Quantum Mechanics — Introduction & Overview

Quantum Mechanics: Quick Guide to Quantum Mechanics

We can use theoretical calculations based on quantum mechanics to predict the electron's behaviors through probability.

A Brief History Leading to Quantum Mechanics

In 1801, Thomas Young performed his famous double-slit experiment. The double-slit experiment showed that light is a wave. In the early 1900s, scientists could not explain blackbody radiation. The wave model of light did not explain the experimental results. To solve the problem, Max Planck, proposed that an atom can only absorb a set amount of energy. He called the set amount of energy a quantum (the plural is quanta). A few years later, Albert Einstien explained the photoelectric effect using Planck's idea. He proposed that light is a particle. He called a particle of light a photon and explained that each photon has a specific energy. A photon's energy is quantized.

Neils Bohr furthered the work of Planck and Einstien. He proposed that electrons' energies are quantized. Bohr proposed that an electron orbits the nucleus at specific circular radii. He called these radii orbits. Bohr also explained how electrons move between orbits. An electron in one orbit has a specific energy. To get to an orbit further from the nucleus, the electron has to absorb a quantum of energy. For an electron to return to an orbit closer to the nucleus it must release a quantum of energy as a photon. Bohr had explained the existence of black body radiation! Bohr's model also helped to explain the atomic spectrum of the hydrogen atom. Unfortunately, Bohr's model was unable to explain the line spectra of larger atoms. Something was missing!

In 1914 scientists had a problem. Robert Millikan had finished his experiments confirming the photoelectric effect. Einstein was right. Light behaved like a particle. But, Young's double-slit experiment showed light behaved like a wave. To solve the problem, Louis De Broglie, proposed the wave-particle duality. The wave-particle duality says that all matter has a wave nature and a particle nature. Werner Heisenberg realized the implications of the wave-particle duality. It is impossible to know both a particle's momentum and location. We call Heisenberg's idea, the Heisenberg uncertainty principle. De Broglie and Heisenberg set the stage for quantum mechanics.

What is Quantum Mechanics?

The quantum mechanical model explains how an atom's electrons behave. Erwin Schrodinger proposed an equation that explained an electron's behavior. His equation accounted for the wave-particle duality and the Heisenberg uncertainty principle. We creatively call the equation, the Schrodinger Equation.

Because electrons have a wave nature, we cannot know their exact location. The Schrodinger equation gives us a probability function. A probability function tells us the likelihood that we will find an electron within a region. The Bohr model assumed that an atom had circular orbits. Circular orbits are two-dimensional. The Schrodinger equation accounts for atoms being three-dimensional. We call the three-dimensional region an orbital. To be more specific, an orbital is a region where we are ninety percent confident we will find our electron. We visually represent an atom's orbitals with an electron cloud diagram. Three quantum numbers describe an orbital. The first quantum number (n) tells us an electron's distance from the nucleus. The second quantum number (l) tells us the shape of the orbital. The third quantum number tells us the orientation of the orbital along the x, y, or z-axis.

Why Do Chemists Need Quantum Mechanics?

It may seem like quantum mechanics is too theoretical. Don't chemical reactions occur whether we know quantum mechanics or not? While true, quantum mechanics helps chemists to understand how bonding works. For example, we know that a covalent bond forms when atoms share valence electrons. But, what does it actually mean to share electrons? Quantum mechanics shows us that bonding happens when two orbitals overlap. Overlapping orbitals is how we get Molecular Orbital Theory and Valence Bond Theory.

Theoretical chemistry uses quantum mechanics to predict the properties of atoms and molecules. In biochemistry, proteins are so large that we cannot determine how they fold. Theoretical chemists have used quantum mechanical models to predict protein folding. Quantum mechanics has become a powerful tool for chemists!

(Video) Schrödinger's cat: A thought experiment in quantum mechanics-Chad Orzel

by TED-Ed

In this video, Dr. Orzel utilizes Schrodinger's cat thought experiment to explain quantum mechanics. Additionally, he explains the dual particle and wave nature of everything. He uses the duality characteristic to demonstrate chemical bondings between atoms. At last, he highlights the implementation of quantum mechanics in developing electronics.