Covalent Bonds -- Which type of force?

[ruivocanadense]

When you study physics deeper you learn there are 4 types of atomic forces that represent all other in the atomic level (i.e. electromagnetic force, gravitational force, strong nuclear force, weak nuclear force).

I always try to see how they manifest in the the atomic interactions. For example, collisions are electron clouds repelling each other when they get too close, electric force. Molecular bonds are caused by induced dipoles (Van der Waals forces), magnetic force. Ionic bonds, electric force.

But covalent bonds are dictated by the octant rule princeple and I don't know how that fits into these 4 forces. Weak and strong nuclear forces play a part in it? Even ionic bonds are related to it, because before they become ions the atoms try to follow the octant rule, hence creating the ionic force (electric force) between them.

So what kind of elementary force causes this octant rule to happen?

 

[TeethWhitener]

The electromagnetic force.

 

[mjc123]

Octet, not octant.

You might find this of interest: https://www.chemicalforums.com/index.php?topic=77729.msg283639#msg283639

 

[ruivocanadense]

The electromagnetic force.

And how is that exactly? can you elaborate on it?

 

[ruivocanadense]

Octet, not octant.

You might find this of interest: https://www.chemicalforums.com/index.php?topic=77729.msg283639#msg283639

Yes Octet, sorry.

I don't see them taking a physics approach to forces on it. There must be a way to break it down. And say it is such force because such is acting on it.

 

[TeethWhitener]

And how is that exactly? can you elaborate on it?

The physics of the molecule is governed by the molecular hamiltonian:
wiki
One can predict the existence of covalent bonds solely by looking at Coulomb (that is, electromagnetic) interactions. A very simple example is the hydrogen molecule ion ($H_2^+$):
https://en.wikipedia.org/wiki/Dihydrogen_cation
As you can see in the above page, the potential energy term is simply the Coulomb operator, yet it predicts accurately that there is a covalent bond between the two hydrogen atoms.

 

[DrDu]

The question about the covalent bond is quite subtle. Usually, it is a bond between neutral atoms, so it is not immediately clear, why electromagnetism should lead to an attractive force. As first pointed out by Hellmann, the possibility to of delocalization of the electrons in a bond over two atoms results in a reduction of kinetic energy (remember position-momentum uncertainty and T ~p^2/2m).
This is also how e.g. bonding is explained in the Hubbard model ("hopping terms").
Actually, the situation is more complex, as kinetic and potential energy are connected via the virial theorem. The decrease of kinetic energy in bond direction is more than compensated by an increase of the kinetic energy perpendicular to the bond axis as the orbitals of the electrons shrink in this direction. However, this is made up by the gain in potential energy of the electrons being nearer to the nuclei on average.

 

[Dr_Nate]

Yes Octet, sorry.

I don't see them taking a physics approach to forces on it. There must be a way to break it down. And say it is such force because such is acting on it.

Hi @ruivocanadense. I'll give something closer to a high school-esque answer to understand the octet 'rule'. You can't understand it with one fundamental force. There are other rules of physics that are involved in chemistry. First, I am going to 'lie' to you for simplicity and then make a brief mention of molecular orbital theory.

You need to understand electron configurations, what core electrons are, and what valence electrons are. For example, nitrogen has an electron configuration of 1s2 2s2 2p3. The 1s2 electrons are the two core electrons, and the 2s2 2p3 electrons are the five valence electrons. We say that the two core electrons make a helium core, represented like this [He], and the core electrons are not involved in chemistry because the energy difference between the core electrons and valence electrons is large. The core electrons are more tightly bound, so their atomic orbitals are disturbed very little.

To fill the rest of the 2p orbitals, you would need three more electrons. With the five in nitrogen, that would bring you to eight. An octet! This is where the number eight comes from. Have you noticed that in the covalent bonding you are learning at this introductory level they are always s- and p-block elements? It's that way because 2s electrons and 6 p electrons get you to eight. Other orbitals don't get involved because their energies tend to be to far above or below the valence electron orbitals. Beware, though, there are quite a few exceptions as the octet rule isn't a widely applicable rule. Some molecules are radicals with unpaired electrons, and there are 'expanded octets' such as sulfur hexafluoride.
Now how do they bond? There is a rule of the universe that says that systems will tend to minimize their energy. When the atoms come together, their electron orbitals distort to minimize the system's energy. These distorted and blended atomic orbitals are called molecular orbitals, which extend over the whole molecule. The details involve the electromagnetic force and quantum mechanics. We still retain the same number of valence electrons, so we retain the number eight in cases where the octet rule works.

 

[snorkack]

Importantly, what turns electromagnetic force into covalent bonds seems to be zero point motion and exchange forces. Or rather, this is why not all atoms are strongly bound to each other. (octet and pair rules). Same things (zero point motion and exchange forces) are reason why only some isotopes are bound, not all of them.