Illustrated · 2 min
History
By the late nineteenth and early twentieth centuries, many famous scientists had developed various models depicting atomic structure. We started with the plum pudding model. But Ernest Rutherford's famous gold foil experiment disproved it. So, Rutherford developed the nuclear model. But, it seemed to predict that all atoms are unstable. Why? The electrons should fly into the nucleus. The electrons and protons would annihilate each other! Something wasn't quite right.
By this time, we were beginning to understand that energy is quantized. Planck's quantum theory used quantization to explain Blackbody Radiation. Einstein used it to solve the photoelectric effect. Neils Bohr applied quantization to the angular momentum of orbiting electrons. Now, the electrons didn't spiral into the nucleus! Bohr's atomic model also explained the absorption and emission spectra of the hydrogen atom unbelievably well. But it didn't work for more extensive elements. We were still missing something!
While the Bohr model did express the electrons' particle-like movements, it missed the wave-like characteristics. At that point in time, we understood that light behaved like a particle and a wave. So, we developed the wave-particle duality. But there was more! Louis de Broglie combined Planck's quantization equation with Einstien's relativity equation. What was the result? The de Broglie equation shows that all matter has a wavelength.
But what does the wave-particle duality have to do with atomic models? Enter Erwin Schrödinger. In 1926, he began to develop an entirely new understanding of physics. It would describe the movement and forces (mechanics) of quantum particles. So, we would creatively call it quantum mechanics. Eventually, it would lead to a new model of the atom called the quantum mechanical model. It focuses on the wave-like nature of electrons. For chemists, the new quantum mechanical model would develop into theoretical chemistry.
Quantum Mechanical Model Overview
The quantum mechanical model disregards the idea that an electron orbits around the nucleus. Instead, electrons are waves in space. So, we can only describe the electron using probability. For example, the exact location of an electron cannot be determined. Instead, we calculate the most likely zone for finding the electron. So, we might say, "we are 95% confident our electron is within this region." We call the region an atomic orbital. Orbitals are 3D spaces and have different shapes. We calculate an orbital's size and configuration using mathematical expressions called wavefunctions. When the wavefunction equals zero, that space has zero probability of being occupied by an electron. We call that location a node. To help you visualize an orbital, you can look at an electron cloud diagram.
Here is the difference in the way atomic models were described. The quantum mechanical model includes electrons moving in a space called an electron cloud.
Image source: By Cassie Gates
In quantum mechanics, the shape of the orbital is determined through equations.
Image source: By Cassie Gates
