Introduction
Ask most chemists who discovered the Periodic table and you will almost certainly get the answer Dmtri Mendeleev. Certainly Mendeleev was the first to publish a version of the Table that we would recognise today but does he deserve all the credit? What would have happened without Mendeleev, and is it really appropriate to use the word discovered?
No one can give definitive answers to these questions but it is certainly true that a number of other chemists before Mendeleev were investigating patterns in the properties of the elements that were known at the time and it is surely true that had Mendeleev never lived modern chemists would be using a Periodic Table.
This material looks at the work of other chemists such as John Newlands, Lothar Meyer and Alexandre-Emile Béguyer de Chancourtois as well as Mendeleev himself in developing the Periodic Table. / You may find other spellings of Mendelev’s name, Mendeleyev, Mendelejeff, Mendeleeff or Mendelayev, for example. There is no ‘correct’ spelling in English because the Mendeleev himself would have spelt it using the Russian (cyrillic) alphabet.
Some scientists, and many science fiction writers, have speculated that the Periodic Table might be the basis of communication with an alien civilisation if the human race were ever to meet one. If we make the reasonable assumption that the properties of the elements are the same everywhere in the universe, then it seems inevitable that a technologically advanced race would have their own version of the Periodic Table which would contain the same information as ours (even if expressed very differently).
John Newlands
Could the original formulation of the Periodic Table be regarded as British? Just four years before Mendeleev announced his Periodic Table, John Alexander Reina Newlands (see box) wrote in Chemical News
‘If the elements are arranged in order of their equivalents [ie relative atomic masses in today’s terminology] with a few transpositions, it will be seen that elements belonging to the same group appear in the same horizontal line. Also the numbers of similar elements differ by seven or multiples of seven. Members stand to each other in the same relation as the extremities of one or more octaves of music. Thus in the nitrogen group phosphorus is the seventh element after nitrogen and arsenic is the fourteenth elements after phosphorus as is antimony after arsenic. This peculiar relationship I propose to call The Law of Octaves'.
Surely this was a prediction of patterns in the properties of the elements and described a Periodic Table?
Newlands thought that patterns were connected with the relative weights of atoms (we would now call them relative atomic masses – they were then called atomic weights) of different elements. Fortunately, in 1860 there had been a conference in Karlsruhe (Germany) which had made a more accurate list of these atomic weights than had previously been available. Not only had some values been slightly wrong through inaccurate measurements but some were half or a third of the correct value through false reasoning. See box Problems with relative atomic masses.
One difficulty was that only about 60 elements were known then (there are over 100 now), although fortunately most of the undiscovered ones were of higher relative atomic mass. Newlands listed those known in order of their atomic weight putting their position in this sequence alongside the symbol. He did not give a name to this position number. A copy of his table is shown below using the symbols Newlands used.
H 1 / F 8 / Cl 15 / Co/Ni 22 / Br 29 / Pd 36 / I 42 / Pt/Ir 50Li 2 / Na 9 / K 16 / Cu 23 / Rb 30 / Ag 37 / Cs 44 / Tl 53
Gl 3 / Mg 10 / Ca 17 / Zn 25 / Sr 31 / Cd 34 / Ba/V 45 / Pb 54
Bo 4 / Al 11 / Cr 18 / Y 24 / Ce/La 33 / U 40 / Ta 46 / Th 56
C 5 / Si 12 / Ti 19 / In 26 / Zr 32 / Sn 39 / W 47 / Hg 52
N 6 / P 13 / Mn 20 / As 27 / Di/Mo 34 / Sb 41 / Nb 48 / Bi 55
O 7 / S 14 / Fe 21 / Se 28 / Ro/Ru 35 / Te 43 / Au 49 / Os 51
The pattern was perfect up to calcium then became less convincing as some metals appeared unlike the non-metals to their left. However a further seven elements later there was a greater similarity. Then Newlands was forced to sometimes put two elements in the same box so that after this similar elements would be in the same horizontal line. Di stood for didymium, which we now know is not an element at all but a mixture of two elements.
