, which is in a way a physical bond but doesn't have much to do with chemistry.

The definition you quoted is fine, and leaves quite clearly those kinds of bonds that are not included as "the physical bonds". For instance, Van der Waals forces that do not bind molecules into a rigid crystal structure, particularly London dispersion forces, are usually not considered chemical bonds, neither are the forces that hold together atomic nuclei or neutron stars.

Of course, none of these bind molecules together; if you restrict it to that you're right that every bond is physical and chemical at the same time.