Why do atoms form molecules? The quantum physics of chemical bonds explained

Why do atoms combine to form molecules? The quantum mechanics of chemical bonds and chemistry. How is it that we can have so many chemical substances with only about 100 relatively simple building blocks, called atoms? This is because atoms are rarely found alone. They are mostly found in combinations with other atoms, through the process of chemical bonding.

Why does this happen? Why is the universe not full of just atoms floating around? The answer to this important question lies in understanding the role that energy plays in the formation of molecules, and its roots in quantum mechanics. All natural systems tend to adopt a state of lowest energy.

A marble at the top of a hill has high potential energy due to gravity. If given the opportunity, it will roll naturally to the bottom of the hill where it will have a lower potential energy.

The hydrogen atom by itself will be in its lowest energy state, called the ground state. But when a second hydrogen atom is introduced, some things begin to happen. Both atoms are in their respective ground state. But as they come closer to each other, first, the electrons since they are both negatively positively charged repel each other, but the electron of hydrogen atom 1 also starts to get affected by the positive charge of the proton in hydrogen atom 2. Similarly the electron of atom 2 starts to get attracted to the proton of atom 1.

So the electrons of each of the 2 atoms tend to get pulled slightly to the other one’s proton. And if they get close enough, the cloud begins to spread to the space between the two atoms. if the atoms get too close, then the protons begin to repel each other and push each other apart. So there is an optimal distance that the two protons prefer to be in.

Shouldn’t the electron clouds be repelling each other, and not allow them to get anywhere near each other? No, there are other interactions that affect the energy of the system:

To calculate the lowest energy of this two atom system, or molecule of hydrogen, we have to take into account:
1) The kinetic energy each atom
2) The potential energy between the two protons
3) The potential energy between the two electrons
4) The potential energy between each electron and each proton

The sum of the possible outcomes of kinetic and potential energy of this entire system in quantum mechanics is referred to the Hamiltonian, represented by capital H. This Hamiltonian is an operator corresponding to the energy of the system, and once you plug it into the time-independent Schrodinger equation, you can solve to get possible values for energy. This is not a trivial equation to solve. But it can be represented for simplicity by the morse potential graph.

The energy two atoms system is less than the energy of two separate one atom systems. This is the reason if a bunch of hydrogen atoms are near each other, they will naturally combine to form a molecule of H2 rather than float around by themselves. This sharing of electrons by two atoms of hydrogen is called a covalent bond.

Not all atoms form bonds with atoms of their own kind, nor with just any other atom. All the substances strive to achieve remarkable stability by sharing or having magical numbers of electrons – 2, 10, 18, 36, 54, or 86 electrons in so called shells around the nucleus of atoms. These numbers correspond to the number of electrons contained in the 6 naturally occurring noble gases. These are inert elements because they already contain the number of electrons needed to form highly stable shells around the nucleus.

Other elements strive to contain a full set of electron in their outer shell called the valence shell. Any element with an unfilled outer shell has a much higher chemical potential energy than these noble gases.

But what's so special about these numbers? Chemists will say, all atoms strive to form a valence set of electrons. And this attractive force for atoms to share electrons in order to form a full valence shell, is balanced by the repulsive forces of their electron clouds and protons.
#hamiltonian
#schrodingerequation

Quantum mechanically, it all has to do with potential energy of multi atom systems. The Schrodinger equation and the Pauli exclusion principle are the underlying principles.

[ArvinAsh]

Also, yes, as many of you have pointed out - Methane is NOT a smelly gas! My mistake. Interesting fact: The United States has one of the largest supplies of natural gas, (which is almost pure methane - CH4) in the world. About half of all homes in the US have natural gas piped in from a central source, similar to the way water is piped in to many homes around the world. And all natural gas supplied to homes in the U.S. has a smelly compound called mercaptan added to give it a very foul, "rotten egg" smell. This is done so that any leak, which can be quite dangerous, does not go undetected by human noses. Growing up, we sometimes had leaks in our house, which I could smell very strongly. So as a kid, I always thought, Methane smelled like rotten eggs. lol.

[thebeautyinbeyond]

Always asked my teacher why atoms wanted a full outer shell and she could never give me an answer, thank you so much!

[manashejmadi]

Mom: Why did you fail in the test?
Me: It all boils down to Quantum Mechanics!

[genericname]

This video is so beautifully put together that anyone who is even slightly knowledgeable of chemistry, physics or quantum physics, will absolutely understand. The most complex subject is so easily and patiently explained. Only if we were this fortunate to have had such teachers to teach us or such videos available to us on the palm of our hands when we were studying. In our school/uni days the teachers got upset when you asked complex yet most fundamental question, or even worse - made fun of you so you r permanently discouraged. I am saving this video in my playlist to visit it over and over, whenever i need to refresh this topic. In many aspects, kids these days are very lucky.

[fritt_wastaken]

They should theach that in schools before even talking about chemistry! It doesn't really matter that kids wouldn't fully understand that, it would at least give them a clue of what is going on.
My most hated class was chemisry, I hated to learn these seemingly arbitrary rules. But since I found out the reason behind it all, I started to love chemistry. But it was way too late - classes are already over.

[Picasso_Picante]

Once again you did it. I have a better understanding of chemical bonding than I did at college 30 years ago. Thank you sir.

[nerdexproject]

This channel is freaking good!!
Thank you as always for your work!

[Bill..N]

ANOTHER outstanding science class Arvin ! In chemistry studies WAY BACK when, My "Ground state" was a slack jaw... This time around, I only sensed it going Slack a few times..Well done friend..

[TheSaferHouse]

If I'd had you for a teacher I most certainly would not have failed chemistry in high school. I love all your videos, keep up the great work my friend!

[hpz]

Damn, I learned much more in a 12 minute youtube video than 4 years of chemistry taught in school and college.Keep up the good work arvinash, deserve much more subs than you currently have.

[rc]

Boom! These super high quality videos are exciting to watch. Arvin Ashe is teaching us the real deal. No misleading pictures, no misleading analogies, and NOT dumbing it down. THIS is cutting edge short form physics communication! ❤️

[ZubairKhan-vsfe]

Your explanations are simple, beautiful and elegant.

Can only be done by somebody who fully understands the concepts

[exoplanet]

"WHY do atoms form molecules?" 

Anyone learning about chemistry should ask this question, but few do.
molecules

So, Thanks so much for tackling this topic. 

I would like to challenge all college level science teachers (including myself) to REFRAIN from pretending that atoms have feelings. In this video we heard words like "atoms strive to..." "atoms want to ..." "atoms are happy when"...etc. These pedagogical constructs may help grade school students visualize what's happening.  

But ultimately they are as accurate as saying "the Sun moves through the sky because it is pulled by a chariot"

[Aniruddha_godbole]

In school they didn't teach "why" fundamentals of chemistry they just tecach "what", always been curious about these fundamentals later come to know about quantum physics the base of all!
Great explanation as always

[Samgurney]

"Consciousness that we enjoy". Speak for yourself...

[SirArthurTheGreat]

There were always so many why and how questions I had as a curious kid with ADHD in school that the teachers would never be able to answer, thanks for your videos!

[yoyo-]

Its been 6yrs completing my schooling. Today I understand the concept of chemical bonds. There’s no reason why one shouldn’t subscribe this channels.

[prateekgupta]

He is like my grandad who loves to teach me

[sidd]

You have a very nice way of starting a video. The sentence "Think about this" aroused curiosity in me.

[rhouser]

You found the sweet spot for my low energy brain to understand how a molecule is formed;) Such a good visible explanation! Thank you