Note that Newlands did not always stick to a strict increase in number. He exchanged the positions of Zn and Y, presumably because he realised that Y resembled Bo (modern symbol B). The modern Periodic Table does not always show an increase in relative atomic masses for successive elements but it is a less common occurrence than in Newlands’ table.
On 1st March 1865, he described his ideas at a lecture at the Chemical Society (a forerunner of the Royal Society of Chemistry). The lack of spaces for undiscovered elements and the placing of two elements in one box were justifiably criticised but an unfair suggestion from Professor Foster was that he might have equally well listed the elements alphabetically. Foster was on the Publication Committee which refused to publish his paper, supposedly because it was of a purely theoretical nature. Humiliated, Newlands went back to his work as chief chemist at a sugar factory.
John NewlandsNewlands was British - his father was a Scottish Presbyterian minister. His unusual third name, was a family name showing his mother's Italian roots, indeed he fought on Garibaldi's side during the campaign to reunify Italy. He was born in London on 26th November 1837.
He was educated by his father at home rather than at school then studied for a year (1856) at the Royal College of Chemistry which is now part of Imperial College London. Later he worked at an agricultural college trying to find patterns of behaviour in organic chemistry. However he is more remembered for his search for a pattern in inorganic chemistry. /
John Newlands. Reproduced courtesy of the Library and Information Centre, Royal Society of Chemistry.
The house where Newlands was born, at 19, West Square, London, near the Elephant and Castle underground station. Reproduced courtesy of Gordon Woods.
Four years later Mendeleev, unaware of Newlands' ideas, formulated an improved Periodic Table which gained acceptance, particularly because he left spaces for undiscovered elements, some of which were soon found with properties he predicted. As the Periodic Table became accepted, Newlands, understandably, claimed its first publication. However the Chemical Society did not back his claims. Indeed the final years of his working life were spent running a family chemical business with his brother.
The Chemical Society made some amends for discrediting him by asking him in 1884 to give a lecture on the Periodic Law. However its full recognition of his discovery waited until 1998, the centenary of his death, when the Royal Society of Chemistry oversaw the placing of a blue commemorative plaque on the wall of his birthplace. Note its inscription.
Reproduced courtesy of Gordon Woods
Problems with relative atomic masses
What was the faulty reasoning that led to inaccurate relative atomic masses (atomic weights)? There were two main faults. First chemists were not distinguishing between the weights of atoms and of molecules. Seven common elements exist as diatomic molecules (molecules containing two atoms, such as oxygen, O2), of special importance being hydrogen, the original standard for atomic weights. If a molecule of H2 is given a relative mass of 1 instead of 2, then when other elements are compared with it, their relative atomic masses are halved.
Second, at the time chemists used a term called equivalent, or combining weight. This was the number of grams of an element that combined with 8 g of oxygen (They used this because 8 g of oxygen combine with 1 g hydrogen so 8 g of oxygen was equivalent to 1 g hydrogen.) Chemists used this because it is in general easier practically to measure the weight of an element that combines with oxygen than the weight that combines with hydrogen. Atomic weights were then found from the equivalent weight using the relationship:
Equivalent weight x valency = atomic weight
where valency is the combining power of an element (the number of atoms of hydrogen that would combine with an atom of the element).
For example the equivalent weight of carbon is 3 g, because 3 g of carbon combine with 8 g oxygen. The valency of carbon is 4 because it forms the compound methane, CH4. So the relative atomic mass of carbon is 3 x 4 = 12.
Even when the equivalent weight was accurately determined, if the valency was wrong then a simple fraction of the correct atomic weight was obtained. (In the above example, if the valency of carbon was thought to be two, the value for carbon’s atomic weight would be 6.) The combining (or equivalent) weights were generally accurate but sometimes an element was given the wrong valency. Thus beryllium, combining weight 4.6, was given the valency 3 because it was chemically similar to aluminium. This gave an atomic weight of 13.8, placing it between carbon and nitrogen where there was no space.
