For the past few weeks, we have been discussing the periodic table which celebrates its sesquicentennial in 2019.
The development of the table was based on chemical observations such as the stoichiometry of reactions or the formation of specific minerals. It was a great achievement derived from an understanding of how atoms and molecules react.
But when Mendeleev developed his table with its peculiar shape, there was no explanation for why it looks the way it does. At a casual glance, it is certainly not a simple structure.
The explanation for the periodic table emerged in the early 1900s. It began when Ernest Rutherford, Ernest Marsden and Hans Geiger were exploring the structure of the atom. Their experiment was very simple - they shot alpha particles at gold foil and measured the results.
To their surprise, most of the alpha particles passed right through the foil but a small fraction bounced back.
They correctly assumed the mass of an atom was located in a small nucleus and most of the surrounding volume was filled with electrons.
They came up with the "solar system model" of the atom which most of us are familiar with as the symbol for an atom.
Determining the structure of the atom raised more questions - why was it stable?
Why didn't the negatively charged electrons crash into the positively charged nucleus? And more importantly, since accelerating electrons give off X-rays and a circular orbit requires the electrons to be constantly accelerating, why didn't electrons continually emit X-rays and run out of energy?
Niels Bohr was a post-doctoral fellow working with Rutherford. He had an interest in theoretical physics and was the first to afford an explanation for the strange behaviour of the atoms. In 1913, using two postulates (one proposing quantum energy jumps; the other quantizing angular momentum) he derived a theoretical basis for the atom.
Further, his work provided a very convincing theoretical explanation for the spacing of line spectra observed by the likes of Bunsen and Kirchhoff. He argued each orbit in an atom was an integer multiple of a fundamental wavelength and only integer values were allowed.
Quantum mechanics at the deepest theoretical levels is a complex subject but the concept of quantized transitions is easy to observe on a daily basis. Consider something as simple as climbing the stairs. Each stair is a separate and distinct integer level.
That is, there is a first stair and a second stair but none in-between. Further, when walking up a flight of stairs, it is impossible to step just halfway or three-quarters of the way up to the next one. You either make the step or fall flat on your face.
This happens with electrons in the atom.
They are either in the first orbit or the second orbit but nowhere in between. Shifting from one orbit to the other involves energy.
The input of energy lifts an electron to a higher orbit; the release of energy results in an electron dropping to a lower orbit.
Since electrons can only be in quantized orbits, Bohr's model explained almost everything. But it soon became apparent that "almost" wasn't enough. Other researchers realized the orbits the electrons occupied were elliptical and electrons have spin. It is a pair of electrons which occupy each atomic orbit.
Over the two decades which followed, quantum mechanics was born and our explanation of the atom was refined. Electrons don't actually orbit the nucleus but occur as wave-functions occupying a region of energy-space around the nucleus called an orbital. The orbitals have shapes and forms based on their angular momentum and principal quantum number. Each orbital can only hold two electrons and within an atom individual electrons are uniquely defined. And the nucleus contains both protons and neutrons.
From the perspective of the periodic table, we now understand the basis for its shape and why the chemistry for the elements in each column is so similar as it is based on the orbitals occupied by electrons.
The first two columns on the left - headed by hydrogen and beryllium - have an
s-orbital as their outer orbital which is filled with either one or two electrons, respectively. The block of elements on the right hand side feature three different p-orbitals and a total of six possible electrons giving a group six columns wide.
The odd structure of the table reflects the underlying theoretical structure. But is important to remember the periodic table pre-dated the explanation.
Is the table finished? By no means.
With the discovery of the structure of the atom and nucleus, scientists have been able to fill in the remaining blank in the heart of the table (technetium) and extend the table to 118 elements.
Most artificial elements are incredibly short-lived with half-lives only microseconds long but this work means it is doubtful the table will ever be complete.
