The evolution of the periodic table

The periodic table is known the world over, to scientists and non-scientists. While only scientists might notice if some elements were misplaced, the overall arrangement of little squares containing a number and element symbol is immediately recognisable. It didn’t always look like this though, so let’s explore some of the highlights of its development.

In your class

This article describes some of the work of the early scientists that led to the modern periodic table. It introduces students to the importance that knowing reasonably accurate atomic masses played in successfully ordering the elements, and shows that the early scientists were not always correct in their assumptions.

The article provides a historical background that fits nicely alongside the curriculum and will be of interest to anyone studying the chemistry of the modern periodic table. The activity will bring a piece of scientific history to life.

1789: Simple substances

French chemist Antoine Lavoisier drew up the first list of so-called ‘simples’ – substances which at the time could not be broken down further. He included elements known as ‘earths’, such as magnesia and barytes, but left out soda and potash since he suspected they could be broken down.

1805: Atomic masses

The first table of what we now call atomic masses appeared in print, produced by John Dalton.

Chemists had also begun to establish which elements combined to make certain substances, and what proportions combined to make those substances. This led to hypotheses about the relative masses of elements. By modern standards the values seem poor, but it was difficult to measure combining quantities of elements when they reacted.

1807: Sodium, potassium and other metals

Humphry Davy broke down soda and potash, discovering the new metals sodium and potassium had remarkably similar properties. Scientists around this time also recognised similarities between magnesium, calcium, strontium and barium.

1862: Grouping similar properties

French mineralogist Alexandre-Émile Béguyer de Chancourtois developed a system to arrange all the then-known elements according to their relative atomic masses. He marked the positions of the 60 known elements on a scale showing increasing atomic mass, but then wrapped the scale in a helical fashion around a cylinder of circumference 16 mass units. When plotted this way, elements with similar properties (ones we would now say are in the same group) were, to a good approximation, aligned in vertical columns. Lithium (7), sodium (23) and potassium (39) all differed by 16 units (almost) and were aligned on successive turns of the helix. Rubidium (87) being 48 units heavier than potassium was aligned with, but three turns below, potassium.

Unlike Mendeleev, Béguyer did not predict missing elements’ properties, but since he was plotting a graph, there was space for additional discoveries. Béguyer called his system ‘vis tellurique’ or, in English, the ‘telluric screw’ because the element tellurium was located in the centre of the chart, and, since he was primarily a mineralogist, because the name had a connection to the earth.

1869: Dmitri Mendeleev

Russian chemist Dmitri Mendeleev published his first periodic table. Nor was he alone; a number of independent workers came up with early version of the periodic system. It was thanks to the work of others like Béguyer that these early tables were possible. Mendeleev could not have created his table until a reasonably accurate set of atomic masses was known and there were enough elements to recognise patterns within their arrangement.

We now know elements in the periodic table are ordered by atomic numbers. Apart from a few instances (such as tellurium and iodine which have caused considerable confusion), this order is the same as the order of elements’ relative atomic masses.

2019: Happy anniversary!

International Year of the Periodic Table of Chemical Elements, proclaimed by the United Nations to raise awareness of its beauty and significance.

Article by Peter Wothers a teaching fellow in the department of chemistry, University of Cambridge and a fellow and director of studies in chemistry at St Catharine’s College, Cambridge, UK. Teaching resource by Catherine Smith, a secondary school teacher at Hinckley Academy and John Cleveland Sixth Form Centre, an 11–18 academy in Leicestershire.