Questions
Q 1. Newlands was the first person to give elements a 'position number'.
(a) What do we call this today?
(b) What property of the atomic nucleus is it related to?
Q 2. (a) Which modern chemical group is missing from Newlands’ list of elements?
(b) Apart from the omission of this whole group which element is the first omitted?
Q 3. There are some unfamiliar symbols. Give the modern symbols and names of Gl and Bo.
Q 4. Give the symbols of two elements which are at least two columns out of position because of extremely inaccurate atomic weights.
Q 5. What do columns in the Newlands’ table almost correspond to in a modern Periodic Table?
Q 6. Where do the alkali metals Li, Na, K etc appear in Newlands’ table? Give the names of the extra elements mixed with them. Why do you think Newlands thought they had a similarity with the alkali metals? (Hint; what important property do elements in the same group share?)
Q 7. A musical octave goes from a note in one octave to the corresponding note in the next octave, eg from C to C, counting the notes at both ends. How many elements are there between Li and Na, or Na and K? What should this figure be? Explain the difference.
Q 8. Give two ways in which the Newlands’ table is inferior to that of Mendeleev.
Julius Lothar Meyer – the first identifier of periodicity?
Most people regard Mendeleev as the initial formulator of the Periodic Table with the work of his contemporaries often ignored. One such chemist was the German Julius Lothar Meyer, (1830 - 1895), see box, who was just four years older than the Russian.
In 1864, five years before the first announcement of a Periodic System by Mendeleev, Meyer had produced a table of just 28 elements which he listed by their valence. [The term valence is now called valency and represents ‘combining power’ of an element. For example sodium forms a chloride NaCl and has a valency of one; magnesium forms MgCl2 and has a valency of two and so on.] The 28 elements were almost entirely main group elements. He incorporated transition metals in another table in 1868 which listed the elements in increasing weight order with elements with the same valence in a given column. This was earlier than Mendeleev's table (1869) but unfortunately Meyer's was not published until 1870.
Mendeleev and Meyer were unaware of each other’s work until after this. Later, Meyer admitted that the Russian had first published about the Periodic System by saying that his ideas matched those of Mendeleev.
However Meyer’s main contribution was his recognition of periodic behaviour, ie a repeating pattern of a property shown on a graph. In the case of Meyer, the property he chose was the atomic volume of an element and he plotted against its atomic weight.
The graph below clearly shows a periodic pattern as the atomic volume rises to peaks and then falls again.
Atomic volume is the volume of a mole of atoms (using modern terminology) and is therefore a measure of the size of an atom of a particular element provided that the element is in the solid state where the atoms are packed closely together.
Meyer and Mendeleev had a great deal in common. They both attended the crucial first world chemistry conference at Karlsruhe which produced a significantly more accurate list of atomic weights than had previously existed. This was the key to arranging the elements in sequence. They both trained at Heidelberg University under Bunsen and Kirchoff. Thus they would certainly have known each other although neither was aware of all the work done by the other. Meyer's roots, however, were firmly in Germany. /
Julius Lothar Meyer . Reproduced courtesy of the Library and Information Centre, Royal Society of Chemistry.
Activity
You can investigate Meyer’s graph using a spreadsheet. The data on the spreadsheet that accompanies the web site is modern and is therefore more accurate than that which Meyer had. It also includes the elements discovered since Meyer’s time. You could eliminate these by using the interactive Periodic Table to find which elements were known in Meyer’s time (1868). You can use the spreadsheet to calculate the atomic volume of each element and then to plot this quantity against relative atomic mass. You should also omit from your graph elements that are not in the solid state at room temperature. This is because in liquids and especially in gases, the atoms are not packed closely together so the volume of a mole of atoms does not tell us much about the size of the atoms of an element (it actually tells us about the spacing between them).