150 years on from Mendeleev’s periodic table: why it is still a work in progress

Science and Technology

“I saw in a dream a table where all elements fell into place as required. Awakening, I immediately wrote it down on a piece of paper, only in one place did a correction later seem necessary.” – Dmitri Mendeleev

On 6th March 1869, Dmitri Mendeleev stood before the Russian Chemical Society and presented ‘The Dependence between the Properties of the Atomic Weights and the Elements’ in which he demonstrated an apparent periodicity to the behaviour shown by elements, when ordered by their atomic weight.

150 years on, in the ‘International Year of the Periodic Table’ as designated by the United Nations, the results of his work can be found in every science classroom, surrounded by principles and facts discovered much later, such as the makeup of the atom. It is perhaps the symbol of science.

The periodic table has been through many iterations since 1869 as our finer understanding of chemistry has progressed. Although there are still ongoing debates as to what is the best way to present the periodic system, the current iconic form looks like it’s here to stay. But work on the periodic table never stops: in 2016 four new elements were named, taking the total to 118 and completing the seventh row.

These all are extremely unstable, existing for only fractions of a second before decaying, so the pursuit to make them may seem like a useless endeavour. For many scientists though, there is a greater goal in mind.

It is proposed that beyond this region of instability of the elements lies the ‘Island of Stability’. The elements here, predicted to begin element 126, contain ‘magic’ numbers of protons and neutrons, filling complete shells within the nucleus. As a result, they are more stable towards fission and also to alpha decay. It’s unknown how long these elements may be stable for: some say days, some say millions of years.

Work on the periodic table never stops: in 2016 four new elements were named, taking the total to 118.

By now, atomic synthesis is something which we understand, but it is the technical barriers which are hard to overcome. For example, a method where Calcium (atomic number 20) projectiles are shot at a heavy element target to create an even heavier element, would require Einsteinium (atomic number 99) as the target in order to generate element 119; but Einsteinium itself is difficult to make. One solution may be to use heavier projectiles, but the issue with this is that the probability of successful collisions is much lower.

In Dubna, Russia, a building suitably named the ’Superheavy Element Factory’ was recently built, with a particle accelerator, detectors and spectrometers to separate particles of different mass. Elements 119 and 120 are next on the list, beginning the eighth row of the table. It’s been calculated that this row, as well as any more beyond it, would contain a larger number of elements than the seventh. These rows would house a new ‘g-block’, where valence electrons would sit in previously unoccupied g-orbitals.

It’s unknown how heavy we can go. In 1911 Chemist Elliot Quincy Adams predicted that no element could weigh greater than 256 atomic mass units. But elements 99 and 100, with weights around 256, were made over 60 years ago. Richard Feynman calculated that an atom with more than 137 protons would violate special relativity: a larger nucleus means a great pull on the electrons, meaning they must orbit the nucleus at a faster and faster speed in order to not collapse, but of course this cannot extend beyond the speed of light. Feynman’s predictions were flawed though, as he treated nuclei as a single point, whereas massive nuclei behave very differently to what this assumptions suggests. When this is accounted for, the atomic number limit becomes 173.

In the 150 years since Mendeleev’s laid the foundations, advances in our scientific knowledge have extended our insight into the world around us and the experimental limits of our endeavours. The future will no doubt see us push these further, and although the new elements we make may have no great practical use, they will at least represent an ability to manipulate the building blocks of our world.

Image Credit: Serge Lachinov