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What is Quantum Chemistry?

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson, we will get a brief overview of quantum chemistry, a sub-discipline of chemistry that focuses on the properties and behavior of subatomic particles, especially electrons. Without a foundational understanding of electron behavior, it would be impossible to understand the concepts of chemical bonding.

Small Particles with a Big Purpose

What's the smallest thing you've ever held in your hand? A single grain of sand? A spec of fabric? Did you know that if we lived in a universe where a single electron was the size of an apple and everything else was scaled to that size, a human being would have to be about 2.16 x 1013 meters tall! That's how small an electron is though, so small it's almost incomprehensible.

When most people hear the word 'quantum' they go running for the hills or they think of an awkward scientist in a lab coat with a dozen pens in a pocket protector. In this lesson that's exactly what we are going to be talking about, quantum chemistry. Quantum chemistry is a field of chemistry that tries to explain the strange and unusual behavior of the smallest particle we know of, the electron. Unlike what some people presuppose, you don't have to be a genius in order to get a basic knowledge set of this important sub-discipline of chemistry. Let's start our journey!

What is Quantum Chemistry?

If we had to summarize what quantum chemistry was in simple terms, it's basically the study of the very small. A long time ago before the scientific instrumentation we have today, scientists thought that the atom was the simplest form of matter (anything with a mass, regardless of how big or small). As time went on, however, it turned out they were wrong. There seemed to exist particles that actually made up atoms, things called subatomic particles.

The field of quantum chemistry emerged when this discovery was made, and there were three very important subatomic particles that were found to make up every single atom. These particles are of course protons, neutrons, and electrons.

The proton was discovered by Ernest Rutherford in 1919. It was found to carry a +1 charge and was determined to be located inside the nucleus of the atom. Neutrons were discovered by James Chadwick a few years later in 1932 and have no charge (neutral). They were also found inside the nucleus of the atom. Electrons, however, were discovered first by J.J. Thomson in 1897. They have a -1 charge and are found outside the nucleus of the atom in things called orbitals.


Structure of a helium atom
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Notice that in this picture, the nucleus of the helium atom contains the protons and neutrons, and the area outside of the nucleus is where the electrons are found.

All About those Electrons

When electrons were discovered, the field of quantum chemistry really came center stage, since electrons interacting with one another are what ultimately result in chemical bonds being formed. Erwin Schrödinger was the scientist who really is responsible for the way we think about modern atomic structure and how we view electrons.

Schrödinger suggested that the electrons in an atom existed in orbitals and that it is impossible to know exactly where an electron is at any given point and time. Instead, we can only know the probability of finding an electron at a given location. As it might stand to reason, the likelihood of finding an electron will always be the greatest near the nucleus. As we move away from the nucleus, the probability goes down. Why would that be the case?

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