Why do antibonding orbitals have more energy than bonding orbitals?

[Frigus]

Suppose their are two electrons which are out of phase and they interact to create node due to which those electrons will be automatically present in anti bonding molecular orbital but we say antibonding molecular orbital have high energy...now there problem is that if their is no energy available and the electrons are out of phase then if they interact they will be in higher energy state but due to absence of energy they can't go their but they have created node in between nuclei so remaining orbital is antibonding orbital so where will electrons go and I also have another question that why electrons are always in two states that is in phase and out of phase why can't they have some other phase difference in between them.
Their is also one thing which is itching me is that "we say due to aufbau's rule electrons go in bonding orbital first due to low energy so does this means that the way electrons will interact is depended upon the situation because to have bonding orbitals electrons should be in phase which means that state of electron was not decided earlier."
These things have blown my mind.

Thanks
Hemant

 

[DrClaude]

Suppose their are two electrons which are out of phase and they interact to create node due to which those electrons will be automatically present in anti bonding molecular orbital but we say antibonding molecular orbital have high energy...now there problem is that if their is no energy available and the electrons are out of phase then if they interact they will be in higher energy state but due to absence of energy they can't go their but they have created node in between nuclei so remaining orbital is antibonding orbital so where will electrons go and I also have another question that why electrons are always in two states that is in phase and out of phase why can't they have some other phase difference in between them.

What you wrote there is very confusing. What exactly do you mean by "two electrons which are out of phase."

Molecules are not built up with electrons "of a specific phase" coming together.

Their is also one thing which is itching me is that "we say due to aufbau's rule electrons go in bonding orbital first due to low energy so does this means that the way electrons will interact is depended upon the situation because to have bonding orbitals electrons should be in phase which means that state of electron was not decided earlier."

The Aufbau principle is a method to figure out the electronic configuration of the ground state of the molecule. It has nothing to do with how molecules are form in reality from the collision of atoms or molecules.

 

[Frigus]

What you wrote there is very confusing. What exactly do you mean by "two electrons which are out of phase."

Molecules are not built up with electrons "of a specific phase" coming together.

I was watching this video and at 1:14 he said that if electrons are in phase they will do constructive interference and if they are out of phase they will do destructive interference.
From this a doubt came to my mind that why does electrons can always be in phase or out of phase why doesn't some another combination occurs between them.
Can you please specify some part which I am not able to explain properly or should I rewrite this whole question again.

 

[DrClaude]

I was watching this video and at 1:14 he said that if electrons are in phase they will do constructive interference and if they are out of phase they will do destructive interference.

The video tries to present things in such a simplified manner that the result is basically wrong. I understand where the guy is coming from, but this approach can lead to a lot of confusion.

When constructing molecular orbitals for the hydrogen atom, you can consider them (= approximation) as the sum of two atomic orbitals, one from each hydrogen atom. Two linear combination are possible,
$$
\Psi_\mathrm{bonding} \propto \phi_a(1) + \phi_b(2)
$$
and
$$
\Psi_\mathrm{anti-bonding} \propto \phi_a(1) - \phi_b(2)
$$
where a/b labels the orbitals and 1/2 labels the atoms. The resulting molecular orbitals appear to show, in-between the nuclei, constructive or destructive interference.

I think it is better to consider the molecular orbitals as mathematical constructions first, and then populate them with electrons (like you do with atoms) then to conceptualize them as electrons interfering with each other.