The periodic table contains all the elements that have been discovered. The periodic table shown below contains all the elements up to element 88, radium. There are 118 known elements but after element 94, Plutonium, these elements are artificial and do not occur naturally.

The early periodic table

The modern periodic table is of great value to scientists. The elements are arranged according to their atomic number and placed into groups according to their chemical properties. This enables scientists to predict and work out how elements will react and behave even if they have never seen or used these elements before. However it took many years to arrive at this version of the periodic table and early periodic tables were very different to the one we are familiar with today.

You may have wondered why the periodic table is the shape it is, why not arrange the elements in say a square, a rectangle or even a circle! Well believe it or not once you get to know your way around the periodic table you will quickly realise why it is a great asset to any chemistry student and why it is set out the way it is. Imagine for a moment you were starting to build a jigsaw but without a picture of what you are trying to build. You might start with all the edge pieces, or put all the pieces of say the sky together or any pieces of say a building that you thought might be in the jigsaw picture.

Well when trying to construct the periodic table over 150 years ago scientists faced a similar problem. New elements were being discovered all the time, they had some limited information about these elements, some of which was correct and some incorrect and of course they knew nothing about protons, neutrons or electrons or our ideas about the structure of atoms. Some of the elements that were put in early versions of the periodic table were in fact compounds and lots of elements still had not been discovered yet. So no wonder it was proving difficult to find any sense or pattern in how the elements reacted and how they might be grouped. They had to piece all this new information together and try to find any common patterns in how the elements reacted. Like many new ideas they did not always get it right first time! They used the information they had available at the time to build the best models they could, like most scientific ideas as more evidence/information became available their ideas changed.

John Dalton - An early attempt at constructing a periodic table

In 1803, John Dalton, an English School teacher, who was interested in physics, meteorology and chemistry (atomic theory) produced a periodic table of sorts. It contained a number of observations:

• Atoms of a particular element are identical and they join with other elements to form compounds.
• Elements were made of small particles called atoms.
• Atoms of different elements varied in mass.
• Compounds are made up of elements that combine in whole number ratios only.
• In chemical reactions atoms rearrange themselves into compounds. The atoms are not changed in these compounds and it is possible to break the compounds back up into the elements that make them up.
• Dalton also made an attempt at calculating the masses of some elements, though his calculations were not always correct.

Over 20 years later Dalton published a revised volume of work which contained his periodic table of elements, in fact some of his elements were actually compounds! A revised version of John Dalton's periodic table is shown below. His symbols look at little odd when compared to those used today, they are a little difficult to remember and they have not been used since. But it was a start at constructing a periodic table and trying to group the elements together in some way and spot patterns in their chemical reactions.

The triads

A German chemist named Johan Wolfgang Dobereiner made some interesting observations in the late 1820s which would help chemists in developing a periodic table, but his ideas were not given enough weight and dismissed as a curiosity or coincidence. Dobereiner noticed that the properties of bromine seemed similar to those of chlorine and iodine. Bromine is a liquid and he placed it between chlorine, a gas and iodine, a solid. Dobereiner also noticed that the mass of bromine was approximately half-way between that of the element above it and the one below it- chlorine and iodine. Dobereiner also noticed similar patterns in the masses of the elements calcium (Ca), Strontium (Sr) and barium (Ba), as well as sulfur (S), selenium (Se) and tellurium (Te). These triads (groups of three elements), together with the atomic masses of the elements should have helped scientists identify patterns in the elements and to construct a periodic table to group them together in some related way.

John Newlands- The Law of Octaves

Around 1865 an English chemist named John Newlands made a breakthrough. He published his "Law of Octaves". Newlands took all the known elements and arranged them in order of their atomic mass. He noted that every eighth element had similar chemical properties e.g. starting with hydrogen and counting 8, we come to the element fluorine, similar eight elements after fluorine is chlorine and so on.... However his observation that every eighth element had similar properties only worked for the first 20 elements, after that it failed. For example the elements lithium, sodium and potassium all react violently with water, but Newlands put the element copper, which does NOT react with water, in the same group as these elements.

There were other problems with his periodic table.
In places Newlands has more than one element in a box in his table and he put elements like iron in the same group as oxygen and sulfur when these elements have little in common. Newlands grouped these elements together in order to get them to fit his ideas even though it was apparent that some elements in his groups were not similar at all. He assumed that all the elements had been discovered when in reality new elements were being discovered fairly regularly. His ideas were rejected by his peers and he was even ridiculed by some!

Dmitri Mendeleev

In 1869 the Russian scientist Dmitri Mendeleev improved on Newlands periodic table. Like Newlands he arranged the elements in order of their atomic masses and grouped elements with similar chemical properties together in their own groups, but unlike Newlands he assumed that there were still elements to be discovered and he left gaps for these undiscovered elements in his table. Not only that, he predicted the properties of some of these as yet undiscovered elements based on their position in his periodic table. When these unknown elements were finally discovered it was found that their properties were very similar to those predicted by Mendeleev

Part of Mendeleev's periodic table is shown opposite. It contains the elements boron(B) and aluminium (Al) in one group with their atomic masses. Underneath aluminium Mendeleev left a gap in his table for an element that he believed was as yet undiscovered. He named this undiscovered element eka-aluminium. Similarly underneath carbon (C) and silicon (Si) was yet another undiscovered element that Mendeleleev named eka-silicon. In 1875 the element gallium (Mendeleev's eka-aluminium) was discovered and in 1886 the element germanium (Mendeleev's eka-silicon) was discovered. These two new elements had chemical properties and physical properties that were very close to Mendeleev's predicted properties. This suggested that Mendeleev's hypothesis and ideas on the structure of the periodic table were probably correct.

Key Points

• John Dalton suggested that atoms of different elements had different masses and that they combine to make compounds in fixed proportions.
• Dobereiner made some important obervation regarding the masses of elements which suggested how they could be arranged in a periodic table.
• John Newlands arranged the known elements in a table and made an observation that patterns in chemical behaviour appeared to repeat every eighth element. However his table only contained just under 70 elements and his patterns of chemical reactivity only really worked for the first 20 elements. Newlands made the mistake of assuming that all the elements had been discovered, when in fact many more were being discovered fairly frequently.
• Mendeleev's periodic table like Newlands had the elements arranged by their masses, but he left gaps for what he considered was for elements that had yet to be discovered. He also made prediction about the properties of these undiscovered elements which turned out to be quite accurate.

Check your understanding - Questions on the periodic table

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