Mar 062018

On this date in1869 Dmitri Mendeleev presented his first periodic table to the Russian Chemical Society. There were previous periodic tables and Mendeleev’s publication almost coincided with German chemist Julius Lothar Meyer’s table. They both constructed their tables by listing the elements in rows or columns in order of atomic weight and starting a new row or column when the characteristics of the elements began to repeat. This ordering of the elements was a significant breakthrough in the evolving understanding of the structure of elements, and is still the basis for the periodic table in use today. In modern terms, the rows represent the electron shells, and the columns represent the number of electrons in the outer shell starting with one in the column of the left and ending with a full shell in the column on the right. On the far left the elements are highly reactive, on the far right they are virtually unreactive. Thus, in the first row, Hydrogen, on the left, is very reactive and Helium, far right, is inert. That’s why you fill a balloon, or zeppelin, with Helium and not Hydrogen.

In 1789, Antoine Lavoisier published a list of 33 chemical elements, grouping them into gases, metals, nonmetals, and earths. Chemists spent the following century searching for a more precise classification scheme. In 1829, Johann Wolfgang Döbereiner observed that many of the elements could be grouped into triads based on their chemical properties. Lithium, sodium, and potassium, for example, were grouped together in a triad as soft, reactive metals. Döbereiner also observed that, when arranged by atomic weight, the second member of each triad was roughly the average of the first and the third; this became known as the Law of Triads.

German chemist Leopold Gmelin worked with this system, and by 1843 he had identified ten triads, three groups of four, and one group of five. Jean-Baptiste Dumas published work in 1857 describing relationships between various groups of metals. Although various chemists were able to identify relationships between small groups of elements, they had yet to build one scheme that encompassed them all.

In 1857, German chemist August Kekulé observed that carbon often has four other atoms bonded to it. Methane, for example, has one carbon atom and four hydrogen atoms. This concept eventually became known as valency; different elements bond with different numbers of atoms.

In 1862, Alexandre-Emile Béguyer de Chancourtois, a French geologist, published an early form of periodic table, which he called the telluric helix or screw. He was the first person to notice the periodicity of the elements. With the elements arranged in a spiral on a cylinder by order of increasing atomic weight, de Chancourtois showed that elements with similar properties seemed to occur at regular intervals. His chart included some ions and compounds in addition to elements. His paper also used geological rather than chemical terms and did not include a diagram; as a result, it received little attention until the work of Dmitri Mendeleev.

In 1864, Julius Lothar Meyer, a German chemist, published a table with 44 elements arranged by valency. The table showed that elements with similar properties often shared the same valency. Concurrently, William Odling (an English chemist) published an arrangement of 57 elements, ordered on the basis of their atomic weights. With some irregularities and gaps, he noticed what appeared to be a periodicity of atomic weights among the elements and that this accorded with “their usually received groupings”. Odling alluded to the idea of a periodic law but did not pursue it. He subsequently proposed (in 1870) a valence-based classification of the elements.

English chemist John Newlands produced a series of papers from 1863 to 1866 noting that when the elements were listed in order of increasing atomic weight, similar physical and chemical properties recurred at intervals of eight; he likened such periodicity to the octaves of music. This so termed Law of Octaves was ridiculed by Newlands’ contemporaries, and the Chemical Society refused to publish his work. Newlands was nonetheless able to draft a table of the elements and used it to predict the existence of missing elements, such as germanium. The Chemical Society only acknowledged the significance of his discoveries five years after they credited Mendeleev.

The recognition and acceptance afforded to Mendeleev’s table came from two decisions he made. The first was to leave gaps in the table when it seemed that the corresponding element had not yet been discovered. Mendeleev was not the first chemist to do so, but he was the first to be recognized as using the trends in his periodic table to predict the properties of those missing elements, such as gallium and germanium. The second decision was to occasionally ignore the order suggested by the atomic weights and switch adjacent elements, such as tellurium and iodine, to better classify them into chemical families. Later, in 1913, Henry Moseley determined experimental values of the nuclear charge or atomic number of each element, and showed that Mendeleev’s ordering actually corresponds to the order of increasing atomic number.

The significance of atomic numbers to the organization of the periodic table was not appreciated until the existence and properties of protons and neutrons became understood. Mendeleev’s periodic tables used atomic weight instead of atomic number to organize the elements, information determinable to fair precision in his time. Atomic weight worked well enough in most cases to (as noted) give a presentation that was able to predict the properties of missing elements more accurately than any other method then known. Substitution of atomic numbers, once understood, gave a definitive, integer-based sequence for the elements, and Moseley predicted (in 1913) that the only elements still missing between aluminium (Z=13) and gold (Z=79) were Z = 43, 61, 72 and 75, all of which were later discovered. The sequence of atomic numbers is still used today even as new synthetic elements are being produced and studied.

I learned, much to my surprise as a young boy that cooking is a form a chemistry. My mother explained to me that when the dry ingredients of a cake are mixed you can take your time, but when you add liquid you have to be quick because the chemical process producing CO2 has started and does not last indefinitely. This was a major turning point in my life – all knowledge can me integrated !!  Here’s a video on chemistry and baking soda. After it I will comment.

The basic idea is fine: make two cake batters except one has baking soda and one does not. Bake them and see what happens. No surprises. The one without baking soda does not rise as much. Note that it does rise a little. Why? Also note she says that the cake without baking soda is heavier than the one with soda. This means she did not conduct a proper experiment. She should have used the same weight of batter for both types of cake. The resultant cakes should be the same weight. The one with baking soda will be fluffier, but not lighter. This experiment gets a C- from me.

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