To many students of chemistry, the Periodic Table of Elements seems to be a monolithic, unchanging pillar of the discipline. However, a recent measurement of the properties of lawrencium (element 103) has rekindled an old controversy regarding its position on the table (1).
According to William Jensen, a chemistry professor who specializes in chemistry education, many modern chemists and students fall victim to fundamental misconceptions regarding the nature and purpose of the Periodic Table. He argues that the table is more than a list of elements by their atomic number.
To him, the most important criterion that the table must meet is that it is actually periodic as its name implies. The behavior of an element is closely related to its atomic number, but the correlation is not perfect, and Jensen feels that the table should be changed to reflect the actual behavior of certain heavy elements, namely lawrencium and lutetium (2).
In the eighties, Jenson was a proponent of replacing lanthanum and actinium with lawrencium and lutetium respectively. Columns in the table share similar trends in chemical properties such as atomic radii, ionization energies, melting points, and electronegativity values. Jenson argued that the Sc-Y-Lu series mirrored the trends of other columns in the d-block (the large central block on the table) more closely than the current Sc-Y-La arrangement. He made a similar argument in favor of putting lawrencium in the position currently occupied by actinium (2).
Jenson’s arguments found new support in a recent measurement of lawrencium’s ionization energy. An element’s ionization energy, measured in electron volts (eV), is the energy required to strip its outermost electron, and is related to its chemical reactivity. The measurement itself was very difficult to accomplish since lawrencium is not present in nature, and has half-life of just 27 seconds when created in a lab setting (1).
First, the team of chemists created individual atoms of the element using a particle accelerator. Then they placed the atoms on very hot surface, and by counting the number of electron knocked loose by the heat energy, they were able to measure lawrencium’s ionization energy (3). They measured 4.96 eV, a surprisingly low value, which hints that the element may belong closer to the left side of the table (1).
The International Union of Pure and Applied Chemistry (or IUPAC) will ultimately decide where to place the elements, but they currently have no official position on the matter. According to Jan Reedijk, the president of IUPAC’s inorganic-chemistry division, the organization may come make its decision this summer (1).
1. Castelvecchi, D. (2015, April 8). Exotic atom struggles to find its place in the periodic table. Retrieved April 19, 2015.
2. Jenson, W. (2015). The positions of lanthanum (actinium) and lutetium (lawrencium) in the periodic table: An update. Foundations of Chemistry. Retrieved April 19, 2015, fromhttp://link.springer.com/article/10.1007/s10698-015-9216-1/fulltext.html
3. Sato, T. (2015). Measurement of the first ionization potential of lawrencium, element 103. Nature, 520(7546), 209-211. Retrieved April 19, 2015, from http://www.nature.com/nature/journal/v520/n7546/full/nature14342.html