History of the periodic table

Since the beginning of time, people have known about basic chemical elements such as gold, silver and copper, as these can all be discovered in nature in native form and are relatively simple to mine with primitive tools.

Aristotle, a philosopher, theorised that everything is made up of a mixture of one or more of four elements. They were fire, air, earth, and water. He also theorised that they change into new substances to form what we see.

Hening Brand was the first person to officially discover an element. Brand was a bankrupt German merchant who was trying to discover the Philosopher’s Stone — an object that is supposed to turn silver into gold. He experimented with distilling human urine until he finally got a glowing white substance which he named phosphorus. He kept his discovery secret, until 1680 when Robert Boyle rediscovered it and it became public.

By 1809, a total of 63 elements had been discovered. As the number of known elements grew, scientists began to recognize patterns in the way chemicals reacted and began to devise ways to classify the elements.

In 1817, Johann Dobereiner noticed that strontium had similar properties to calcium and barium, and that its atomic weight fell between them. He placed these three elements into a group, which he called a triad.

After compiling these, Dobereiner proposed that nature was made of triads of elements. He inferred that in a triad of elements, the middle element had the atomic weight of the average of the top and bottom elements' atomic weights. From this law, Dobereiner went on to discover the halogen triad composed of chlorine, bromine, and iodine and the alkali metal triad of lithium, sodium, and potassium.

This idea of triads became a popular area of study. Between 1829 and 1858, a number of scientists discovered that chemical relationships can extend beyond triads. During this time, fluorine was added to the halogen group; oxygen, sulphur, selenium, and tellurium were grouped into a family; and nitrogen, phosphorus, arsenic, antimony, and bismuth were classified as another.

John Newlands was an English chemist, who in 1863 wrote a paper which classified the 56 elements that had been discovered at the time into 11 groups which were based on similar physical properties. He noted that many pairs of similar elements existed which differed by some multiple of eight in atomic weight.

Newland took Dobereiner's ideas and expanded on them. He also organized his elements by mass and property, but he added a twist. Dobereiner had worked only in small groups, but Newlands wanted to relate all the elements to each other.

Newlands arranged the known elements by atomic weights. 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, the first elements in each of those groups were similar to one another, as was the second element in each group, and the third, and so on. There was a certain pattern in the properties of elements that became even more apparent as time went on.

Dmitri Mendeleev, a Siberian-born Russian chemist, was the first scientist to make a periodic table much like the one we use today. Mendeleev arranged the elements in a table ordered by atomic mass. His table was published in Principles of Chemistry in 1869.

Mendeleev also predicted the discovery of other elements and pointed out that some of the then-current atomic weights were incorrect.

Mendeleev provided for variance from atomic weight order, left space for new elements, and predicted three undiscovered elements. His table did not include any of the noble gases, which hadn't been discovered.

In 1913, Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number. Previous to this, atomic numbers were just random numbers based on an element's atomic weight. Moseley's discovery showed that atomic numbers were not arbitrary but had an experimentally measurable basis.

Mosley's research also showed that there were gaps in his table at atomic numbers 43 and 61 which are now known to be radioactive and not naturally occurring. Following in the footsteps of Dmitri Mendeleev, Henry Moseley also predicted new elements.

The modern periodic table we use almost daily in chemistry has changed a lot since Johan Dobereiner put a limited amount of elements in groups of three all those years ago.

The six people discussed have all played very important roles in not just creating the periodic table, but in evolving chemistry as a whole. Without these people’s research, who knows where we’d be today.

• WebElements periodic table