Covalent Bonds ( split from Atom anatomy and chemical bonds )

[QuantumT]

Basically I just want to know, what exact force it is, that holds molecules together?

I've read a fair share of articles about it, and learned about electrostatics and Coulomb's Force. (Electrostatics is a part of wikipedia's series on electromagnetism.) But none of the molecule articles anywhere mentions electromagnetism!

So, is it (basically) electromagnetism that does the job? Or is electrostatics not equal to electromagnetism?

Edited March 4, 2019 by QuantumT

[studiot]

2 hours ago, QuantumT said:

Basically I just want to know, what exact force it is, that holds molecules together?

I don't know who split this off or why,

But I do wonder why they didn't provide at least a basic answer to the question.

 

QuantumT,

This question is best considered in terms of energy not force.
Are you happy with what potential energy is?

If you think about force you then ae faced with the question

If there is a force pulling the atoms together in a molecule, what stops them banging together.

ie what holds them apart at a certain distance?

[QuantumT]

6 minutes ago, studiot said:

I don't know who split this off or why,

I made this thread as a split, due to the following reply from swansont:

Quote

If you wish to discuss thus further, please open up a new thread.

 

10 minutes ago, studiot said:

If there is a force pulling the atoms together in a molecule, what stops them banging together.

ie what holds them apart at a certain distance?

I was wondering the exact same thing. But is it EM or not EM?

Edited March 4, 2019 by QuantumT

[Sensei]

Imagine you have free particles (or molecules) A and B. They have (rest) masses m_a and m_b. There are possible two cases:

1) Together they have smaller (rest) mass m_ab than free constituents.

m_a + m_b > m_ab

and energies:

m_ac² + m_bc² > m_abc²

dE = ( m_ac² + m_bc² ) - m_abc²

Energy released during joining them is dE.

Joining them is exothermic/exoenergetic reaction (e.g. fusion)

Separation of them requires external source of energy and is not spontaneous.

It's example of stable isotope. Minority of isotopes are stable.

 

Sometimes it is reversed..

2) Together they have bigger (rest) mass m_ab than free constituents.

m_a + m_b < m_ab

and energies:

m_ac² + m_bc² < m_abc²

Free particles A and B have smaller (rest) masses than joined together.

 dE = m_abc² - ( m_ac² + m_bc²)

Which means that there is needed external source of energy dE to join particles A and B.

Joining them is endothermic/endoenergetic reaction.

Separation of them releases energy and can be spontaneous.

It's example of radioactive isotope. Majority of isotopes are unstable.

 

6 hours ago, QuantumT said:

Basically I just want to know, what exact force it is, that holds molecules together?

Did you read this article?

https://en.wikipedia.org/wiki/Chemical_bond

 

[QuantumT]

Got it! Thank you.

To make an analogy:

It seems like I'm asking "Is this Shakespeare?"
And you are replying: "It's Hamlet!"

Am I right?

Edit: Or is there more than "Shakespeare" (EM) involved? Like the weak force?

Edit #2:
I am aware that metal and non-metal make ion bonds, called salts.
I should have called this thread Molecule Bonds instead, sorry.

Edited March 5, 2019 by QuantumT

[studiot]

22 hours ago, QuantumT said:

I was wondering the exact same thing. But is it EM or not EM?

OK but I also said that you need to consider energy and asked you about potential energy.

 

Since the powers that be here don't appear to want to help you, I will continue.

 

Let us start with the simplest atom - hydrogen.

This has one proton and one electron.

So the must be an electric force acting between them.

This force is purely electrostatic.
That is it is the same force that would appear if both the proton and the electron were stationary.

There is then no magnetic part to this force, since magnetic effects only occur when charges are in motion.

So this is the force that holds the atom together.

Since the proton is over 1800 times more massive than the electron we consider it stationary and allow the electron to be in motion.

The kinetic energy of the electron is then just sufficient to balance the electrostatic attraction between unlike charges.

Note I have switched to energy to describe this. Using forces doesn't easily work out.

 

Now let us introduce a second atom.

This also has a proton and an electron.

So the second proton weakly attracts the electron from the first atom, but more and more strongly as the two atoms approach each other.

So the electron has potential energy due to each proton.

So also does the second electron.

As we bring the protons ever closer, a repulsion between the protons grows.

The potential energy of this has the opposite sign to the electron proton potential energy.

So again a balance at a minimum energy cost can be achieved.

So both electrons belong to both protons which are fixed at the minimum energy distance from each other.

 

As an aside extending this to a lattice of positive nuclei is is how metallic bonding works.

 

 

 

[QuantumT]

28 minutes ago, studiot said:

So the must be an electric force acting between them.

This force is purely electrostatic.
That is it is the same force that would appear if both the proton and the electron were stationary.

There is then no magnetic part to this force, since magnetic effects only occur when charges are in motion.

Thank you! I finally understand the difference between EM and ES, and why EM does not apply to atoms and molecules.

Thank you!

[studiot]

Remember this theory (the Bohr theory) is fine for the nuclei since they are considered stationary, their potential field has no magnetic effect.

 

The difficulty (which ushered in Quantum Mechanics) is that the charges (well the electrons) are in motion.

 

So the classical equations that describe the motion of a charged particle (the electron) in an electric potential field (that of the proton) require a magnetic effect (EM radiation).

But this is not observed in practice.

 

This is why we have a quantum equation of motion - the Schrodinger equation.

 

[swansont]

On 3/4/2019 at 6:39 PM, QuantumT said:

 I was wondering the exact same thing. But is it EM or not EM?

All bonding within molecules is electromagnetic in nature. However, there are a lot of ways that can happen.

12 hours ago, studiot said:

Remember this theory (the Bohr theory) is fine for the nuclei since they are considered stationary, their potential field has no magnetic effect.

 

The difficulty (which ushered in Quantum Mechanics) is that the charges (well the electrons) are in motion.

 

So the classical equations that describe the motion of a charged particle (the electron) in an electric potential field (that of the proton) require a magnetic effect (EM radiation).

But this is not observed in practice.

 

This is why we have a quantum equation of motion - the Schrodinger equation.

 

The fact that the Bohr theory is ultimately incorrect (it is based on classical physics) is why many of the comments in the previous explanation are actually wrong. Electrons are not in motion (in aa classical sense) and there are magnetic effects that affect the energy the electron has, which are intrinsic to the electron and proton.