Developing a modern periodic table: From spirals to the stars | Science Museum (2024)

The earliest attempts to classify matter, from metals to minerals, were based on the idea of essential qualities, or elements. Ancient Greek philosophers had four: air (hot and wet), water (cold and wet), earth (cold and dry) and fire (dry and hot).

Grouping chemical substances together according to similar qualities had practical value for alchemists and apothecaries, but there was no consensus or underlying order.

Things began to change at the end of the 18th century, as chemists introduced classification based on measurable quantities, like weight.

In 1789 one leading moderniser, the famed French chemist Antoine Lavoisier, produced a list of 33 ‘simple substances’ that made up the 300 or so chemicals known at the time, categorised by gases, metals, non-metals and earths.

John Dalton, a Manchester schoolteacher, introduced the idea of classifying elements by the relative weight of their atomsin 1806—a revolutionary concept at the time, which would be crucial to the invention of periodic tables.

Today we know of 118 elements that can be combined to form over 70 million different chemical compounds, so having a system to classify these at an elemental level is crucial.

In 1862 the French geologist Alexandre-Émile Béguyer de Chancourtois published a paper describing a classification of the elements.

De Chancourtois''telluric screw', as he called it, placed the elements in order of atomic weight in a spiral pattern around a cylinder.

De Chancourtois' periodic system.

Like Mendeleev’s table, but seven years before, de Chancourtois had stumbled upon a periodic repetition of chemical properties in his arrangement.

Unluckily for him, the groundbreaking paper escaped the attention of chemists for decades, largely because the publisher did not include a visual diagram of his screw system.

In 1926 the Science Museummade a working model of de Chancourtois' periodic screw.

By turning the handle, you can see chemical groupings of elements, such as lithium, sodium and potassium (three of the alkali metals),align vertically down the rotating column.

Atomic mass, weight or number?

• An element can come in different forms (isotopes), depending on how many neutrons it contains. Atomic weight is the average mass for an element across all its isotopes.
• Atomic mass is the mass of a particular atom or isotope.
• An element's atomic number is the number of protons it has. Atomic number replaced atomic weight as the value that determines the order of the elements in the periodic table.

Perhaps the most important periodic system post-Mendeleev was a three-dimensional double spiral system by English scientist William Crookesin 1888.

Crookes added the newly-discovered noble gases (helium, neon, argon, krypton, xenon, radon) to his system, which helped persuade Mendeleev that his table could be adapted to accommodate them.

But more interestingly, Crookes’ periodic system illustrated his theory of the evolution of the elements. In this, elements formed in a plasma (a hot state of matter) inside stars. As temperatures cooled, they evolved from the lightest element, hydrogen, at the top of his spiral, to the heaviest, uranium, at the bottom.

Crookes' original model physically embodies one of the first evolutionary theories of the elements.