History of the Periodic Table

The development of the periodic table began in the 1800s as chemists started to recognize similarities in the properties of various elements and place them into families. Johann Döbereiner started with a few family groups each containing three elements. John Newlands put the elements of those families in repeating groups called octaves (as with musical notes). Lothar Meyer noted additional repeating properties. Dmitri Mendeleev put it all together into what has become the periodic table of the elements. The sections on this page give you some information about what they did.

Döbereiner's Triads

Some of the first elemental similarities were noted by a German chemist named Döbereiner in 1829.

His observations began with bromine which had just been discovered. He noticed that the properties of bromine were similar to chlorine and iodine. Not only were they similar but various properties of bromine, including the atomic mass, fell midway between the properties of chlorine and iodine. Not only were there similarities in the properties but also there was a pattern or trend within the group of regularly increasing atomic masses.

Newlands' Octaves

Somewhat later, about 1864, a chemist by the name of John Newlands came up with what he called the law of octaves. This idea was a bit more developed than Döbereiner's triads.

Newlands arranged the known elements by atomic mass. In doing so, he noticed some recurring patterns, and the patterns were such that if he broke up his list of elements into groups of seven (starting a new row with the eighth element), the first element in each of those groups were similar to one another. So was the second element in each group and the third and so on. Note below that helium, neon, argon, etc. (the inert gases) were not known at the time and were left out.

Meyer's Periods

In 1870, Lothar Meyer, a German chemist, made a chart that plotted atomic volumes against atomic mass. He measured the volume of one atomic mass's worth of each element - that is, one mole - and figured that since the number of atoms in each amount was the same, the volumes measured must represent the relative volumes of the individual atoms. By plotting those volumes against the atomic masses you can see that there is a recurring pattern - sort of like waves with the crests and the troughs, or hills and valleys. We can start with the element at the top of each one of those peaks. They are lithium, sodium, potassium, rubidium, and cesium. Following each of those there is a repeating pattern.

An important observation that Meyer made was the change in length of that repeating pattern. Unlike Newlands’ octaves, these groups were not all the same length. Hydrogen was sort of a group all by itself, lithium through fluorine was another group, sodium through chlorine was another, potassium through bromine, and so on. Notice that there are small groups at the beginning and then larger groups afterwards. In summary, there is repeating periodicity of the atomic volume, but the periods changed in size. The first period is one element in length - hydrogen. The second and third periods are seven in length. The fourth and fifth periods are seventeen elements in length. Subsequent to Meyer's work the inert gases were discovered, so we now have one more element in each period - making 2, 8, 8, 18, 18.

Mendeleev's Table

The most famous work that was done in developing the periodic table was done by a Russian chemist, Dmitri Mendeleev. Mendeleev developed his periodic table in 1869. It was a table, not a graph. It was a two-dimensional arrangement in which elements having similar properties were placed adjacent to one another. It took several forms as it developed.

First, like Newlands, he started a new line when the elemental properties repeated themselves. Li, Na, K, Rb and Cs each started a new line or a new period.

Second, he changed the order of some of the elements because the chemical and physical properties of those elements fit into the pattern better if their atomic masses were ignored (e.g. Te and I). In doing so, he foreshadowed the importance of the atomic number rather than the atomic mass.

Third, he left gaps in places where the properties of the "next" element did not fit the pattern. Then he predicted that elements would be discovered that had the properties that fit into the empty spots on his table. They were discovered within 15 years after his predictions. You can see this below in the blank spots beneath Al and Si. These are the elements Ga and Ge. Not only did Mendeleev predict their existence, he used his new table to predict their properties with startling accuracy. He was able to do this because elements were placed together in columns on the basis of shared properties, and those properties varied regularly throughout the table. We will learn much more about these variations in the coming sections.

In the decades after Mendeleev's first table, many advancements in chemistry changed the periodic table. New elements continued to be discovered, some of which fit Mendeleev's predictions and some of which did not. The discovery of protons and atomic number resolved the irregular spacing of atomic masses (for example, B and C differ by only 1 amu, whereas O and F differ by 3 amu; however, in both cases, the difference in atomic number is precisely one proton). Finally, the development of orbital theory and electron configurations revealed the deep mathematical symmetries that cause elements to share properties in regular repeating ways.

Today, the patterns of the periodic table are extremely well-understood, however it is still evolving. Nuclear physicists use ever-more-powerful tools to create heavier and heavier elements never previously observed. In the 1930's the table did not go past uranium. Just within the last decade or so, elements 113, 115, 117, and 118, still given placeholder names and symbols due to their recent discovery, have been named nihonium, moscovium, tennessine, and oganesson. It is likely that in coming years the table will need to add an eighth row.

A final note about this modern table: it is usually displayed with two rows separated out, floating like an island below the main table. However, this method of display is purely for convenience (it is difficult to fit the table neatly onto standard letter paper otherwise). Those rows fit into the main body of the table, following after barium and francium, with lutetium and livermorium sitting in the same column as scandium and yttrium (see below)