# What is the difference between physical and chemical bonds?

If you characterize the chemical bonds to two categories physical and chemical bonds, how do you do it? Aren't all bonds chemical and physical?

From the freedictionary.com, chemical bond:

Any of several forces, especially the ionic bond, covalent bond, and metallic bond, by which atoms or ions are bound in a molecule or crystal.

What is the physical bond then? How should the atom be bonded to molecule if these bonds are not the ones. What are these "several forces" excactly?

In the answer I would like to see the lists of chemical and physical bonds.

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Makes you think... the ionic bond in salts and the bond you get when charged paper bits stick to a comb are both essentially electrical attraction, yes? But one is a physical bond, and one is a chemical bond... –  user95 May 4 '12 at 10:26

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:

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:

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…

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Now, 'What explicitly defines a Hydrogen bond?' 3.. 2... 1... GO! :D –  LordStryker May 4 '12 at 12:25
@LordStryker easy! ;-) IUPAC Recommendations published in Pure Appl. Chem., 2011, Vol. 83, No.8, pp 1637-1641, doi:10.1351/PAC-REC-10-01-02 –  F'x May 4 '12 at 13:45
Ah yes. I knew that the H-bond definition was going to be updated but I had not actually seen the paper. Very good! I am still somewhat dissatisfied though. One example I can give is the criteria regarding H-bond angle. The closer to 180 it gets, the stronger the bond. So controversy then lies in, how acute can the angle be before it is no longer considered an H-bond? I've seen so many people (professors and students included) argue about what is and isn't an H-bond due to the angle being 'too far' from 180. Sure one can say 'H-bond like' but that starts to sound like hedging to me. –  LordStryker May 4 '12 at 14:23
+1 for such an extensive discussion. Number 5 is indeed a nice compact definition, though to be pedantic one would still need to put in a quantitative threshold for what's meant by "strong" and crystal: otherwise a group of many oppositely-charged ballons would classify as a chemically bound crystal. –  leftaroundabout May 4 '12 at 15:07
What is the justification for drawing the "dividing line" at e.g. 50 kJ/mol? –  Eric Brown May 4 at 12:30
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"Physical bonds" can actually be termed as "intermolecular forces". They hold the atoms or molecules together, or even repel them.

Types of Physical bonds are:

1. Van der Waals force
2. London dispersion force (part of the van der Waals forces)
3. Dipole–dipole interactions
4. Hydrogen bonding
5. Debye force (induced dipole)

Your definition of chemical bonds is correct.
The several types of chemical bonds are (there might be more; these are the one I know):

1. Covalent bond

2. Ionic bond

3. One- and three-electron bonds - These are the bonds which are made up of an odd number of electrons, for example $H2^+$ is a one electron bond while $ClO2$ has a 3 electron bond (see here)

4. Bent bonds (Banana Bonds)

5. 3c-2e and 3c-4e bonds (Sub category of Banana bonds)

6. Aromatic bond (Resonance stabilized bonds)

7. Metallic bond

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Hi Ashu, your list of chemical bonds is exactly (word-for-word and in the same ordering) that of Wikipedia… Did you take it from there (in which case it should be mentioned)? Otherwise, a reference would be appreciated. –  F'x May 4 '12 at 11:18
One can further classify covalent bonds into "regular" covalent bonds and dative, or coordinate covalent bonds; in the former, the two atoms bonded both contribute electrons to the bond, while in the latter only one of the two atoms contribute the two electrons. –  user95 May 4 '12 at 11:46

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.

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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.

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