| The development of the Periodic Table (post-16) |
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Julius Lothar Meyer – the first identifier of periodicity? |
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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), 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.
ActivityYou can investigate Meyer’s graph using a spreadsheet. The data on the spreadsheet 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). QuestionsQ 1. (a) What is the valency of the metal in the following compounds: CrCl2, CrCl3, TiCl2, TiCl4? (b) Where do most of the elements with more than one valency come in the Periodic Table? (c) What concept do we use nowadays to deal with the idea of valency? Q 2 (a) What sorts of elements are at the peaks of Meyer’s graph? (b) Explain in terms of their atomic structures why these elements have large atoms? (c) What trend in atomic volume does Meyer’s graph show as we cross a Period? (d) Use atomic structures to explain this trend. Q 3. Explain why oxygen, nitrogen and fluorine are missing from Meyer’s graph. Q 4. Explain in terms of atomic structure why the relative atomic masses of elements potassium and argon place them in a different order than do their atomic numbers. Q 5. (a) Calculate the atomic volume of potassium in cm3 mol-1 using the data Ar = 39.9, density = 0.86 g cm-3. (b) Convert your answer into nm3 mol-1 (c) What is the volume of a potassium atom in nm3 atom-1? (d) Using the formula that the volume of a sphere = 4πr3/3, calculate the radius of a potassium atom in nm3. (e) Is this in the right range for the size of an atom? |
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