When adjacent non-bonded atomic orbitals overlap with each other or are next to each other, they create more favorable molecular orbitals. A molecular orbital is simply the overlap of a few atomic orbitals. If you want to know what the molecular orbital is going to look like, we can use a system very common in organic chemistry called the linear combination of atomic orbitals (LCAO), which helps you predict what the molecular orbital will look like.
Now that I've hopefully convinced you that atomic orbitals like to share electrons, I want to talk about what those electrons look like after they share. We're going to take the example of ethene. Recall that one orbital has one electron, another orbital has another electron—these are the atomic orbitals, and this is typically how we represent them. Each of the conjugated atoms will receive one atomic orbital. Notice that I have 2 conjugated atoms, so I draw 1, 2 atomic orbitals next to each other and place however many free electrons there are into those orbitals. Atom 1 is donating one electron, which is why I put one electron there. Atom 2 is donating another, which is why I add another electron.
Remember, an orbital is just a region of space that is statistically probable to have electrons. It's like a cloud of electron density where there's a high chance we'll find electrons, but it's not actually a particle. When you bring atomic orbitals close together, they don't collide; they interfere with each other like waves, not like particles. This interference can be constructive or destructive.
Constructive interference means that the waves of those atomic orbitals build on each other, increasing their amplitude and the chances of finding electrons between them. This is called an in-phase overlap, forming a bonding interaction, which means that the chances of finding electrons between these two atoms is unusually high.
On the other hand, destructive interference occurs when atomic orbitals are out of phase, causing the waves to cancel each other out, creating a node where there is no mathematical chance of finding electrons. This leads to an antibonding interaction, making the atoms unstable and likely to repel each other.
It is important to note that the positive and negative lobes of an atomic orbital do not relate to electrical charges; it's just a way to think about orbitals. During constructive overlap, the whites (or positives) and grays (or negatives) are on the same side, aligning properly; during destructive overlap, they are opposite each other.
According to the Pauli exclusion principle, you can only put two electrons in each orbital. When we make our new molecular orbitals, molecular orbital pi 1 and molecular orbital pi 2, based on the overlapping atomic orbitals, both electrons can fill the lowest energy orbital, creating a bonding interaction with no electrons in the antibonding region. If we had an extra electron, it would go into the higher energy, antibonding orbital, destabilizing the bond.
In conclusion, the reason alkenes can form such good double bonds is that they have exactly two electrons to share constructively in one molecular orbital, effectively making it look like a single, low-energy molecular orbital that promotes bonding between the two. I hope this is a good start to understanding molecular orbital theory, and I will follow up with more videos explaining exactly what you need to know so you can apply this theory to solve problems.